|Information from the CISN|
|Date & Time (Local):||2003/02/22
|Date & Time (UTC):||2003/02/22
|Location:||5 km (3 miles) N of Big Bear, CA
February 22, 14:00 PST (updated 23 Feb): A ML5.4 mainshock occurred at 04:19 am on 22 February, located 2 miles north of Big Bear City in San Bernardino County at a depth of 3.7 miles. It was not preceded by foreshocks. It was followed by strong aftershock activity of 116 aftershocks during the first 6 hours, with the five largest aftershocks (local time):
We use seismic records to determine the orientation of the fault on which an earthquake occurs. The mainshock exhibited strike-slip faulting (horizontal movement) on a steeply dipping (80 degrees) plane striking N40W, sub-parallel to the local strike of the Helendale fault. This mechanism is consistent with the mainshock being near the Helendale fault, a more than 40 mile long, late Quaternary fault in the Mojave Desert. Preliminary locations of the aftershocks appear to form a 2 miles long, northwest striking trend, located 2 miles to the west of the main surface trace of the Helendale fault. Hence, this sequence may be occurring on a small sub-parallel fault, adjacent to the Helendale fault.
This sequence is located in a very seismically active area. It is located about 6.2 miles due north of the epicenter of the M6.2 Big Bear aftershock of Landers, which occurred on 28 June 1992. Two M5.3 and M5.4 Landers aftershocks occurred about 3.1 miles to the northwest of the current activity in November and December 1992. In February 2001 a M5.1 event occurred about 5 miles to the west of the current activity.
The 1992 Mw7.3 Landers and the 1999 Mw7.1 Hector Mine earthquakes occurred in the Eastern California Shear Zone (ECSZ) that cuts across the Mojave desert forming a 40 mile wide swath of crustal deformation. The 1992 Landers earthquake was located towards the middle of this zone, while 1999 Hector Mine earthquake occurred near the eastern edge of the ECSZ. In contrast, the Helendale fault forms the western edge of the ECSZ. It has contributed to the generation of past temporal clusters of large earthquakes in the ECSZ.
Egill Hauksson, Kate Hutton, Lucy Jones, and Doug Given
The CISN Southern California Management Center produces ShakeMaps for events of M3.5 and higher. ShakeMaps are based on the observed ground motions from seismic instruments combined with predicted motions in areas without sensors. Several ShakeMaps have been generated for events in this sequence.
The CISN Engineering Data Center produces reports showing details of ground motions for events of interest. The have issued an Internet Quick Report (IQR) for the M5.4 showing records from the Big Bear Fire Station (8 km away) as well as other sites of interest:
The M5.4 and many of its aftershocks have been widely felt in southern California. People with Internet access can report their observations of shaking and damage through the Did You Feel It? Web site. In the links below, you can see the maps illustrating the reported intensities for several events in this sequence. Unlike the ShakeMaps - which are based on data from seismic instruments - these maps are based on human observations. You can contribute to these data by filling out the form for each event.