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
- 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:
from UC Berkeley Seismological Laboratory
of M>2 Earthquakes since 1969
First Motion Mechanism 1