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Bolinas: Seismic

Bolinas lies west of the San Andreas Fault, which runs the length of Bolinas Lagoon and continues northward through Olema Valley and Tomales Bay. Bolinas and the Point Reyes peninsula are on the Pacific Plate, moving north relative to Stinson Beach and the North American Plate at an average rate of about 1 inch (2.5 cm) per year.

The magnitude 5 earthquake occurred on Tuesday, August 17 at 6:06PM local time 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. The August 17th, 1999 event probably resulted in slip of a few cm on a small patch of fault at depth.

The Bolinas earthquake is interesting to scientists for two reasons:

  1. 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.
  2. 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.

source: 8/17/99 Magnitude 5 earthquake near Bolinas, CA

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