Seismo-Watch
Earthquake Alert Bulletin No: 98-290

Preliminary data indicates a significant earthquake has occurred:

Regional Location:

Preliminary Magnitude:

Moment Magnitude:

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

SAN FRANCISCO BAY AREA

4.1 ML (NCSN)

4.0 Mw (NCSN)

98/12/04
12:16:07 (4:16 a.m. PST)
37.92N
122.29W
5.8 km
A

Notations:

An earthquake registering a preliminary magnitude of ML 4.1 (NCSN), Mw 4.0 (NCSN) occurred at 12:16:07 UTC (4:16 a.m. PST), December 04, in the San Francisco Bay Area near Richmond, California. The focal depth was placed at 5.8 km beneath the surface and the fault plane solution indicated predominantly strike-slip motion along a northwest trending fault plane. The quake was centered about 3 miles southeast of Richmond and 4 miles northwest of Berkeley in El Cerrito, along the northern branch of the Hayward fault. It was followed by a small M1.7 tremor 15 minutes later.

Seismo-Watch received several reports from people in the East Bay being jarred from sleep by the quake. Felt reports have also come in from as far away as San Francisco and Concord. There has been no reports from residents in the North Bay (Marin County). Light items toppled from tables and shelves and there are some news of some commercial damage. The quake was not strong enough to cause any structural damage.

This morning's quake was the largest quake along the northern Hayward fault since a M4.0 jolt on June 26, 1994 centered in much the same location. The M4.1 magnitude matched a March 29, 1986, event as the largest magnitude for an event on the northern Hayward fault in the last 35 years. The strongest earthquake along the entire length of the Hayward fault since the 1960's measured M4.3 and was centered near Hayward on March 27, 1983. In 1868, a strong earthquake registering M6.8 ruptured the southern branch of the Hayward fault from about San Leandro to south of Fremont, causing widespread damage throughout the Bay Area. Until the Great 1906 earthquake, it was referred to as the Great San Francisco earthquake. Sketchy reports of a similar magnitude event as the 1868 temblor along the northern Hayward fault in 1836 have proved to be false.

Activity along the Northern Hayward fault occurs in four distinct clusters which are separated by equally distinct seismic gaps. The most active cluster is located near Berkeley and the least active cluster is in North Richmond. This morning's quake triggered in the El Cerrito Cluster (Cluster No. 3). Click here or on the figure for an enlarge image.

 


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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)
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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