Preliminary Report 22 Feb. 2003 

California Seismic Network (CISN): Cooperative Project of Caltech, USGS, CGS, & UCB 

Southern California Seismic Network Caltech/USGS 

Egill Hauksson and Kate Hutton, Caltech, Pasadena, CA91125
                [email protected]
Lucy Jones and Doug Given USGS, Pasadena, CA 91106
                [email protected]

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): 

                22 Feb 04h 20 min M4.0
                22 Feb 04h 21 min M4.3
                22 Feb 04h 25 min M4.0
                22 Feb 06h 16 min M4.1
                22 Feb 11h 33 min M4.5

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�) plane 
striking N40�W, sub-parallel to the local strike of the Helendale fault. 

This mechanism is consistent with the mainshock being nearthe Helendale fault, a more than 40 mile long, 
late Quaternary faults 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 December1992. 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 a cross 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.