Special Earthquake Report No: 01-006

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ML 4.4 (NCSN)

Mw 4.4 (U.C. Berkeley)

23:18:22 (03:18:22 p.m. PST)
7.8 km

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SAN JOSE, CALIFORNIA, February 25, 2001 (Seismo-Watch) -- A moderate earthquake measuring ML 4.4, Mw 4.4 occurred at 23:18 UTC (3:18 p.m. local time), Sunday afternoon in the East Bay hills east of San Jose. California, shaking much of the South San Francisco Bay area but not causing any damage. (See Seismo-Watch EQ Alert Bulletin / Subscription info)
The quake was centered along the Calaveras Fault about 11 miles east of San Jose at the base of Mt. Hamilton, in Halls Valley (Maps). The UC. Berkeley fault plane solution indicates strike-slip motion along a northwest trending plane which correlates well with the type of slip commonly associated with the Calaveras Fault.
The quake was widely felt in the South San Francisco Bay Area and as far away as Marin County, San Joaquin Valley, Santa Cruz Mountains and Monterey Bay area. There have been no reports of items toppling off tables and shelves. Residents nearest to the epicenter said windows jiggled, pictures on walls rattled, and hanging plants swung.
Some people heard a roar just before the ground began to shake. Depending on where people were located, most said they felt a sharp jolt followed by a brief rolling motion.
The aftershock sequence appears rather mild, posting only four tremors in the first hour, the largest registering M2.1 at 3:37 p.m. There were no significant foreshocks.
This segment of the Calaveras Fault shows a fair number of microseismic movements each month, yet most are register M3.0 or less. The regular seismic activity has scientist refering to this portion of the fault as the creeping segment of the Calaveras Fault. Further to the southeast near Hollister, tectonic creep has displaced recent construction features, such as roads, curbs, fences, buildings, etc., is is an exciting example of ongoing earth processes.
It was the second M4 quake in the Central California in two weeks as a M4.0 triggered on February 11 along the San Andreas Fault near San Juan Bautista.
The last time a M4 jolt occurred along this segment of the Calaveras Fault was on June 14, 1998 with a M4.0 temblor. The strong Morgan Hill M6.2 earthquake struck the Calaveras Fault in Santa Clara Valley on April 24, 1984 and caused more than $6 million in damages. Here is a list of M4+ events in the last six years:
1995/09/13 20:36:46.53  37.0962 -121.5120   8.23  4.10 coda 
1995/12/13 05:45:12.75  36.9758 -121.4708   7.57  4.00 coda 
1996/05/21 20:50:20.16  37.3593 -121.7227   8.14  4.50 coda   
1998/06/15 01:59:22.39  37.0385 -121.4790   8.54  4.01  MLm 
2001/02/25 23:18:22.50  37.3330 -121.6988   7.81  4.44  MLm
(See list of M4+ quakes from 1970 - 2000)
More information will be added shortly.
Additional Resources: (Sites will appear as pop-up windows)
NCSN Recent Earthquakes Page for South San Francisco Bay Area.
USGS - Did You Feel It?.
Sources: USGS, Caltech, personal communications.
Update: 12:30 p.m., February 26, 2001.
Information contained within this report may not be disseminated without prior written consent from Seismo-Watch, Inc.
Copyright (c) Advanced Geologic Exploration 2001

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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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