Special Earthquake Report No: 99-018

Regional Location:

Preliminary Magnitude:

Moment Magnitude:

Greenwich Mean Date:
Greenwich Mean Time:
Focal Depth (km):
Analysis Quality (A-D):


5.0 ML (NCSN)

4.7 Mw (UC Berkeley)

6.9 km

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An earthquake registering a preliminary magnitude of ML 5.0 (NCSN), Mw 4.7 (UC Berkeley) was detected 01:06:18 UTC, August 18, (6:06 p.m. PDT) along the coast of Marin County of Northern California. The focal depth was placed at a depth of 6.9 km and and the fault plane solution suggested predominately reverse motion along a northwest trending fault plane, with a slight strike-slip component. (See fault plane solution)
The quake was centered about 17 miles north-northwest of San Francisco at Bolinas Bay and the small coastal town of Bolinas. (Location Maps) It was centered either along the northern segment of the San Andreas Fault where it comes briefly back on shore north of San Francisco or a nearby fault splay. The northern segment of the San Andreas Fault is considered locked and has not produce a M5+ earthquake north of the Golden Gate since the powerful 1906 San Francisco ~M8 earthquake. In 1957, however, a M5.3 jolt occurred just west of Daly City but it occurred along the peninsula segment of the San Andreas Fault and not the locked northern segment. The Daly City quake caused over $1 million in damage and injured 40 people.
Preliminary reports indicate the quake was strongly felt in the epicentral area and as far away as Gilroy, Vacaville, Livermore, and Santa Rosa. Items toppled from tables and shelves in homes, hanging objects swung, and pictures shook on walls. Commercial items, like bottles and other light objects, are said to have toppeled in one Safeway store and there is a report of a few windows being cracked or broken. Most people reported the shaking lasted about 5-10 seconds. There have been no reports of injuries at this time.
One of the most common comments witin the hundreds of "Felt Earthquake Reports" received by Seismo-Watch was the noise that accompanied the quake. Most in the epicentral region heard a loud explosion-like sound, manying thinking it was a bomb or an accident of some sort. Sounds are common with moderate to large earthquakes, however, most sounds are commonly described as freight trains passing by or through the house, of course, depending on how close one is to the epicenter.
While the fault motion is rather surprising, the 1957 Daly City earthquake also had reverse-slip motion. For the most part, the San Andreas Fault is a strike-slip type of fault.
Curiously, there have been only a few aftershocks at this time. The largest was a M2.6 at 11:44 p.m., August 17, which was locally felt. As a precaution, the USGS have issued an aftershock warning which states the following probabilities: 10% chance of a M5.0 or larger quake in the next 7 days; 5-10% chance of a quake larger than the main quake; and there is a likelihood of between 3-12 M3+ aftershocks.

Update: 9:30 p.m. PDT, August 18, 1999.

Other News Reports:
San Francisco Chronical
KTVU - TV News Channel 2
KRON - TV NewsCenter4 - Video (requires REAL Player)
Marin Independant Journal
San Francisco Examiner

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Subtract 8 hours from Greenwich Mean Time to obtain PST or 5 hours for EDT
Location Quality: A (good), B (fair), C (poor), D (bad)
Magnitude: Ml (local or Richter magnitude), Lg (mblg), Md (duration), Mb (body wave), Ms (surface wave), Mw (moment)

Standard Sources Include:
U.S. Geological Survey, National Earthquake Information Center, Golden, CO (NEIC)
Harvard Geophysical Observatory, Harvard University, Cambridge, MA (HRV)
U.S. Geological Survey, Northern California Seismic Network (NCSN)
University of California, Berkeley, Seismological Laboratory (UCBSL)
Southern California Seismic Network (USGS & Caltech), Pasadena, California (SCSN)
California Institute of Technology, Pasadena, California (Caltech)
various wire reports and/or personal communications

All data are preliminary and subject to change.
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