California
Built in the 1930s, three-mile-long Golden Gate Bridge has been retrofitted to withstand an earthquake up to magnitude 8.3.
San Francisco Bay is perhaps the most seismically intriguing hot spot in North America. It parallels and crosses two of the world’s most notorious faults: the San Andreas Fault to the west and the Hayward Fault to the east. As a result of Earth’s crust downwarping (broad, downward folding) between the two faults, San Francisco Bay has an odd semicircle shape, visible in satellite images. The downwarping forms a bowl-shaped depression that holds the curving bay.
Viewed from the air, the San Andreas Fault runs along the Pacific coast, marked by a series of long, narrow lakes north and south of the entrance to San Francisco Bay. The bright orange Golden Gate Bridge spans this entrance. The Hayward Fault runs roughly along the interface of the gray urbanized zone and the green mountains to the east of the bay. Brown and tan colors in the bay’s waters are caused by salt marshes and artificial ponds for harvesting salt.
San Francisco Bay’s earthquake history goes back at least 28 million years, when the San Andreas Fault was born out of forces generated by the Pacific Plate moving relative to the North American Plate. The San Andreas is a transform fault, one that occurs when two tectonic plates scrape past each other. In some places, the plates in this type of fault move smoothly, lubricated by fluids and ductile rocks; in other spots the two sides of the fault get stuck and then move violently all at once, in an earthquake. Here, the offshore Pacific Plate is moving northwest and the North American Plate is moving southeast at a rate of about an inch and a half per year. Around 650,000 years ago, movement between the San Andreas Fault and the Hayward Fault opened a depression between the two faults. This area remained a valley until the last ice age, when increased runoff from the Sierra Nevada mountains filled the valley with water, creating the three-lobed bay we see today.
In 1906, a magnitude 7.8 quake struck along the San Andreas Fault just offshore San Francisco, setting off widespread fires in the mostly wood-built city. Fire would go on to destroy over 80 percent of the city’s buildings and more than 3000 people died. On October 17, 1989, a magnitude 6.9 earthquake struck approximately sixty miles south of San Francisco, killing sixty-seven people and causing more than $5 billion in damage.
The prospect of a quake striking this region again is not a matter of if, but when. The thirty-year forecast for the area estimates a 72 percent probability that at least one earthquake with a magnitude of 6.7 or greater will strike before 2043.
Saltwater meets freshwater in the San Francisco Bay, where 40 percent of California’s rivers drain to the Pacific Ocean. Golden Gate Bridge stretches across the inlet to the bay (Golden Gate Strait) at its narrowest point. Salt marshes and artificial ponds create the muddy colors in the bay’s inland waters.
Keep an eye peeled for the three-lobed inlet. You will fly over the San Francisco Bay en route to San Francisco International Airport or Oakland International Airport.
