Special Earthquake Report .
Mendocino Fault Earthquake Swarm

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Saturday, May 4, 2002

An earthquake swarm consisting on twelve quakes registering M 3.0 or stronger, including one as large as M 4.3, occurred during the last 72 hours just off the coast of Petrolia and Cape Mendocino, California.

According to the Northern California Seismic Network (NCSN), the earthquake swarm was centered between 10 and 17 miles west of Petrolia in the Pacific Ocean. See USGS preliminary data set.

The activity began with a M 3.1 at 9:58 p.m. on Friday night, May 3, but picked up during the 5 o'clock hour the next morning, producing seven M 3.0s events in the next three hours. It sputtered several quakes registering M 2.7 or stronger the rest of the morning, including two more in the M 3.0 range, and a few more were recorded in the next 36 hours.

The largest quake thus far measured M 4.3 and came at 6:56:31 a.m. on Saturday and was centered 17 miles west of town. See NCSN preliminary data set. It was the furthermost event of the sequence. UC Berkeley determined a moment tensor solution of Mw 4.3 and their fault plane analysis suggest the quake occurred along a generally east-west trending plane, which correlates well with the orientation of the Mendocino Fault.

Since the activity was centered an appreciable distance off shore, it will take further analysis to resolve activity registering less than M 2.5. See a NCSN helicorder of the activity. << very cool!

Many of the temblors were felt at Petrolia and the larger ones were widely felt along the Humbolt County coast and as far inland as Redway. Most reported brief ground shudders that caused windows to rattle, walls to vibrate and hanging plants to swing, but nine of the shaking was strong enough to topple items from tables or shelves. Did you feel any of these quakes?

A resident on the coast just west of Petrolia told Seismo-Watch he heard a whistle, then a bang prior to the M 3.9 jolt that happened at 5:17 a.m. on Saturday morning. Still lying awake when the M 4.3 struck just before 7 a.m., he said the ground learched one way then the other, but the vibrations were less intense -- probably because it was centered a little further away.

After a month of relatively quiescence and relatively low activity prior to that, earthquake activity off the coast of Cape Mendocino began to increase last week when a M 4.4 temblor struck on April 28 west of Ferndale in the Gorda plate. See report. This latest event brings the total of M 4.0s recorded off the North Coast this year to three.

In April of 1992, three earthquakes registering M 7.1, M 6.6 and M 6.5 rocked the region, causing more than $60 million in damages but no fatalities. See Notable Earthquake Report.

Report update: 7:29:04 AM, Monday, May 6, 2002

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