Humboldt County is shaken by M 5.1 quake

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Sunday, August 15, 2003

An earthquake registering M 5.1 occurred in the wee hours of the morning off the Northern California Coast, jarring people from sleep and toppling a few light objects from tables and shelves, but not causing any damage or injuries.

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According to the Northern California Seismic Network (NCSN) the M 5.1 earthquake struck at 2:22 a.m. PDT (09:22:13 GMT), Friday, August 15, about 76 miles west of Eureka in the Pacific Ocean. (See USGS location map) (See USGS earthquake data page)

The earthquake occurred north of the very active Mendocino Fault in the Gorda tectonic plate.

The focal depth was shallow, only 3.7 km deep (2.3 miles) and the type of fault motion was strike-slip. Quakes usually in this location have a left lateral motion, that is as you cross the fault, the rocks moved to the left.

The earthquake was felt over a wide area of the Humboldt County coast, from Trinidad to Petrolia, and in the Coast Range as far east as Kneeland and Carlotta (See USGS Community Intensity map). The most intense shaking was felt around the soft sediments of Humboldt Bay, where a few light items topped from tables and shelves.

Earthquake activity on the Humboldt County coast and along its off shore seismic zones has been rather quiet of late, producing only four earthquakes measuring M 4.0 or stronger this year and just six in the last 14 months, making this one of the quietest seismic periods in the last 20 years. See list.

The number of M 4.0+ quakes per year is usually in the double-digit figures (+10 eq/yr), but on three occasions, it has fallen below this mark: 1993 (6), 1998-1999 (7 and 8), and 2003 (4).

You can follow weekly earthquake activity on the California North Coast on the following Seismo-Watch earthquake reports:

• Northern California Coast Report

Three newspapers feature Seismo-Watch earthquake reports in the area of this earthquake, the Eureka Times Standard, the Santa Rosa Press Democrat, and the Ukiah Daily Journal. Be sure to let their editors know how much you enjoy it.