![]() ![]() ![]() But the Lake Elsman quakes did not bring the Loma Prieta epicenter location closer to failure. These smaller quakes did indeed stress the fault, unclamping the portion of the fault that would undergo the greatest slip during Loma Prieta, perhaps enabling fluid infiltration and so making it more slippery than the surrounding patches. Were they foreshocks signaling something important, or just seismic flotsam and jetsam? The answer is murky. Even the term, ‘recurrence’ should be abandoned, as it implies a regularity that does not hold up to scrutiny except in some isolated cases.įour months and then two months before the Loma Prieta earthquake, the M5.4 and M5.3 Lake Elsman shocks struck several miles off the San Andreas. Faults not only quake M~7’s on their own timescales, but there must be many more M~7’s than M~8’s. So, we’ve come to understand that the San Andreas doesn’t just produce M~8’s. Since the likely mean time between M~8 shocks is about 250 years on the San Andreas, why would we see a large San Andreas shock only 83 years after the 1906? But now we know from extensive trench excavations that there was also a M~7 in 1838 in Woodside, 68 years before the great 1906 event. Thirty years ago, most of us regarded earthquake recurrence as more regular and periodic than we do now. In fact, the bend in the San Andreas near Loma Prieta is what built the Santa Cruz Mountains and uplifted the spine of the peninsula. ![]() Much of the San Andreas is neither vertical nor right-lateral, and there can be multiple closely-spaced rupture surfaces. Fault bends produce contortions in the fault geometry (such as its inclination to the surface) and slip direction. We’ve since come to understand that the quake did occur on the San Andreas and we’ve learned a lot more about why. Because the epicenter locates to the west of the San Andreas Fault proper, and because the north part of the rupture involved thrusting rather than the expected right-lateral slip (whichever side you are on, the other side moves to the right), seismologists argued fiercely for a decade after the quake about whether Loma Prieta occurred on the San Andreas. ![]() The epicenter of the Loma Prieta quake was in the Santa Cruz mountains, about 90 kilometers (55 miles) from San Francisco and Oakland. Collection of Oakland Museum of California.ĭid Loma Prieta rupture the San Andreas Fault? Victims being helped to climb down from the collapsed Cypress Freeway. Somehow, the worst that Mother Nature could hurl at us brought out the best in us, just as it did in 1906. There was calm compassion, without any looting, rioting or fighting. Bystanders helped fight fires in the Marina district of San Francisco and extricate car passengers in the collapsed Cypress Street Viaduct in Oakland. All told, the magnitude-6.9 quake was the strongest to shake the Bay Area since 1906, killing 63 people and injuring more than 3,700.įirst and foremost, the earthquake triggered extraordinary acts of heroism, courage, cooperation and community. The game was postponed and the series didn’t resume for another 10 days due to damage across the Bay Area.ĭuring the quake, a double-deck section of the Nimitz Freeway (called the Cypress Street Viaduct) collapsed, killing 42 people, part of the Bay Bridge collapsed, killing one person, and many buildings in both Oakland and San Francisco crumpled. Concrete fell onto the upper deck, and power was knocked out to the stadium. and those in the stadium thought the crowd was exceptionally raucous but it soon became clear that an earthquake had struck. On, Candlestick Park in San Francisco started shaking during a World Series game between the Oakland Athletics and San Francisco Giants. Stein (2019), Holding a mirror to the Loma Prieta earthquake 30 years later, Temblor, But the quake has been followed by an unexpected three-decade period of seismic quiet.Ĭitation: Ross S. Marks the 30th anniversary of the Loma Prieta earthquake, the Bay Area “wake-up call” that revealed our weaknesses and catalyzed our preparation. ![]()
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