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Earthquakes shake up Bay Area before Loma Prieta anniversary

Pleasant Hill still rumbling after 4.5 surprise

A finger pointing to the jagged lines of a seismograph readout. Photo by NurPhoto/NurPhoto via Getty Images

For the 30th anniversary of the Loma Prieta earthquake—the 6.9 tremor struck October 17, 1989 shortly before game three of the World Series between the Oakland Athletics and the San Francisco Giants—Curbed SF will feature quake-related coverage looking back at that fateful day, and what you can do to prepare for the next big one.

The Bay Area has been unsteady on its feet in the weeks leading up to the 30th anniversary of the Loma Prieta earthquake (October 17), with many relatively small but significant quakes setting the region on edge.

On Wednesday morning, the U.S. Geological Survey (USGS) recorded a magnitude 2.6 quake near Gilroy and a small series of quakes ranging from 2.5 to 3.4 further south in San Benito County.

This comes after a 4.5 quake centered around Pleasant Hill jolted the East Bay late Monday night (actually the second Pleasant Hill quake that night), followed by a 3.4 aftershock in almost the exact same place one day later.

Quakes centered around Colma, Dublin, and San Jose rattled the region earlier in the month too. In all, USGS records nine earthquakes of at least 2.5 magnitude in the Bay Area in the last 30 days.

Of course, this is hardly unusual; minor quakes happen weekly somewhere in the region. And the Los Angeles Times notes that an average of 25 earthquakes of magnitude 4.0 to 5.0 happen every year in the fault systems underneath California and Nevada; it’s only natural that a few pop up in the Bay Area.

It’s equally natural for a quake of such a magnitude to have aftershocks of varying degrees; according to USGS, there’s about a two percent chance of an aftershock even larger than the Pleasant Hill quake in the period of about a week afterward, with the largest aftershocks happening closest to the epicenter.

Nobody was hurt and there’s been no significant property damage reported from the latest tremors.

In 2007, USGS projected a 63 percent chance of a magnitude 6.7 or greater earthquake in the Bay Area over a 30-year span, adding that “the two faults in the San Francisco Bay Area most likely to have a damaging earthquake are the Hayward-Rodgers Creek fault system (31 percent) and the San Andreas Fault (21 percent).

The likelihood rose to 72 percent by 2014, the increased odds driven, in part, by more data and also the fact that a big quake did not happen in the seven years prior.

The odds of such a quake somewhere in the state of California, north or south, are as high as 99 percent by 2038.

To see where the major liquefaction zones are in the Bay Area, check the latest map. And be sure to read Curbed SF’s tips on putting together an earthquake kit.