Special Reports
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M5.1 (SCSN), Mw 4.7 (SCSN)

15.2 km (9.4 miles)

Moderate M5.1 earthquake strikes Southern California

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Page 2, Anza M5.1 earthquake report, continued...

Foreshocks and aftershocks

There were no apparent foreshocks in the immediate area, however, a couple of small tremors registering M2.4 and M1.6 were detected to the south near Ocotillo Wells a few hours earlier.

A robust aftershock sequence has followed the main shock and at the time of this report more than 80 quakes have been recorded, but all have been weak and none have measured M3.0 or stronger. See the USGS quick list (good for only seven days following the quake. We will summarize the activity later).

SCSN has issued aa Aftershock Forecast in response to this event.

Felt Reports

The earthquake shook the epicenteral area hard and a wide area of Southern California from the coast to the Mexican border. Did you feel this quake? Many people were jarred from sleep, some were frightened, and even a few thought it was a terrorist attack. There have been no reports of any damage nor injuries at this time.

Many people said there were two distinct jolts. First one (the P-wave) was a sharp giggle lasting just a brief moment, perhaps a second or two. Some near the epicenter reported a light grown or rumble sound. Most said the shaking and the noise was enough to catch their attention.

After a short pause, longer the further away they were, the ground began to shake again. This time a little more vigorously and lasted any where from a couple of seconds to maybe 15-20 seconds. Some said it seemed to go on for about a minute which is probably a little excessive. This was the S-wave. P-waves move through the rocks quicker than S-waves. S-waves shake the ground more.

While there have been a wide range of observations, including windows and glassware rattling, hanging plants swinging, pictures on walls giggling and crystal chandeliers clanking, there have been no reports of anything toppling from tables of shelves. This is unusual for a quake with magnitude of ML 5.1 and may suggest the lower moment magnitude analysis of Mw 4.7 is closer to a more acceptable value considering the damage, or lack thereof.

Some pictures were knocked askew and objects on tables shifted.

Some people noticed that their animals acted strangely before, during and after the main shock. Dogs barked, cats were spooked, and birds chirped or squawked. One person in the epicentral region noticed the crickets had stopped churping prior to the shaking.

A number of people were frightened by the quake and many were also charged with excitement. One resident reported her teenage son exclaimed, 'whoopee, all right!' One in Victorville was chrged with excitement and could calm down for 15-20 minutes. Also of note is that many people slept through the shaking.

Some people noted they could tell a direction to the shaking and a few noticed that some walls shook more than others.

There have been no reprts of rock falls or landslides and there were no power outages.

Continued on Page 3...

Report update: 11:29:47 PM, Wednesday, October 31, 2001

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