Earthquakes, Tsunamis, Volcanoes, Landslides, Coastal Erosion, and Land Subsidence
What You’ll Learn
• Types of geological hazards
• How earthquakes happen
• Scales used to rate earthquakes
• Causes and impacts of tsunamis
• Types of volcanoes and volcanic eruptions
• Causes of landslides and debris flows
• The consequence of excessive groundwater removal
• How human activity may cause geological hazards
Goals and Outcomes
• Become familiar with the terminology that applies to geologic hazards
• Compare geologic events according to magnitude and intensity
• Evaluate how certain geologic events are connected
• Compare the potential impacts of major geological hazards on a community
• Consider effective ways to inform communities about geologic hazards
• Predict the risk of geological hazards for an area using knowledge of past occurrences
• Evaluate decisions made about real communities based on geologic research
Many geological hazards share one thing in common: the buildup of pressure over time. In this chapter, we describe several geological hazards that have serious consequences for humans. Earthquakes, which can have far-ranging and catastrophic effects, are examined in detail. The chapter also covers tsunamis, volcanoes and landslides and the potential impacts these hazards can have on communities. The implications of coastal erosion are discussed in relation to patterns of building and human activity, as well as a section on land subsidence and sink holes. We also touch on ways human activity may lead to some types of geological events, including mine collapse, fracking, subsurface withdrawal of water, oil, and gas, as well as underground injection of wastewater.
The Earth is composed of the core, mantle, and crust.
• The crust is a paper-thin layer on the surface, extending to about 5 kilometers (3.1 miles) below the oceans and about 30 kilometers (18.6 miles) on average beneath the continents.
• The mantle is the middle layer, estimated to be 2900 kilometers (1802 miles) thick.
• The core is the innermost region of the Earth, over 3500 kilometers (2175 miles) thick.
Much of the action associated with the geological hazards described in this chapter occurs in the mantle, but it greatly affects the crust, where we live. Quite literally, the “pressure is on” for the crust of Earth, which is also the most brittle of the Earth’s three layers.
This chapter describes geological hazards that occur at the mercy of these shifting layers over time and that have very serious consequences for human settlements, posing threats to both people and property. It is important to gain at least a basic understanding of the science behind these hazards in order to take steps to protect our communities from their impacts.
EVIDENCE OF WHAT’S INSIDE
Earth is about 6370 kilometers (3959 miles) deep from surface to the center, but we have been able to drill wells only a few kilometers deep. So how can we tell what’s inside the Earth? Several kinds of evidence have been used, including
• Study of once-deeply buried rocks that have been uncovered by erosion
• Study of interior materials that emerge as lava and gases from volcanoes
• Analysis of earthquake waves that pass through Earth
• Study of the curious configuration of Earth’s continents and ocean basins
• Study of Earth’s movements through space
• Study of meteorites
Collectively, this research has provided evidence of Earth’s structure and composition.1
SELF-CHECK
• Define crust, mantle, and core.
• List five geologic hazards that are related to the Earth’s shifting layers.
• List several pieces of evidence about the composition and structure of Earth’s interior.
The Greek word seismos means to quake; seismology is the study of earthquakes. Earthquakes are geologic events that involve movement or shaking of the Earth’s crust and upper mantle, otherwise known as the lithosphere. The upper part of the mantle is cooler and more rigid than the deep mantle below it; thus it behaves similarly to the overlying crust. The two layers together are broken into the moving plates that contain the continents and oceans. It is the movement of these thin tectonic plates that is so influential in an earthquake.
Earthquakes are usually caused by the release of stresses accumulated as a result of the rupture of lithosphere rocks along opposing boundaries, or fault planes, in the crust. These fault planes are typically found along borders of the Earth’s tectonic plates. The plate borders generally follow the outlines of the continents, with the North American plate following the continental border with the Pacific Ocean in the west, but following the mid-Atlantic trench in the east.
4.3.2 Tectonic Plate Movement: A Constant Wrestling Match
On average tectonic plates move only a few inches a year. However, when the seven large plates that encompass entire continents and giant portions of oceans move in opposing directions, a few inches can be highly significant.
The areas of greatest tectonic instability occur at the perimeters of the slowly moving plates because these locations are subject to the greatest strains from plates traveling in opposite directions and at different speeds. Deformation along plate boundaries causes strain in the rock and the consequent buildup of stored energy.
Tectonic plate activity is similar to a constant arm wrestling match underneath our feet, with each side exerting tremendous force against the other. Eventually the built-up stress exceeds the rocks’ strength and a rupture occurs. When the rock on both sides of the fault snaps, it releases the stored energy by producing seismic waves. These waves are what cause the ground to shake in an earthquake.
The variables that characterize earthquakes are ground motion, surface faulting, ground failure, and seismic activity. Ground motion is the vibration or shaking of the ground during an earthquake. When a fault ruptures, seismic waves radiate causing the ground to vibrate. The severity of the vibration increases with the amount of energy released and decreases with the distance from the causative fault, or epicenter. Accordingly, damage is generally most severe at or near the epicenter, although this is not always the case.
• Surface faulting is the differential movement of two sides of a fraction—the location where the ground breaks apart. The length, width, and displacement of the ground characterize surface faults.
• Liquefaction is the phenomenon that occurs when ground shaking causes loose soils to lose strength and act like viscous fluids. Liquefaction causes two types of ground failure: lateral spread and loss of bearing strength.
• Lateral spreads develop on gentle slopes and entail the sidelong movement of large masses of soil as an underlying layer liquefies.
• Loss of bearing strength results when soil liquefies causing buildings and other structures to tip and even topple over.
Earthquakes can occur in sequence. The first impact is generally the strongest, but aftershocks, or subsequent earthquakes may follow. Aftershocks usually occur within 2 days following the initial quake, but may continue for weeks, months, or even years. Their size, strength, and frequency usually diminish with time. If an aftershock is larger than the main shock, the aftershock is re-designated as the main shock and the original main shock is re-designated as a foreshock.
Why do aftershocks occur? Even though the major strain between two plates is released by the initial quake, their touching edges still need to adjust to new positions. The edges may not be able to pass each other smoothly, and this additional realignment creates the smaller shocks.
When the earth’s plates finally stop grinding against each other, the surface landscape may have changed over huge areas around the earthquake’s center. This happened after the 1906 earthquake in San Francisco, California, when a piece of land 430 kilometers (267 miles) long shifted north by 6 meters (20 feet).
Aftershocks can be very destructive. They are often of sufficient intensity to crumble roads, bridges and buildings that had been weakened by the initial earth movement.
4.3.5 Rating Earthquakes: Scales of Magnitude, Intensity, and Acceleration
Seismologists use several different standardized scales to measure the severity of earthquakes. These include the Richter magnitude, the modified Mercalli intensity (MMI), and peak ground acceleration (PGA), among others.
4.3.5.1 Measuring Magnitude with the Richter Scale
The Richter scale is an open-ended logarithmic scale that describes the energy release of an earthquake through a measure of shock wave amplitude. The instrument that records the amplitude of the seismic waves is called a seismograph. The corresponding earthquake magnitude is expressed in whole numbers and decimal fractions. Each unit increase in magnitude on the Richter scale (e.g., increasing the magnitude rating from 4 to 5) corresponds to a 10-fold increase in wave amplitude, or a 32-fold increase in energy.
The Richter scale does not have an upper limit. Some earthquakes can have magnitudes of 8.0 or higher. The Sumatra–Andaman Earthquake that caused the December 26, 2005 tsunami in the Indian Ocean had a magnitude of 9.15 on the Richter scale, making it among the highest magnitude ratings to ever be recorded by a seismograph. At the other extreme, an earthquake with a magnitude of less than 2.0 is called a microearthquake. Quakes can be detected when they are relatively light in strength; however, the seismograph may need to be closer to the center of the earthquake for measurement purposes.
4.3.5.2 Measuring Intensity with the Modified Mercalli Intensity Scale
The measurement of earthquake magnitude does not specifically address the various levels of damage that could result from an earthquake event. Damage potential is measured by intensity. Earthquake intensity is most commonly measured using the modified Mercalli intensity (MMI) scale, a 12-level scale based on direct and indirect measurements of seismic effects. The scale levels are typically described using roman numerals, ranging from
• I |
Imperceptible events (can only be detected by seismographic instruments) |
• IV |
Moderate events (can be felt by people who are awake at the time) |
• XII |
Catastrophic events (total destruction occurs) |
A detailed description of the MMI scale of earthquake intensity and its correspondence to the Richter scale is shown in the following table. The MMI scale incorporates the types of damages that can be expected with various earthquake intensities (see Table 4.1).
4.3.5.3 Measuring Acceleration with Peak Ground Acceleration
PGA is a measure of the strength of ground movements. The PGA measurement expresses an earthquake’s severity by comparing its acceleration to the normal acceleration due to gravity. For example, an object dropped from a height on the Earth’s surface will fall (ignoring wind resistance) toward the Earth faster and faster, until it reaches terminal velocity. This principle is known as acceleration and represents the rate at which speed is increasing. The acceleration due to gravity is often called “g,” a term popularly associated with roller coasters, rockets, and stock car racing. The acceleration due to gravity at Earth’s surface is 9.8 meters (980 centimeters) per second squared. Thus, for every second an object falls toward the surface of the Earth, its velocity increases by 9.8 meters per second.
Consider this analogy: a driver presses the gas pedal of a car forcefully, causing groceries in the trunk to smash against the back of the car. The quicker the driver presses on the gas, the more eggs are likely to be damaged. This effect is caused by the fast acceleration that forces the contents of the trunk of the car to shift rapidly and violently, not slowly or smoothly. A slower acceleration allows the groceries to remain stationary within the confines of the trunk. The groceries-in-the-trunk scenario approximates the behavior of an earthquake. If ground acceleration is rapid, more structures and objects experience damage than if the shaking is relatively slow, even if the ground moves the same distance.2
TABLE 4.1 Modified Mercalli Scale of Earthquake Intensity
Scale |
Intensity |
Description of Effects |
Maximum Acceleration (millimeter/second) |
Approximate Corresponding Richter Scale |
I |
Instrumental |
Detected only on seismographs |
<10 |
N/A |
II |
Feeble |
Some people feel it |
<25 |
<4.2 |
III |
Slight |
Felt by people resting; like a truck rumbling by |
<50 |
N/A |
IV |
Moderate |
Felt by people walking |
<100 |
N/A |
V |
Slightly strong |
Sleepers awake; church bells ring |
<250 |
<4.8 |
VI |
Strong |
Trees sway; suspended objects swing, objects fall off shelves |
<500 |
<5.4 |
VII |
Very strong |
Mild alarm; walls crack; plaster falls |
<1000 |
<6.1 |
VIII |
Destructive |
Moving cars uncontrollable; masonry fractures, poorly constructed buildings damaged |
<2500 |
N/A |
IX |
Ruinous |
Some houses collapse; ground cracks; pipes break open |
<5000 |
<6.9 |
X |
Disastrous |
Ground cracks profusely; many buildings destroyed; liquefaction and landslides widespread |
<7500 |
<7.3 |
XI |
Very disastrous |
Most buildings and bridges collapse; roads, railways, pipes, and cables destroyed; general triggering of other hazards |
<9800 |
<8.1 |
XII |
Catastrophic |
Total destruction; trees fall; ground rises and falls in waves |
>9800 |
>8.1 |
4.3.6 Earthquake Impacts on People and Property
Direct impacts from earthquakes include damage to structures and infrastructure, such as buildings, pipelines, roadways, and bridges. Impacts to a community from an earthquake event can include injury and death; loss of vital services including electricity, gas, and communications systems; lost revenue and economic damages; and increased demand on public safety, health and emergency facilities, and critical government services. Secondary impacts are common following earthquakes and include fire, loss of water supply and water pressure, hazardous material releases, explosions of gas and other flammable material and related incidents. Earthquakes can also trigger tsunamis and landslides.
Most injuries and deaths, as well as the majority of property damage from an earthquake are caused by ground shaking. Hard, brittle structures such as buildings and roadways tend to break and crack when they are forced to move due to the underlying ground movement. In contrast, resilient or flexible objects are less likely to suffer breakages. Structures can fail due to both horizontal and vertical shaking. Tall buildings tend to sway or vibrate, depending upon the construction materials and height as well as their distance from the epicenter. The top floors of buildings that are located close to one another may even collide during the swaying motion triggered by a large earthquake. Pancaking occurs when high-rise buildings collapse in on themselves because the poured concrete flooding separates from the corner fastenings, causing the floors to drop vertically down.
4.3.7 Earthquake Experience in the United States
Earthquake risk in the United States is significant in many regions of the country. The area of greatest seismic activity is along the Pacific Coast in California and Alaska, but as many as 40 states can be characterized as having at least a moderate earthquake risk. In the past 20 years, scientists have learned that strong earthquakes in the central Mississippi Valley are not freak events but have occurred repeatedly in geologic past. The New Madrid seismic zone is an area of major earthquake activity that experiences frequent minor shocks. Although earthquakes in the central and eastern United States are less frequent than in the western part of the country, these areas represent considerable seismic risk.
4.3.7.1 Notable Earthquakes
The United States has experienced earthquakes of all levels, some of which have caused catastrophic damage and loss of life. Of the top 10 largest earthquakes to occur in the United States, nine have taken place in Alaska with magnitudes of 7.9 or higher.* The earthquake with the second highest magnitude in the world took place in Prince William Sound, Alaska in 1964, with a magnitude of 9.2. This event took 125 human lives and generated a tsunami wave that reached an estimated height of 200 feet. Other significant earthquakes include the following:
• The San Francisco Earthquake of 1906 occurred on April 18, 1906, at 5:12 in the morning. The epicenter was near San Francisco, and the quake lasted approximately 45–60 seconds. The quake is believed to have been between VII and IX on the MMI scale. The quake could be felt from Southern Oregon to south of Los Angeles, and as far east as central Nevada. Reports of deaths vary from 500 to 700 people, although these figures may be underestimated. Significant damage occurred to the city as a direct result of the quake, but the event is remembered more for the devastating fires that broke out city-wide following the ground shaking. Gas lines snapped, and wood and coal burning stoves overturned. Water mains broke, hampering the work of city fire-fighters. Firebreaks were created throughout the city by blowing up buildings and entire neighborhoods to stop the spread of the flames.
• The Loma Prieta, California Earthquake occurred on October 18, 1989, at 5:04 p.m., the height of rush hour, measuring 6.7 on the Richter scale, and lasting for 15 seconds. As a result of the quake 63 deaths occurred, with over 3700 people injured. Property damage reached nearly $6–$8 billion, making this the most costly U.S. disaster up to that time. Over 1800 homes were destroyed, 2600 businesses were damaged, and 3000 people were made homeless. The two-tiered Bay Bridge and Nimitz Freeway both collapsed, crushing cars underneath and killing and trapping hundreds of motorists. Fire damage to infrastructure and buildings was extensive, gaping cracks were created in most of the main roads, and landslides were triggered in surrounding areas. Fans waiting to see the World Series baseball game in Candlestick Park rushed onto the field as the whole stadium swayed; the event was televised live nationwide until the broadcast was abruptly disconnected.
• The Northridge Earthquake occurred on Monday January 17, 1994 at approximately 4:30 a.m.; it measured 6.7 on the Richter scale and lasted 15 seconds. The epicenter was located 20 miles northwest of Los Angeles beneath the San Fernando Valley. Between 57 and 72 deaths and more than 9000 injuries were reported. The death and injury toll would have undoubtedly been much higher had the event not occurred on the Martin Luther King, Jr. holiday, when many commuters were at home. Nearly $44 billion in property damage occurred, with 25,000 dwellings made uninhabitable and thousands more severely or moderately damaged. The damage was so widespread that nine hospitals were closed, nine parking garages collapsed, 11 major roads into Los Angeles were closed, two bridges on the Interstate 10 Santa Monica Freeway collapsed, and countless other bridges and overpasses were damaged, paralyzing the city for weeks, and severely delaying rescue and recovery efforts.
4.3.8 Earthquakes: Lessons Learned
What can we learn from these past earthquake experiences? The 1906 San Francisco Earthquake, which caused millions of dollars in damage, was considered merely ill fortune at the time. San Francisco’s citizens thought little of the possibility of future quakes. The city was rebuilt following the disaster in almost identical fashion. Yet, that earthquake provided a wealth of information to scientists and, eventually, to planners and builders, about the nature of earthquake damage and the geological processes involved. In fact, the 1906 Earthquake is considered by some scholars as one of the most significant quakes in modern history because of the scientific knowledge gained. Damage reports from the 1906 event gave an early indication of the significance of the fault, although a large horizontal displacement and the great rupture length it displayed were not fully appreciated at the time. Although the theory of plate tectonics was not developed for another half century, reports from the 1906 quake laid a sound foundation for later studies in seismology.
Subsequent earthquakes have provided even more opportunities to learn additional information about how earthquakes happen, and, more significantly for the purposes of mitigation, what types of construction design, techniques, and materials are best suited to withstand earthquake impacts. Damage reports indicate that in many of these events, structures and infrastructure (roads, overpasses, bridges, gas and pipe lines, etc.) that were constructed or retrofitted (strengthened) to handle earthquake impacts fared much better on average than older and less substantially constructed buildings and facilities. Engineers and architects have also learned that building design in earthquake-prone areas should incorporate safeguards against both horizontal and vertical shaking. Structural supports should be carefully designed to avoid potential disasters like upper floors collapsing onto the floors below or onto open space, such as parking decks, that are located on the lower levels. Much research has been conducted on transportation infrastructure (roads, bridges, overpasses, and interchanges) to discover the types of damages that have occurred to the transportation network during past quakes (see Figure 4.1). Bridge performance is particularly critical, because the failure of support columns can cause entire spans of highway to collapse. In California, bridges and overpasses built prior to 1971 are especially vulnerable; following the 1971 San Fernando Earthquake, standards for earthquake design and construction were toughened considerably.
FIGURE 4.1 Damage to Interstate 5 from the Northridge Earthquake.
CHILE VERSUS HAITI: HOW RICH AND POOR NATIONS EXPERIENCE DISASTER DIFFERENTLY
In 2010, two earthquakes hit in rapid succession—the first in Haiti on January 12 and the second in Chile on February 27. The earthquake in Chile was far stronger than the one that struck Haiti—magnitudes 8.8 and 7.0, respectively, yet the death toll and level of property damage in the Caribbean nation were much higher.
The reasons for the differing experiences are tragically simple, and illustrate the value of mitigation and preparedness and the need for adequate capacity and resources for disaster relief and rebuilding.
Chile is a wealthy, developed country with a history of earthquakes; it is infinitely better prepared, with strict building codes, robust emergency response and experience in handling seismic catastrophes. In 1960, Chile suffered the worst earthquake in recorded history, a 9.5 magnitude quake which killed thousands. As a result, its people have an “earthquake consciousness,” and are well versed in what to do should a quake strike. Although nearly 700 people died in the 2010 quake, that number is undoubtedly much lower than would have occurred if the higher standards had not been implemented in Chile years earlier.
Haiti, in contrast, is a third-world country with widespread poverty—its population is the poorest in the western hemisphere. Since the end of the colonial period, the Haitian government has been largely dysfunctional, suffering from rampant corruption and periods of insurrection. Haiti has no national building code and no means of checking building safety, and there are no domestic disaster relief or recovery planning services. These factors have led to a population without the capacity to construct adequate housing, and without the resources to rebuild when tragedy strikes. Haiti’s death toll from the 2010 earthquake was approximately 230,000, which makes it the third deadliest since 1900, behind the Indonesian quake of 2004 and one in Tangshen, China, in 1976 which killed 655,000.
Studies of past earthquake disasters have also shown that many injuries and deaths occur because of breaking glass, falling objects, and insecure furnishings. Fairly simple and relatively inexpensive retrofitting activities can be implemented to combat these dangers, such as bolting furniture to the wall, anchoring bookcases, and securing appliances like televisions and computers so they remain stable during earthquake shaking.
We have also learned that areas prone to earthquakes can be delineated on maps. Within these areas, seismologists can create various zones of potential earthquake severity based on the underlying geological characteristics of the Earth. This ability to locate earthquake risk potential is critical to hazard mitigation and preparedness efforts. Seismic risk zone information is essential for steering future development away from areas that are most at risk from ground shaking and for targeting at-risk structures for retrofitting.
SELF-CHECK
• Define seismology, lithosphere, tectonic plate, fault plane, seismic wave, ground motion, epicenter, surface faulting, liquefaction, lateral spread, loss of bearing strength, seismograph, microearthquake, intensity, acceleration, and pancaking.
• Name the three scales used to measure the severity of an earthquake; explain the unit measurement of each.
• Describe the effects of a level VI earthquake.
• Discuss ways of reducing human deaths and property damage caused by earthquakes.
• Identify the geographic areas of the United States that are potentially impacted by earthquakes.
A tsunami is a sea wave generated by a disturbance on the seabed that displaces the overlying water. Although often referred to as “tidal waves,” this name is very misleading, for the daily rise and fall of the tides has nothing to do with generating these waves.1 Tsunamis are primarily associated with earthquakes in oceanic and coastal regions. Landslides, volcanic eruptions, nuclear explosions, and impacts of objects from outer space (such as meteorites, satellites, and “space trash”) can also generate a tsunami.
As a tsunami crosses the deep ocean, its length from crest to crest may be a hundred miles or more, yet its height from crest to trough will only be a few feet or less. Consequently, a tsunami does not capsize vessels at sea—in fact, in the open ocean these waves are so low and broad that they pass virtually unnoticed beneath ships and cannot be seen from the air.
In the deepest oceans, such as the Pacific, a tsunami can travel very long distances—from the shores of one continent to another at the speed of a jet plane, or up to 600 miles per hour (970 kilometers per hour). When a tsunami enters the shallow waters of a landmass in its path, the velocity of the wave diminishes to just tens of miles per hour, but the height of the wave increases dramatically. In shallow waters, a large tsunami can crest to heights exceeding 100 feet (30 meters) and strike with devastating force. The word tsunami is from the Japanese meaning “harbor wave” because of the impact the waves have when they enter harbors and other narrow coastal areas (Figure 4.2).
Tsunamis do not strike the coastline with a single blow. As a tsunami enters the shoals, water along the beach commonly recedes from the shore before the wave hits. This water rises up offshore to form the first tsunami wave to strike the coast. Successive waves may be much bigger, and hit the coast at intervals ranging from 15 minutes to several hours. Depending upon the topography of the land mass, a tsunami can reach far inland, causing damage from inundation and the force of the water rushing forward miles from the shore.
There have been reports of beachgoers who, intrigued by the unusual sight of the exposed shore as the ocean recedes, run to gather shells or rescue fish and other sea creatures left stranded as the ocean runs “backwards.” This phenomenon, combined with the fact that tsunamis involve a series of waves with periods of relative calm between, has led to many deaths that might otherwise have been prevented if people had evacuated in a direction away from the shoreline to higher ground at the first warning signs of the impending threat.
FIGURE 4.2 This National Oceanic and Atmospheric Administration (NOAA) map shows the speed of the tsunami generated by an earthquake off the coast of Japan. Note that the wave arrived at the coast of Chile in a mere 21 hours.
In many cases, the first tsunami wave to hit the shore is not the biggest. At Crescent City, California, the first two waves of the 1964 Alaska earthquake tsunami swept along the coast 23 minutes apart, about 4 hours after the earthquake. These waves caused only minor flooding and gave a false sense of security to some residents, who returned to their places of business to clean up or save their merchandise. However, some of these people and a significant part of the town’s waterfront, then later were smashed by successive waves, which were much higher. These damaging waves reached 21 feet (6.5 meters) in height. Eleven people were killed and the damage exceeded $7.5 million.1
Worldwide, the impact of tsunamis has been profound and far-reaching, with deaths in the tens of thousands. The coastal areas of countries where very large tsunamis have occurred experience multiple effects, including: total destruction of buildings and infrastructure; loss of livestock, crops and food supplies; leakage of sewage, industrial waste, and hazardous materials into fresh water sources; lost revenue from affected industries including tourism, fishing, shipping, energy, agriculture, and manufacturing.
Other impacts of tsunamis on society include homelessness, unemployment, hunger, injuries, and disease outbreaks, as well as the psychological trauma of survivors. Environmental impacts include deforestation, loss of habitat, dieoff of vegetation and wildlife, wildfire, and saltwater contamination of soils. The economic, environmental, and social impacts of a tsunami can last for generations.
Two International Catastrophes
Two recent tsunamis illustrate the devastating effects that these geologic hazards can cause. On March 11, 2011 a powerful tsunami travelling 800 kilometers per hour with 10 meter-high waves swept over the east coast of Japan. The tsunami was spawned by a 9.0 magnitude earthquake that reached depths of 24.4 kilometers—making it the fourth-largest earthquake ever recorded. The flooding and violent shaking resulted in a nuclear emergency, in which the Fukushima Daiichi nuclear power plant began leaking radioactive steam. The triple disaster of earthquake, tsunami, and nuclear accident killed more than 18,000 people and displaced nearly half a million others.
An undersea earthquake in the Indian Ocean on December 26th, 2004 produced a tsunami that caused one of the biggest natural disasters in modern history. Over 200,000 people are known to have lost their lives. The waves devastated the shores of parts of Indonesia, Sri Lanka, India, Thailand and as far west as Somalia on the east coast of Africa, 4500 kilometers west of the epicenter. Due to the distances involved, the tsunami took anywhere from 15 minutes to 7 hours to reach the various coastlines. The ability to issue warnings of impending tsunami is directly correlated with the location of the quake in relation to populated shorelines. Obviously, a 15-minute window to issue an evacuation order is extremely narrow, resulting in the deaths of thousands of residents trapped in low-lying areas as the tsunami came ashore.
4.4.4 Tsunami Hazards: A Real Risk for the United States
Scientists assess tsunami hazard based on historical evidence combined with earthquake potential. We know that devastating tsunamis have struck North America before, and the likelihood they will strike again is high.* Especially vulnerable are the five Pacific States—Hawaii, Alaska, Washington, Oregon, and California. Of these, Hawaii, which is surrounded by the volcanically active and earthquake-prone Pacific “Ring of Fire,” is the most exposed to tsunami impact.1
Tsunami hazard is moderate to very high in the Caribbean, including the American territories of Puerto Rico and the U.S. Virgin Islands. The Pacific territories of Guam, American Samoa, and the Northern Marianas experience many tsunamis, but historically these have been relatively small.
The U.S. Atlantic coast and the Gulf Coast states have experienced very few tsunami run-ups in the last 200 years. In fact, Louisiana, Mississippi, Alabama, the Florida Gulf coast, Georgia, Virginia, North Carolina, Pennsylvania, and Delaware have no known record of historic tsunami runup. One reported tsunami in the Mid-Atlantic States may be related to an underwater explosion or landslide.3
“Freak Wave” in New Jersey?
In June 2013, the National Oceanic and Atmospheric Administration announced a 6-foot wave that hit New Jersey could actually have been a rare tsunami. A weather system moving through the East Coast may have changed the air pressure enough so that waves generated by the storm behaved like a tsunami. The phenomenon is known as a “meteotsunami”—a tsunami caused by meteorological conditions, not seismic activity.*
* http://oldwcatwc.arh.noaa.gov/previous.events/06-13-13/index.php.
Since 1946, the Pacific Tsunami Warning System based at the Pacific Tsunami Warning Center in Honolulu has provided warnings of potential tsunami danger in the Pacific basin by monitoring earthquake activity and the passage of tsunami waves at tide gauges. However, predicting when and where the next earthquake, and hence tsunami, will strike is currently impossible. Once a tsunami is generated, it is possible through modeling and measurement technologies to forecast the arrival and impact of the tsunami, although we cannot yet make this prediction for a particular coastal location. Monitoring earthquakes gives a good estimate of the potential for tsunami generation, based on earthquake size and location, but gives no direct information about the tsunami itself. Tide gauges in harbors provide direct measurements of the tsunami, but the tsunami is significantly altered by local bathymetry (depth of water and contour of the ocean floor) and harbor shapes, which severely limits their use in forecasting tsunami impact at other locations. Partly because of these data limitations, 15 out of 20 tsunami warnings issued since 1946 were considered false alarms because the tsunami that arrived was too weak to cause damage. Recent developments in tsunami detection—including real-time, deep ocean detectors are improving the ability to forecast, and therefore, issue meaningful warnings to coastal residents.*
Although tsunamis are relatively rare, the potential for catastrophic impact makes it critical that individuals in high-risk areas are educated about and prepared for tsunamis before they strike. The NOAA TsunamiReady program helps local communities increase public awareness and engage in tsunami preparedness activities. To date, there are TsunamiReady communities in Hawaii, Alaska, Washington, Oregon, and California. Since the NOAA TsunamiReady Program was initiated in 2001, local authorities in these states have drawn up tsunami emergency plans, installed sirens and other warning systems to alert people of an approaching tsunami, installed tsunami evacuation route signs and tsunami information signs along the coast and at many state parks, implemented innovative tsunami hazard educational pilot programs, and practiced tsunami evacuation drills (Figure 4.3).
4.4.7 Impact of Climate Change on Tsunamis
A tsunami hitting at high tide is more dangerous than one that hits at low tide because the water level is already elevated, allowing the tsunami to reach even farther inland from shore at higher water levels. Low-lying coastal plains are particularly susceptible to being swamped by the waves. As climate change warms the oceans, sea level rise is expected to impact coastlines throughout the world. Many of these regions are also located in areas of seismic activity, and hence the threat of tsunami. Just as high tide gives a tsunami a “running start” when it hits the shoreline, sea level rise will similarly allow tsunami waters to flood inland from an already-elevated water line, displacing more residents and causing greater damage over a larger area.
FIGURE 4.3 Tsunami warning signs direct people to go inland or to higher ground when an earthquake occurs.
SELF-CHECK
• Define bathymetry.
• Explain how tsunamis are generated.
• Describe some of the impacts of a tsunami that strikes land.
• Discuss ways of reducing human deaths and injury from tsunami.
• Identify the geographic areas of the United States that are potentially exposed to future tsunamis.
A volcano is a vent in the surface of the Earth through which magma and associated gases and ash erupt. The structure that is produced by the ejected material, usually conical in form, is referred to as a volcano as well. The word volcano is derived from the Latin word Vulcan, Roman god of the forge.
4.5.1 Reaching the Boiling Point
A volcanic eruption occurs when superheated rock under the Earth’s surface rises to areas of lower pressure at the surface. During this movement, the rock undergoes a phase change from solid rock to liquid magma (molten rock). Volcanic action can be compared to the action that occurs when water boils and turns to steam, resulting in volume expansion. If water boils while in a closed glass beaker, the container does not have room to hold the additional volume, and the glass will explode to release the pressure. A volcanic eruption works under the same principle, but on a far grander scale. The enormous amount of pressure and heat building up under the surface causes conversion of the rock to magma, and the magma expands. The result is an eruption through the most accessible escape route at the Earth’s surface. Magma that reaches the Earth’s surface is called lava. Lava temperatures can reach 1250°C (over 2000°F).
TABLE 4.2 Types of Volcanic Eruptions
Type of Volcanic Eruption |
Description |
Icelandic |
Gas escapes easily and lava has low viscosity |
Hawaiian |
Gas escapes easily, but lava has slightly more viscosity; builds tall peaks |
Strombolian |
Smaller, more continuous eruptions; its central lava pool is easily triggered due to the pressure that builds quickly under its crust |
Vulcanian |
witch between high viscosity lava and large amounts of ash blown out the top of the volcano |
Vesuvian |
Even more violent high viscosity lava blasts that are due to trapped gases. These gases can blow ash and rock great distances, either vertically or horizontally |
Pilian |
The gas pressure and viscosity of the magma is so high that these volcanoes blow lava and ash laterally, or out of its side (rather than up), to relieve the pressure blockages of the throat of the volcano |
4.5.2 Types of Volcanic Eruptions
There are six major types of volcanic eruptions: Icelandic, Hawaiian, Strombolian, Vulcanian, Vesuvian, and Pilian (see Table 4.2).
Some volcanoes erupt violently, while others flow more peacefully. The variation in volcanic force is due to the following:
• Variance in chemical and mineral content of the rock/magma
• Variance in the viscosity, temperature, and water and gas content of the rock/magma
• Variance in geographic position in relation to tectonic plate edges
Mount St. Helens in Washington State is an example of a Pilian eruption. When Mount St. Helens erupted on May 18, 1980, it erupted with such force that the top 1000 feet of the volcano disappeared within minutes. The blast sent thousands of tons of ash into the upper atmosphere, while simultaneously sending waves of lava, poisonous gas, and mud laterally out the side and down the slopes of the mountain. Within hours, thousands of acres surrounding the volcano were blanketed with the deadly debris. More than 200 square miles of forestland were transformed almost instantly into a gray, lifeless landscape. All living creatures within miles were killed, including one geologist who had been stationed nearby to monitor the volcano’s activity, and one resident who refused to evacuate the area despite official warnings. In all, 57 people died, including loggers, campers, reporters, and scientists, some as far as 13 miles from the mountain itself (Figure 4.4).
FIGURE 4.4 The force of the 1980 eruption of Mt. St. Helens in Washington State flattened trees and killed vegetation and animal life for miles in the perimeter of the volcano.
Areas that are prone to frequent volcanic activity are known as hot spots, such as the one in the Island of Hawaii. In these areas, volcanoes form when magma rises from a stationary heat source—the hotspot—beneath the Earth’s lithosphere. The magma migrates up through fractures in the overriding Pacific plate and extrudes onto the ocean floor, gradually building a mountain of successive lava flows. Over time the mass accumulates until it emerges above sea level, thereby becoming an island volcano.
The Hawaiian Islands are arranged in a chain, and the key to understanding why lies in the relation between the Hawaiian hot spot and the movement of the Pacific plate. The hot spot is a stationary heat source over which the Pacific plate slowly moves toward the northwest. Imagine a conveyer belt with hamburger patties slowly moving over a stationary gas flame, cooking the burgers one after another. The currently active volcanoes in the hot spot on the Island of Hawaii are Mauna Loa, Kilauea, and Loihi.
4.5.4 The Volcano–Earthquake Connection
Some volcanic eruptions are related to earthquakes occurring in the area. It is not understood what the exact triggering mechanism might be, but the subsequent volcanic activity is most likely a response to a pressure change in the magma. This could be a consequence of a change in pressure on the crust in the area of the earthquake, or possibly the severe ground shaking caused by the quake.
DOUBLE WHAMMY: A QUAKE AND A VOLCANO
The eruption of Kilauea in Hawaii in November, 1975, was directly related to an earthquake. A 7.2 magnitude earthquake, along with many aftershocks, occurred just southeast of the Kilauea volcano’s caldera and within its south flank. On that occasion, the volcano erupted for over 16 hours.
4.5.5 Types of Volcanic Structures
There are several varieties of volcanic structures, including:
• Cinder cones: The simplest type of volcano, cinder cones have a single vent and are built of cinders. As lava erupts violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent. Over time, these small fragments build up to form a circular cone. Cinder cones can grow to more than a thousand feet above their surroundings and have a characteristic bowl-shaped crater at the summit.
• Shield volcanoes: These volcanoes can have many eruptions from the rift zones, or fractures that are located along the flanks of their cones. Rather than erupting violently, the lava flows out of a shield volcano in all directions from its vents. These volcanoes are built almost entirely of fluid lava flows that cool as thin sheets and create a gently sloping cone. Some of the largest volcanoes in the world are shield volcanoes. The largest shield volcano (and the largest active volcano in the world) is Mauna Loa. This volcano on the Big Island of Hawaii sits over 13,000 feet above sea level, but the entire volcano measured from the sea floor, rises over 28,000 feet.
• Lava domes: Lava domes are formed by lava piles that are too viscous, or thick, to flow far from the vent. A dome volcano grows by layering the lava upward and outward, largely by expansion from within. Some domes form short, steep-sided lava flows known as coulees. Others form spines over the volcanic vent. Domes commonly occur within the craters or on the flanks of large composite volcanoes.
• Composite volcanoes: Also called stratovolcanoes, these are large volcanoes that are also mountains. Some of the most breathtaking mountains in the world are composite volcanoes, including Mount Fuji in Japan and Mount St. Helens in Washington. Most have a central crater at the summit, but lava can erupt through fissures on the flanks of the cone or flow from breaks in the crater. These volcanoes build up over time by the fortification of the cone when the cooled lava fills the fissures and when cinders and ash are added to the slopes.
4.5.6 Impacts of Volcanic Activity on People and Property
Volcanoes can pose a serious threat to people and property. One of the most common risks associated with volcanic activity involves lava flows. Although lava rarely travels quickly enough to be life-threatening, it does create havoc to the natural and built environment. For example, the flow of lava from volcanoes in southern Hawaii has destroyed numerous homes and buried highways in its path. Because of its excessive heat, lava quickly burns any consumable materials it covers, including trees, houses, roads, crops, and anything else in its way. Lava flows may be slow-moving, but they are unstoppable forces. A few communities in Iceland have tried to stop the advance of lava flows using cold seawater in an attempt to change the lava from a molten to solid condition. The success of this technique is not widely recognized.
Volcanic ash also can be extremely hazardous to plants and animal life. Volcanic ash can form thick deposits on the ground, causing extensive environmental damage. The ash can contain bits of volcanic glass and can be very abrasive. Violent eruptions can bury whole communities, cause houses to collapse, clog engines in airplanes and vehicles, and create breathing problems for humans and animals. The ash plume from an erupting volcano can be dispersed a long distance by high-altitude winds, bringing similar problems to areas far removed from the eruption site.
Pyroclastic flows pose another danger to humans and are experienced in the vicinity of explosive volcanoes. A pyroclastic flow, also called a nuee ardente (French for “glowing cloud”), is an incinerating mixture of gas and volcanic debris, with a temperature from 700°C to 1000°C (1300–1800°F). Pyroclastic flows remain close to the ground, but they can travel downhill from the volcano summit very quickly, up to 150 kilometers (90 miles) per hour, incinerating anything in their path as they roar down the slope. One of the most famous pyroclastic flows occurred when Mount Vesuvius erupted in A. D. 79, engulfing the ancient city of Pompeii, Italy. More recently, the forest on the sides of Mount St. Helens was flattened by a nuee ardent when that volcano erupted in 1980.
Volcanic mudflows also pose a significant threat to human settlements. A mixture of water and volcanic debris, a mudflow can quickly travel downslope, burying all objects on the route downhill. Mudflows created heavy damage during the eruption of Mount St. Helens, as they traveled along stream valleys carrying trees, structures, and soils into the Columbia River below. The U.S. Army Corps of Engineers had to dredge a vast amount of debris that had been deposited in the river by the mudflow so that the river could be used for navigation following the event.
Poisonous gases from volcanic activity also pose a danger to humans and animals. Although volcanic gas is primarily made up of water vapor, other elements that may be present include carbon dioxide, sulfur dioxide, carbon monoxide, hydrogen sulfide, sulfuric acid, hydrochloric acid, and hydrofluoric acid. These toxic gases can impact urban areas and agricultural crops in the vicinity, something that occurs on the island of Hawaii fairly frequently. Vog, or volcanic fog, occurs a short distance downwind from the eruption site, while laze (lava haze) is produced when lava enters the sea and the reaction produces hydrochloric acid fumes.
ICELANDIC ERUPTION GROUNDS AIR TRAVEL
The 2010 eruption of the Icelandic Volcano Eyjafjallajökull caused enormous disruption to air travel when about 20 countries closed their airspace because of an enormous ash cloud that lingered over much of northern Europe for several days. The flight cancellations affected more than 100,000 travelers, creating the highest level of air travel disruption up to that time since World War II.
SELF-CHECK
• Define volcano, magma, lava, hot spot, volcanic ash, pyroclastic flow, nuee ardent, vog, laze, volcanic mudflows, shield volcano, cinders, cinder cones, lava dome, crater, coulee, composite volcano, stratovalcano, and lava flow.
• Name the six different types of volcanic eruptions.
• Identify the United States’ volcanic hot spots.
• List five risks associated with volcanoes.
Landslides occur when masses of rock, earth, or debris move down a slope. The major driving force behind landslides is gravity, assisted by water. Landslides vary in size from relatively small, isolated events to large, widespread ground movement, and can vary in speed from a slow, gradual creep to a rapid rush of earth and debris that rages downhill. Activated by geological hazards, rainstorms, wildfires, and by human modification of the land, landslides pose serious threats to any man-made structures that lie in their path. Landslides can impact fisheries, tourism, timber harvesting, agriculture, mining, energy production, and transportation, as well as community life.
While some landslides move slowly and cause damage gradually, others move so rapidly that they can destroy property and take lives suddenly and unexpectedly. This type of landslide is called a debris flow, which can also manifest itself as a mudslide, mudflow, or debris avalanche. These types of fast-moving landslides generally occur during intense rainfall on water-saturated soil.
Debris flows usually start on steep hillsides as soil slumps or slides that liquefy and accelerate to speeds as great as 35 miles per hour or more. They continue flowing downhill and into channels, depositing sand, mud, boulders, and organic materials onto more gently sloping ground. Consistency ranges from watery mud to thick, rocky mud (like wet cement), which is dense enough to carry boulders, trees, and cars. Debris flows from many different sources can combine in channels, where their destructive power may be greatly increased.
Torrential rainfalls such as those that occur during hurricanes or tropical storms commonly act as triggers for landslides. Several other types of hazards can also put a landslide in motion; consider these examples:
• The Mount St. Helens debris flow was an instantaneous result of the volcanic eruption that took place there.
• A landslide in San Fernando, California, started as a result of a 7.5 magnitude earthquake in 1971.
• A string of wildfires that burned over 4000 acres of vegetative cover in Big Sur, California, exacerbated a landslide on California Highway 1 in 1972 after a series of torrential rainstorms.
• In 1983 land in Thistle, Utah began shifting because of groundwater buildup from heavy rains during the previous fall and the melting of deep snowpack from the winter. Within a few weeks, the landslide dammed the Spanish Fork River, destroying U.S. Highway 6 and the main line of the Denver and Rio Grande Western Railroad. The landslide dam caused flood waters to rise, leading to inundation of the surrounding area. The town of Thistle was completely obliterated. The Thistle landslide was the single most costly landslide event in the United States to date, with damages exceeding $400 million (see Figure 4.5).
• In February 1995, a 1600-foot stretch of popular beach at Sleeping Bear Dunes National Lakeshore suddenly slid into the waters of northeastern Lake Michigan. United States Geological Survey (USGS) and National Parks Service scientists believe that repeated coastal landslides at Sleeping Bear Point may be related to increases in fluid pressure in the spaces between the grains of sand (pore pressure) that make up the bluff at the point. The increased amount of water likely weakened the slope, making it susceptible to the ensuing landslide.
FIGURE 4.5 The Thistle Landslide.
Areas that are generally prone to landslide hazards include existing old landslides, the base of steep slopes, the base of drainage channels, and developed hillsides with steep slopes, particularly where leach-field septic systems are used. Areas that are typically considered safe from landslides include areas that have not moved in the past; relatively flat-lying areas away from sudden changes in slope; and areas at the top or along ridges. These landslide risk areas can be shown on maps, which allows emergency managers, planners and builders to identify locations where construction should be limited or landslide mitigation building techniques can be employed.
According to the USGS, landslides are major geologic hazards that occur in all 50 states, causing $1–$2 billion in damages per year and resulting in more than 25 fatalities annually. Notable landslides include the Canyonville, Oregon, landslide of 1974 that killed 9; the 1980 Mount St. Helens debris flow, which is the world’s largest landslide; and a landslide in Mameyes, Puerto Rico in 1985 that killed 129 people during Tropical Storm Isabel.
SELF-CHECK
• Define landslide, debris flow, and soil slump.
• Identify the two forces that drive a landslide.
• List three types of land areas that are typically prone to landslides.
• Discuss the areas of the United States that are at risk of landslide events.
Coastal erosion is the wearing away of the land surface by detachment and movement of soil and rock fragments. Coastal erosion occurs along sandy shores and barrier islands of the Atlantic and Gulf Coasts, on the rocky coastlines and bluffs of the Pacific, as well as the shores of the Great Lakes. Erosion can occur during a flood or storm event or over a period of years through the action of wind, water, or other geologic processes. Sea level rise can also contribute to the progression of coastal erosion over time.
4.7.1 Shifting Sands, Eroding Bluffs
Wind, waves, and long-shore currents are the driving forces behind coastal erosion. In areas of sandy beach, the removal and deposition of sand permanently changes the structure and shape of the beach. Sand is transported throughout the sea/shore system, and can be transported to land-side dunes, other beaches, offshore banks, and deep ocean bottoms. Rates of coastal erosion can also be affected by human activity, sea level rise, seasonal fluctuations, and climate change.
The beach system is in a state of dynamic equilibrium. Constant movement transfers sand from one location to another; winter storms along the coast can remove significant amounts of sand, creating steep, narrow beaches, while during the summer, milder waves return the sand, widening beaches and creating gentle slopes. The bluffs and cliffs of the Pacific coast experience a dynamism of their own, with wave, wind, and storm action working to wear cliffs faces away slowly over time and more quickly during storm events.
Coastal erosion is a highly localized event that can take place on one end of an island and not at the other. Although much erosion takes place gradually over time, a hurricane or other sudden storm event can drastically increase the amount of coastal erosion that takes place in a short period of time. Episodic erosion is induced by a single storm event. This type of erosion can make structures located along the shore suddenly become unstable and prone to collapse, as scouring occurs around foundation supports and undermines the building.
The average erosion rate on the Atlantic coast is roughly 2–3 feet per year, but the states bordering the Gulf of Mexico, especially in the deltas of Louisiana, have the nation’s highest average annual erosion rate at 6 feet per year. Erosion on the rocky cliffs of the Pacific coast averages 1 foot per year, although large episodic erosion occurs occasionally. The Great Lakes annual erosion rate is highly variable, ranging from 0 to more than 10 feet per year depending on a number of hydrologic and weather-related factors such as fluctuating lake levels and wave action. Rates of 25 feet per year are not uncommon on some barrier islands in the Southeast, and rates as high as 50 feet per year have occurred along the Great Lakes.*
Many coastal states have setback rules that are based on the erosion rate along the shoreline. The erosion setback is a line, measured landward from some specified point (e.g., the first line of stable, natural vegetation), behind which construction must take place. Setbacks are designed to increase the life of a building by avoiding the wear and tear, or sudden collapse, that can happen close to the oceanfront.
4.7.3 Coastal Inlet Hazard Areas
Inlets are areas along the shore where ocean water flows into an estuary or sound. These channels are important for shipping, fishing, and recreation along the southern, Atlantic, and Pacific coastlines as they serve as the ingress and egress point for vessels traveling between the ocean and the inner coastal waterways. Under normal conditions, these areas are very dynamic—the mouth of an inlet typically shifts alongshore because of “everyday” erosion. During a cataclysmic event, inlets can move dramatically, or can close completely, and in some cases entire new inlets can form. These areas are extremely hazardous; when storm-induced erosion shifts an inlet down or up shore, houses, hotels, roads, bridges, and other structures located near the inlet are at risk. During a particularly intense storm, structures can be damaged or destroyed in a matter of hours.
4.7.4 Effect of Climate Change on Coastal Erosion
Despite the differences in erosion potential along the world’s coastlines, there has been a dramatic increase in coastal erosion over the last two decades4 and this is expected to continue as sea level rises and storm frequency and intensity increase. Rather than occurring over the same time scale with sea level rise, erosion of beaches and coastal cliffs is expected to occur in large bursts during storm events as a result of increased wave height and storm intensity. Because of these large events, scientific models predict that shoreline erosion may outpace sea level rise manyfold.5 For example, in 1938 Sakonnet Point, the most seaward point in the state of Rhode Island boasted large sand dunes that stood some 4.6 meters (~15 feet) tall. Today, those dunes are nearly completely submerged during high tide as a result of the combined impact of major storm damage and increased erosional forces. While sea level rose only 0.4 meters during this time period—the 4.6 meter dunes have all but disappeared at high tide.6 This example highlights the potential for erosional forces such as major storm events to outpace the rate of sea level rise. Erosion will have significant effects on coastal habitats, which can lead to social and economic impacts on coastal communities. With the reduction of coastal habitats and the ecological services they provide, coastal communities will experience more frequent and destructive flooding, compromised water supplies and smaller or fewer beaches (Figure 4.6).
FIGURE 4.6 Rising sea level causes beaches to recede and makes oceanfront structures much more vulnerable to storm and erosion damage.
SELF-CHECK
• Define coastal erosion, dynamic equilibrium, episodic erosion, and setback rules.
• Name six forces behind coastal erosion.
• Discuss the effects of episodic erosion on a beachfront community.
Land subsidence is the gradual settling of the Earth’s surface that occurs because of subsurface movement of earth materials and the resulting loss of support below ground. It usually occurs over a period of weeks, months, or years, and often happens so slowly as to be barely perceptible. Collapse occurs more quickly, when the land surface opens up and surface materials fall into cavities below. Collapse can take place over just a few hours. Sinkholes are an especially dramatic example of the collapse process. Subsidence and collapse are serious geological hazards, posing threats to property and human life.
Subsidence is a problem throughout the world. In the United States, more than 17,000 square miles in 45 states, an area roughly the size of New Hampshire and Vermont combined, have been directly affected by subsidence.
4.8.1 Causes of Land Subsidence
Although some subsidence of land is due to natural processes, severe land subsidence is often caused by human activities, such as the removal of groundwater (subsurface water). Other causal factors of land subsidence include the following:
• Thawing permafrost
• Drainage of organic soils
• Dissolving of subsurface limestone rock
• Natural compaction of soils
• Underground mining
• Removal of oil and gas
The construction of levees, such as those at the mouth of the Mississippi River in Louisiana, has caused severe subsidence in Mississippi and surrounding Gulf states. For tens of thousands of years, silt and sand were deposited by the Mississippi River when it regularly overflowed its banks. When the area was settled by humans, however, levees were built to contain the river. While levees stopped the flooding and protected the population, they also stopped the natural replenishing of the land, leading to net loss of land and subsequent subsidence.
More than 80% of the identified subsidence in the United States is a consequence of groundwater exploitation. The increasing development of land and water resources means that existing land subsidence problems will likely worsen in the near future, and new problems will undoubtedly arise.
When large amounts of groundwater are withdrawn from certain types of rocks, such as fine-grained sediments, the rock compacts because the water is partly responsible for holding the ground up. When the water is withdrawn, the rock falls in on itself. Land subsidence can be difficult to notice, because it can extend over large geographic areas and can take place gradually over a period of time. Eventually, however, land sinks to such a degree that houses become off-kilter, roads collapse, and flooding worsens because the elevation of the land has been lowered.
Groundwater is used to irrigate crops, to provide drinking water to municipalities, and for use in manufacturing and various industries. But the withdrawal of groundwater has depleted critical groundwater resources and created costly regional-scale subsidence in many areas of the country. In the Santa Clara Valley in northern California, early agricultural groundwater use contributed to subsidence that has permanently increased flood risks in the greater San Jose area. In nearby San Joaquin Valley, one of the single largest human alterations of the Earth’s surface topography has resulted from excessive pumping of groundwater to sustain the exceptionally productive agricultural business.
Early oil and gas production and a long history of pumping groundwater in the Houston–Galveston area in Texas have also created severe and costly coastal flooding hazards and affected the Galveston Bay estuary—a critical environmental resource. In Las Vegas Valley, Nevada, groundwater depletion and associated subsidence have accompanied the conversion of a desert oasis into a thirsty and fast-growing metropolis. Water-intensive agricultural practices in south-central Arizona have caused widespread subsidence and fissures of the Earth’s surface. In each of these areas, not only have the water resources been dangerously reduced, the action of pumping water out of the ground has created hazardous subsidence conditions.
EFFECTS OF CLIMATE CHANGE: SUBSIDENCE CONCERNS IN LOUISIANA
Owing to subsidence, the state of Louisiana is becoming increasingly more vulnerable to destruction by coastal storms and erosion. The impacts of subsidence on wetlands, the population, and coastal roads and industries in Louisiana are of major concern for residents and officials alike.
Of particular concern is the impact of increased inundation from relative sea level rise, which causes severe wetlands loss. Indirectly, salt water intrusion kills salt-intolerant vegetation, thus making barrier islands and wetlands vulnerable to increased wave action and erosion from coastal storms and hurricanes. Since most of coastal Louisiana is comprised of wetlands, the region is especially vulnerable to land loss.
As much as 50% of Louisiana’s population lives in coastal areas of elevations of three feet or less. As population increases in the region, vulnerability to coastal storms and hurricanes also grows.*
* Subsidence and sea level rise in Louisiana: A study in disappearing land. NOAA Magazine.
Mining of coal, minerals, and other ores is an important economic mainstay in many regions of the United States. However, the process of removing these materials can cause subsidence above the mines. When mining is carried out near the surface of the ground, the supporting structure for overlying rocks is removed. If too much of the supporting material is removed, the surface will collapse into the mine below, creating dangerous collapse pits. The danger of subsidence from abandoned coal mines is particularly acute in Pennsylvania, West Virginia, and Kentucky.
The dangers of mine subsidence can be reduced by filling in the void created when the ores are removed. Leftover mining materials (mine waste) can be dumped into the mining holes to support the overlying roof. When this is impractical, sand or cement can be pumped in through access holes.
Sinkholes are common where rock below the land surface is made up of limestone, carbonate rock, salt beds, or rocks that naturally can be dissolved by groundwater circulating through them. As the rock dissolves, spaces and caverns develop underground, creating what is known as Karst topography (named after an area in Yugoslavia where this occurs regularly). Sinkholes can be dramatic episodes of collapse, because the land usually remains intact up to the point when spaces under the Earth’s surface become too large to support the upper layer of earth. At that stage, a sudden collapse of the land surface can occur. These collapses can be small, just a few feet wide, or they can be huge, swallowing up an entire house. The most damage from sinkholes in the United States tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. Sinkholes are different from soil slumps, which are composed of loose, partly to completely saturated sand or silt, or poorly compacted man-made fill composed of sand, silt, or clay. Soil slumps are common on embankments built on soft, saturated foundation materials, in hillside cut-and-fill areas, and in river and coastal floodplains.
New sinkholes may result from groundwater pumping and from construction and development practices. Sinkholes can also form when natural water drainage patterns are changed and new water diversion systems are developed. Some sinkholes form when the land surface is changed, such as when industrial and runoff storage ponds are created. The substantial weight of the new material can trigger an underground collapse of supporting materials, thus causing a sinkhole.
It is very difficult to predict when a sinkhole might happen. It is critical that homeowners and local officials pay attention to clues on the surface that hint at what lies below, and be particularly aware when limestone is present. Land subsidence and cracks on the surface indicate that the underlying material may have large voids, especially when the cracks occur in a circular pattern. Cracks in walls and foundations of buildings can also indicate that the ground below is becoming frail.
The continental U.S. experiences small earthquakes every day. But over the past few years, their numbers have been increasing. Geoscientists say the new “epidemic” of quakes may be related to industrial wastewater being pumped into underground storage wells. Two potential trigger mechanisms may be setting off the wastewater quakes: other, large earthquakes (some as far away as Indonesia), and the activity at geothermal power plants.
Most of these little quakes in the United States are too small to feel. They tend to happen in “swarms.” Geoscientists have traced some of these swarms to underground faults near deep wells that are often filled with waste fluid from the oil and gas drilling boom. The pressure from the fluid can cause the faults to slip, resulting in earth movement. Large quakes can also cause the faults to slip, resulting in a microearthquake. The production of geothermal energy, which involves extraction of hot water from beneath the earth’s surface to produce steam-generated electricity, may also be correlated with earthquake activity.
Additionally, there is some evidence that the process of hydraulic fracturing—a method of natural gas extraction that involves injecting millions of gallons of water and chemicals deep into the earth—may also produce small earthquakes.
As processes such as fracking, geothermal energy production, and the use of underground wastewater wells are on the rise throughout the country, geoscientists continue to study the potential risks of human-caused earthquakes, especially in areas near known fault lines.
SELF-CHECK
• Define land subsidence, collapse, sinkhole, groundwater, and collapse pit.
• Cite six reasons for land subsidence.
• Discuss the difference between a collapse pit and a sinkhole.
• Name three possible human-induced causes of earthquakes.
The inevitable shifting of the Earth’s layers over time results in equally inevitable geological hazards. This chapter covered earthquakes and their potentially devastating effects on human life and property, particularly in areas along fault lines. Volcanic action can cause significant property damage and risk to human, plant, and animal life during eruptions. This chapter also discussed the risk of tsunami, landslides, coastal erosion, and land subsidence, each of which presents its own dangers and potential for death and property damage. The information on these hazards can be used by emergency managers and those in land use management to better protect communities from their potential impacts.
Acceleration |
The rate at which speed of ground movement increases. |
Aftershock |
The smaller earthquakes that occur after a previous large earthquake, in the same area of the mainshock. Aftershocks can be very destructive and may last for days, weeks or even years after the initial earthquake. |
Bathymetry |
Bathymetry is the measurement of the depth of water in oceans, rivers, or lakes. Bathymetric maps look a lot like topographic maps, which use lines to show the shape and elevation of land features. |
Cinder cones |
The simplest type of volcano; built of cinders, they have a single vent. |
Cinders |
Lava that erupts into the air and breaks into small fragments that solidify. |
Coastal erosion |
The wearing away of the land surface along the coast by detachment and movement of soil and rock fragments. |
Collapse |
When the land surface opens up and surface materials fall into cavities below. |
Collapse pit |
Hole created when too much supporting material is removed during mining; the surface will collapse into the mine below. |
Composite volcanoes |
Large volcanoes that are also mountains; also called stratovolcanoes. |
Core |
The innermost region of the Earth. |
Coulees |
Short, steep-sided lava flows. |
Crater |
Bowl-shaped hole at the summit of a volcano. |
Crust |
The thin layer on the surface of the Earth. |
Debris flow |
Fast-moving landslides that generally occur during intense rainfall on water-saturated soil. |
Dynamic equilibrium |
In the context of coastal erosion, dynamic equilibrium refers to the process whereby sand is moved from one location to another but it does not leave the system. |
Earthquake |
Geologic hazard caused by the release of stresses accumulated as a result of the rupture of lithosphere rocks along opposing boundaries in the Earth’s crust. |
Epicenter |
Causative fault of an earthquake; damage is generally most severe at or near the epicenter. |
Episodic erosion |
Erosion induced by a single storm event. |
Fault planes |
Ruptures in the Earth’s crust that is typically found along borders of the tectonic plates. |
Foreshock |
An earthquake that occurs before a larger seismic event (the mainshock) and is related to it in both time and space. |
Ground motion |
The vibration or shaking of the ground during an earthquake. |
Groundwater |
Subsurface water. |
Hot spot |
Area prone to volcanic activity. |
Inlet |
The connecting passageway between the sea and a bay, sound, lagoon, or other enclosed or posterior body of water. |
Intensity |
How the damage potential of an earthquake is measured. |
Land subsidence Landslide |
A gradual settling of the Earth’s surface. |
Landslide |
When masses of rock, earth, or debris move down a slope. |
Lateral spread |
Type of ground failure in an earthquake that develops on gentle slopes and entails the sidelong movement of large masses of soil as an underlying layer liquefies. |
Lava |
Magma that reaches the Earth’s surface. |
Lava dome |
Type of volcano that grows by layering lava upward and outward, largely by expansion from within. |
Lava flows |
Moving lava; because of its excessive heat, lava quickly burns any consumable material it touches. |
Laze |
Lava haze produced when lava enters the sea and a chemical reaction produces hydrochloric acid fumes. |
Liquefaction |
When ground shaking causes loose soils to lose strength and act like viscous fluid. |
Lithosphere |
The Earth’s crust and upper mantle. |
Loss of bearing strength |
Type of ground failure in an earthquake that results when the soil-supporting structures liquefy. |
Magma |
Molten rock. |
Mantle |
Middle layer of the Earth. |
Microearthquake |
An earthquake with a magnitude of less than 2.0. |
Nuee ardente |
French for “glowing cloud,” an incinerating mixture of gas and volcanic debris; also called pyroclastic flow. |
Pancaking |
When high-rise buildings collapse in on themselves due to earthquake shaking. |
Pyroclastic flow |
An incinerating mixture of gas and volcanic debris; also called nuee ardente. |
Richter scale |
An open-ended logarithmic scale that describes the energy release of an earthquake through a measure of shock wave amplitude. |
Seismic waves |
When the rock on both sides of a fault snaps, it releases this stored energy; waves are what cause the ground to shake in an earthquake. |
Seismograph |
The instrument that records the amplitude of seismic waves during an earthquake. |
Seismology |
The study of earthquakes. |
Setback rules |
Regulations based on the erosion rate along the shoreline. The setback is a line behind which construction must take place. |
Shield volcanoes |
Type of volcano that can have many eruptions from the fractures, or rift zones, along the flanks of their cones. |
Sinkholes |
Spaces and caverns that develop underground as the rock below the land surface is dissolved by groundwater circulating through them. |
Soil slumps |
Loose, partly to completely saturated sand or silt, or poorly compacted human-made fill composed of sand, silt, or clay. |
Stratovolcanoes |
Large volcanoes that are also mountains; also called composite volcanoes. |
Surface faulting |
The differential movement of two sides of a fracture; the location where the ground breaks apart. |
Tectonic plates |
Moving layers of the Earth’s surface that contain the continents and oceans. |
Vog |
Volcanic fog; occurs a short distance downwind from volcanic eruption site. |
Volcanic ash |
Produced by volcanic eruption; can form thick deposits on the ground, causing extensive environmental damage. |
Volcanic mudflow |
A mixture of water and volcanic debris. |
Volcano |
A vent in the surface of the Earth through which magma and associated gases and ash erupts. |
1. The mantle is the outer layer of the Earth. True or False?
2. Which of the following is the most brittle layer of the Earth?
a. Mantle
b. Lithosphere
c. Crust
d. Core
3. Liquefaction causes soils to lose strength and act like fluid. True or False?
4. The Richter scale measures
a. Shock wave amplitude
b. Surface faulting
c. Intensity
d. Ground movement
5. A microearthquake has a magnitude of less than 4.5. True or False?
6. The MMI (modified Mercalli intensity) scale is used to judge earthquake damage potential by measuring
a. Speed
b. Velocity
c. Ground movement
d. Intensity
7. The Richter scale measures earthquakes using values from 1 to 8. True or False?
8. A Vulcanian volcanic eruption is characterized by
a. Low-viscosity lava
b. Large amounts of ash blown off the top
c. Lava and ash blown out of the side
d. Small, continuous eruptions
9. Lava that reaches the Earth’s surface is called magma. True or False?
10. An area that is prone to volcanic activity is called a
a. Hot pocket
b. Lava spot
c. Hot spot
d. Lava dome
11. A stratovolcano is a
a. Composite volcano
b. Coulee
c. Microearthquake
d. Shield volcano
12. Vog is a byproduct of a volcanic mudflow. True or False?
13. A debris flow can be dense enough to carry trees and cars. True or False?
14. Landslides can be caused by volcanic eruptions and earthquakes. True or False?
15. Landslide-prone areas include
a. Bases of drainage channels
b. Areas at the top of ridges
c. Areas along the edge of ridges
d. Areas that have not moved before
16. Episodic erosion takes places over a period of many months or even years. True or False?
17. Coastal erosion becomes a hazard as an increasing number of homes and structures are built in vulnerable areas. True or False?
18. Land subsidence occurs when the land opens up and surface materials fall into cavities. True or False?
19. Most of the land subsidence in the United States is a result of
a. Mine collapse
b. Sinkholes
c. Groundwater removal
d. Earthquakes
1. The Earth is composed of three layers. Name the layers.
2. Which factor is common among the geologic hazards covered in this chapter?
3. In which layer of the Earth does an earthquake form?
4. Explain the role of seismic waves in an earthquake.
5. Name the four variables that characterize an earthquake.
6. California is typically thought of as a particularly earthquake-prone area. Name two other areas that share the threat.
7. What is the name of the geologic event that occurs when solid rock turns to molten rock?
8. Explain what, if any, connection there may be between volcanoes and earthquakes.
9. Lava flows and volcanic ash are obvious dangers to human life and property. Name three other threats from a volcanic eruption.
10. Explain what makes a debris flow different from a typical landslide.
11. Describe how natural hazards other than hurricanes and tropical storms can affect landslides.
12. Coastal erosion is the wearing away of land surface, most notably by wind, waves, and long-shore currents. Name three other causes of coastal erosion.
13. What process causes a beach profile to differ between winter and summer?
14. Removal of groundwater is the most significant cause of subsidence in the United States. What are three other actions that contribute to subsidence?
15. Explain how withdrawal of groundwater causes regional-scale subsidence in many areas of the United States.
1. Using the geologic hazards covered in this chapter, pick those that occur where you live and put them in order of most likely to least likely to occur.
2. Imagine you live in an area that experiences an earthquake measuring 6.1 on the Richter scale. Using the MMI scale, what would be its intensity and what effects would you be likely to experience?
3. Consider the building you’re in and predict its ability to withstand an earthquake that measures 5.4 on the Richter scale.
4. As an emergency manager in a town 100 miles from Washington State’s Mount St. Helens, what kind of information would you provide to members of your community about the risk of volcanoes and other geologic hazards?
5. Landslides occur in all 50 states. Consider the potential for a landslide in your area. Has one occurred in the past? Where would you predict any high-risk areas to be?
6. A block of vacation homes along the shore of North Carolina’s barrier islands has lost many feet of property in the past decade because of the effects of coastal erosion. What information should be presented to potential home buyers in neighboring areas that have not yet experienced such severe erosion? What should they know about regulations for building new structures?
7. Though agriculture has been responsible for a good deal of the groundwater removal and resulting subsidence, the country’s growing population and its need for water (landscaping, drinking water, fire suppression, industry, manufacturing, recreating, etc.) is become a major factor. Describe an area of the country that is experiencing such a potentially hazardous boom in development. What are the community’s water needs?
Know Your Geology
As an emergency manager in your town, what resources would you use to create a map of your town’s geologic high-risk areas? How would you describe the different areas of risk to a homeowner or business owner in your community?
I Feel the Earth Move
Earthquakes are a major concern in many areas of the United States. What is your community’s level of risk from a quake? Use the USGS’s earthquake website, at earthquake.usgs.gov to determine when the last earthquake took place in your state. Consider how earthquake preparedness factors into your own emergency plan. Using the website, determine what you should do during an earthquake. For example, should you head for a doorway? Rooftop? Basement? Also, how are the contents of your home likely to withstand seismic activity?
1. Coch, N. K. 1995. Geohazards: Natural and Human. Englewood Cliffs, New Jersey: Prentice-Hall.
2. FEMA. August 2001. State and Local Hazard Mitigation Planning How to Guide: Understanding Your Risks. Publication 386-2. FEMA.
3. Dunbar, P. K. and C. S. Weaver. 2008. U.S. States and Territories National Tsunami Hazard Assessment: Historical Record and Sources for Waves. Prepared for the National Tsunami Mitigation Program by the National Oceanic and Atmospheric Administration National Geophysical Data Center and the U.S. Geological Survey.
4. Morton, R. A., T. L. Miller, and L. J. Moore. 2004. Open File Report 2004-1043, U.S. Geological Survey.
5. Nicholls, R. J., P. P. Wong, V. R. Burkett, J. O. Codignotto, J. E. Hay, R. F. McLean, S. Ragoonaden, and C. D. Woodroffe. 2007. Coastal systems and low-lying areas. In Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M. L. Parry, O. F. Canziani, J. P. Palutikof, P. J. van der Linden, and C. E. Hanson, Eds., Cambridge, UK: Cambridge University Press, pp. 315–356.
6. Williams, J. B. 2007. Proceedings of the 2007 National Conference on Environmental Science and Technology, Greensboro, NC.
* http://earthquake.usgs.gov/earthquakes/states/10_largest_us.php.
* http://ga.water.usgs.gov/edu/tsunamishazards.html.
