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‘Aseismic’ creeping part of California fault once hosted huge earthquakes

Over 100 ancient temblors in the San Andreas Fault’s central section indicate potential future dangers, scientists say

In October 1989, workers check the damage to Interstate 880 in Oakland, Calif., after it collapsed during the Loma Prieta earthquake. (Paul Sakuma/AP)
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California’s San Andreas Fault is famous for a reason — nearly 800 miles in length, it caused the notorious 1906 San Francisco Earthquake and could eventually unleash “The Big One.”

Its northern and southern tips have long been thought to be much more active, and dangerous, than its supposedly sleepy middle.

Turns out that’s not true. A new study in the journal Geology points to evidence of multiple major quakes in the fault’s central section — past temblors that indicate potential future dangers.

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An international team of researchers analyzed samples of rock core found 1.98 miles beneath the surface of the fault’s central section.

There, the fault is different from the two tips of the system. In the north and south, the plates on either side of the fault line have been building up pressure for years. But in the central section, the plates engage in what is known as aseismic creep: a slow, continuous movement that doesn’t build up the same pressures as elsewhere along the fault.

To determine whether the creeping area produced past earthquakes, the researchers looked for evidence of past temperature spikes that would have resulted from the “slip” that produces an earthquake.

They found signs of more than 100 past quakes, some 6.9-magnitude level or greater. That’s comparable with the 1989 Loma Prieta Earthquake, which caused 63 deaths, over 3,700 injuries and $6 billion in damage near San Francisco.

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The central quakes appear to have happened at least 2,000 years ago, but that doesn’t mean the section won’t produce another large one.

Although the evidence points to more danger in the central section of the fault than previously thought, the researchers say it isn’t cause for alarm because of advances in building codes and growing geologic knowledge.

It does mean that aseismic creep could produce higher magnitude earthquakes than previously thought — and the new information will help seismologists tweak their hazard assessments.

Seismic events are inevitable,” said geologist Stephen Cox of Columbia University’s Lamont-Doherty Earth Observatory in a news release. “Work like this … helps everyone prepare.”