Written in August 1999 by JCP Seismologists
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.
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USGS Report8/17/99 Magnitude 5 earthquake near Bolinas, CA
The magnitude 5 earthquake that occurred on Tuesday, August 17 at 6:06PM local time was located beneath Bolinas Lagoon in Marin County at a depth of about 4.2 miles. It was felt broadly throughout the San Francisco Bay Area and yielded instrumental records of shaking that reflect various ground conditions in the Bay Area.
The earthquake occurred along a stretch of the San Andreas fault that had more than 20 feet of slip in 1906 as indicated by offset fence lines. In contrast, Tuesday's event probably resulted in slip of a few cm on a small patch of fault at depth.
Scientists consider Tuesday's quake to be a small to moderate size event. It would take approximately 4000 such earthquakes to equal the energy released by the 1989 Loma Prieta earthquake or 30,000 such earthquakes to equal the energy released in the magnitude 7.4 earthquake in Turkey which occurred on the same day.
The Bolinas earthquake is interesting to scientists for two reasons:
- It occurred in a region that has had only one previous earthquake in the last 30 years of detailed seismic recording.
- Information obtained from the earthquake waves indicate an up-down direction of slip not the sideways (or lateral) slip expected for the San Andreas fault.
The first observation was somewhat of a surprise. Much of the 300 mile length of the San Andreas fault that ruptured in 1906 (from San Juan Bautista to Cape Mendocino) is currently aseismic (relatively free of earthquakes) and has been since 1906. Scientists consider the San Andreas fault in Northern California to be in a "locked" part of its earthquake cycle.
The second observation suggests that, in detail, this earthquake probably was not on the San Andreas fault, but rather occurred on a small fault adjacent to the main fault at depth.
