Special Earthquake Report No: 99-002

Preliminary data indicates a significant earthquake has occurred:caption

Regional Location:

Preliminary Magnitude:

Moment Magnitude:

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


4.3 ML (NCSN)

4.3 Mw (UC Berkeley)

08:48:36 (12:48 a.m. PST)
10.9 km

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An earthquake registering a preliminary magnitude of ML 4.3 (NCSN) occurred at 08:48:36 UTC (12:48 a.m. PST), January 18, in Central California. The quake was centered about 16 miles south of Hollister and about 17 miles north of the Pinnacles in a remote region along the San Andreas fault, near the intersection of Highway 25 and Gloria Canyon Road.
U.C. Berkeley produced a quick moment tensor solution for the quake and their Moment magnitude (Mw) agreed with the local surface wave analysis (ML). The focal point was placed at 11 km beneath the surface and the fault plane solution indicated strike-slip motion along a northwest trending plane which agrees with the orientation of the San Andreas fault in the area. There were no immediate aftershocks, even in the M1 range, but later that afternoon a M3.4 aftershock shook the region.
The main jolt was sharply felt in the region, waking several people from sleep. There were no reports of items toppling from tables or shelves. Many were surprised by the modest magnitude as the jolt they felt surely registered somewhat higher. The quake was centered about 9 miles southeast of this summer's San Juan Bautista M5.4 earthquake.
This portion of the San Andreas fault is referred to as the Pinnacles segment and typically shows abundant activity in the M1 and M2 range (fault creep). The last M4+ quake on this fault segment occurred September 27, 1995 and registered M4.0. Many people remember a M4.7 that occurred earlier that year on April 23. It was followed by an intense aftershock sequence which lasted for months.
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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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