Earthquake Alert Bulletin No: 98-251

Preliminary data indicates a significant earthquake has occurred:

Regional Location:

Preliminary Magnitude:

Moment Magnitude:

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


4.0 ML (NCSN)

4.2 Mw (UCBSL)

26.1 km


An earthquake registering a preliminary magnitude of Ml 4.0 (NEIC), Mw 4.2 (UCB) occurred at 05:05:46 UTC, October 15 (10:05 P.M. PDT, October 14), in the northern Coast Range of Northern California. The focal point was placed at a depth of 26 km beneath the surface and the Moment magnitude analysis from the University of California, Seismology Laboratory, indicated normal fault motion along a northwest trending fault plane with a slight north-trending dip-slip motion. The quake was centered about 31 miles east of Eureka and about 8 miles south-southeast of Willow Creek, beneath the South Fork Trinity River valley.

Seismo-Watch received felt earthquake reports from several people along the North Coast from Trinidad to Eureka, but none were from the epicentral area. No aftershocks have been measured at this time. Most people reported feeling both the P and the S waves (an initial jolt followed by a later rolling motion), but there were no reports of items toppling from tables or shelves.

The depth and location of the epicenter suggests the quake triggered close to the subduction zone contact between the North American and Gorda tectonic plates, but the normal-slip focal mechanism does not support a thrust motion typical for subduction fault slip. More likely, the quake occurred within either the North American or Gorda plates and was the result of internal deformation of the plate caused by the subduction process.

The quake occurred in a location common for microseismic (M1-2) level activity which usually triggers at depths between 20-36 km below the surface, and more commonly at 26-33 km deep. These depths basically define the subduction zone contact, between the two plates with the shallower quakes occurring within the overriding North American plate. During the past 40 years of detailed seismic monitoring of the region, 29 M3+ earthquakes have occurred in this vicinity, but only two tremors have measured in the M4 range, tonight's Mw 4.2 and a M4.1 on April 9, 1987.

Interestingly, A M3.5 temblor struck the Mammoth Lakes region a little more than an hour after the Northern California jolt. It was the largest earthquake recorded there since a M4.1 struck on August 5 ­ over two months ago!

This earthquake report was faxed to subscribers of the Seismo-Watch Earthquake Alert Bulletins within hours of the event. Click HERE if you are interested in receiving these reports.

Subtract 7 hours to obtain PDT 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)
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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