Quake rocks southern California.

Los Angeles and its environs awoke to a rocking alarm clock on June 28, courtesy of a magnitude 5.8 quake that originated under the San Gabriel Mountains. This is the fourth strong shock to hit the San Gabriel Valley area in the last four years. Seismologists say they don't know why the region has become so shaky of late, but they expect the quake activity to continue in the future.

The recent temblor caused significantly less damage than the magnitude 5.9 Whittier Narrows earthquake that struck the valley in 1987. Why the difference in destruction? Aside from being slightly smaller, last month's quake centered in an unpopulated mountainous region, whereas the earlier quake originated close to Whittier and other cities, explain seismologists Kate Hutton of Caltech and Lucile Jones of the U.S. Geological Survey, both in Pasadena. The Whittier Narrows quake also cleared out many of the weakest buildings and prompted people to reinforce those that survived.

The geometry of the invovled faults may have played a role as well, Hutton and Jones say. Scientists studying the recent quake believe it occurred on the Sierra Madre fault, which dips under the mountains at a 45 [degrees] angle. During the quake, the northern side of the fault slipped up over the southern side, which holds the region's major population centers. The side that slips upward, called the "hanging wall," typically suffers more damage than the downward-slipping "foot wall."