By JCP Geologists

The Magnitude 5.0 Bolinas Earthquake

BOLINAS, CA -- Foundations rocked and rolled as far south as Santa Cruz County -- and shuddered sharply in Bolinas at the epicenter of one of the first significant earthquakes to strike the Bay Area since the Loma Prieta disaster nearly a decade ago. The temblor was felt thoughout much of the North Bay region.

Fortunately, the quake's size was fairly small, at magnitude 5.0. With the exception of a few emptied store shelves and a toppled chimney in the community of Bolinas, 15 miles northwest of San Francisco, there was little damage to speak of.

The shock struck north of the Golden Gate along the San Andreas fault zone near its intersection with the San Gregorio fault, which extends offshore southward from the Bolinas area. Although the San Andreas fault in this area is visited frequently by minor seismic activity, Tuesday's quake was one of the largest to occur along on this stretch of the fault since it ruptured here in the great San Francisco earthquake in 1906.

The U.S. Geological Survey forecast a 1-in-10 chance of strong and possibly damaging aftershocks in the Bolinas area during the week following Tuesday's quake.

Although this earthquake is insignificant in comparison to the disastrous magnitude 7.4 shock in Turkey just the day before, it may prove interesting for local scientists studying the likelihood of future earthquakes in the Bay Area.

The USGS and collaborating scientific agencies are currently updating their estimates of earthquake probability for the region's fault segments, one of which is the San Francisco Peninsula segment. This is a section of the San Andreas fault that extends from Los Gatos northwestward to the Bolinas area, and is immediately north of the Santa Cruz Mountains segment that ruptured in 1989 causing the Bay Area's most recent earthquake disaster.

In its magnitude of 5.0 and its location near the end of a San Andreas fault segment, Tuesday's jolt at Bolinas is not unlike two moderate quakes that closely preceded the Loma Prieta shock. Those events, of magnitude 5.3 and 5.4, occurred near Lake Elsman just south of Los Gatos, about 16 months and 2 months before the earthquake on October 17, 1989.

The Lake Elsman events struck where the northern end of the Santa Cruz Mountains segment and the southern end of the Peninsula segment meet. Their timing and location has led some scientists to regard the Lake Elsman quakes as "preshocks" to the subsequent magnitude 7.1 Loma Prieta rupture -- useful harbingers, if so, had they been recognized as such.

Is the Bolinas quake a preshock of a larger quake to come? On this question the jury hasn't yet had time to convene.