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8/17/99 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 (Did You Feel It?) and yielded instrumental records of shaking that reflect various ground conditions in the Bay Area (ShakeMap - a map of instrumentally recorded shaking intensity).

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.

For more details, see these sites:

Info from UC Berkeley Seismological Laboratory
Aftershock Warning
Map of M>2 Earthquakes since 1969
NCSN First Motion Mechanism 1
California Recent Earthquakes Maps