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M 5.4 (SCSN)

1.2 km (0.7 miles)

Big Bear M 5.4 earthquake shakes a wide area

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Foreword to Page 3

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Foreshocks and aftershocks

There were no foreshocks. The main jolt was followed by a robust aftershock sequence that has included five in the M 4.0 range and several M 3.0s during the first six hours.

The first aftershock came about a minute after the main shock and registered M 4.0. About a minute after that, another quake registering M 4.3 shook. The parade of aftershocks followed thereafter, producing at least 32 events by the end of the 4 o'clock hour, including seven in the M 3.0 range.

The aftershock sequence slowed slightly, releasing 28 events in the 5 o'clock hour, none of which measured M 3.0 or stronger. But at 6: 16 a.m., another M 4.1 aftershock struck, causing the aftershock series to surge slightly, producing 19 events by the end of the hour.

Eleven aftershocks were recorded from 7-8:00 a.m. and then at 8:12 a.m., a M 3.9 event kick started the series again, producing 17 aftershocks by the end of the hour.

This pattern continued for the rest of the day, with the last moderate jolt coming at 11:33 a.m. and registering M 4.5.

As of 8:10 a.m. on Monday morning, February 24, a total of 238 earthquakes registering M 1.0 or stronger had been recorded (or processed), including 16 M 3.0s and five in the M 4.0 range. See a list of M 3.0+ events.

Distant seismographic stations outfitted with digital continuous strip recordings showed spectacular wave forms that captured the events. Two stations located in the Central California Coast Range are shown, one at Black Mountain and the other at Middle Mountain near Parkfield.

Notice the arrival times are slightly later than the initial 4:19:10 a.m. trigger and that the Middle Mountain first arrival is slightly later than recording at Black Mountain. Also notice the initial large aftershocks were lost in the wave train, but a couple others that occurred later were recorded.

Felt Reports
The quake was strongly felt locally, jarring people from sleep, causing buildings to sway, knocking pictures from walls and light objects from tables and shelves, but there have been no reports of damage or injuries at this time.

Did you fell this earthquake. Tell us what you felt. We especially want to hear from people near the epicenter.

The earthquake was widely felt in the Inland Empire region and as far away as the Los Angeles coast, southern San Diego County to the south, Ridgecrest to the north, and Ventura to the west. Several million people felt this earthquake. See the USGS Community Intensity Report.

Residents in San Bernardino Mountains were rocked the hardest, frightening some and causing them to dash outside. The aftershocks came so quickly that some stayed outside, only to feel another one as they considered going back inside.

Continued on Page 3 >>

Report update: 10:03:20 AM, Monday, February 24, 2003

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