Seismo-Watch

Special Earthquake Report No: 99-023

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NORTH SAN FRANCISCO BAY AREA

ML 4.2 (NEIC)

Mw 4.2 (UCBSL)

99/09/22
22:27:13
38.39N
122.63W
7.0 km
A

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An earthquake registering a preliminary magnitude of ML 4.2 (NEIC), M4.2 (UC Berkeley Seismological Laboratory) was detected 22:27:13 UTC (3:27 p.m. PDT), September 22, in the northern San Francisco Bay Area near Santa Rosa (See epicentral map). The focal depth was placed at a depth of 7 km and the fault plane solution indicated reverse-slip motion along a generally east-west trending plane. The quake was centered about 4 miles southeast of Santa Rosa in Bennett Valley east of Taylor Mountain and near the intersection of Bennett Valley and Grange Roads.

The quake was widely felt in the Santa Rosa, Rohnert Park, Napa and Petaluma areas. Residents near the epicenter reported a sharp jolt followed by sharp jiggles which caused windows to rattle and glassware to tinkle, but there were no reports of items toppling from tables and shelves. Residents further from the epicenter said the quake had more of a gentle rolling motion.

The quake was centered close to the surface trace of the Rodgers Creek Fault, one of the prominent faults in the northern San Francisco Bay Area. But because the fault dips slightly to the west and the focal point of the quake was at 7 km beneath the surface, it is likely the quake occurred in the footwall to the Rodgers Creek Fault and probably along some adjacent or sub parallel fault or fracture. Moreover, the typical sense of motion of the Rodgers Creek Fault is strike-slip and not reverse, and the orientation of the fault is more northwest than east-west, adding further to the possibility that the earthquake did not occur along the Rodgers Creek Fault.

It was the second moderate earthquake recorded in the North Bay in about a month and follows the Bolinas M5.0 earthquake on August 17 , which by the way also had a reverse-slip motion. The last M4+ quake this close to Santa Rosa was a M4.4 event on June 4, 1996, and was centered a few miles north of Santa Rosa.


Update: 10:50 p.m., September 22, 1999.

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Subtract 8 hours from Greenwich Mean Time to obtain PST or 4 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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