From The Mercury News:
The strongest earthquake to hit Southern California in nearly 20 years prompted one city to declare a state of emergency Thursday, and shook residents from Las Vegas to Orange County.
The quake, with a magnitude of 6.4, was centered near Ridgecrest, a high-desert community about 150 miles north of Los Angeles. At least 159 aftershocks of magnitude 2.5 or greater were recorded after the earthquake, according to USGS Seismologist Robert Graves. It is a higher than normal number, but not unprecedented, he said. The largest of them were magnitude 4.6.