SCIENCE : Why the Quake Shook Less Here

Caltech seismologists are looking a gift horse in the mouth, and they are stumped by it.

They cannot figure out exactly why the San Gabriel Valley suffered so little damage from last week’s earthquake and why continuing aftershocks are felt less in this valley than other places in the Southland.

“There seems to be a clear boundary where the aftershocks stop,” said seismologist Egill Hauksson. “We don’t know the reason, but they are not moving toward the east.”

Hauksson said distance is part of the reason; the San Gabriel Valley is more than 20 miles from the epicenter in Northridge. But other regions even more distant have suffered greater shaking.

Another reason could be that the fault suspected in the 6.6-magnitude quake runs east-west; land on the south side of the fault moved northward and over the other side. That created north-south movement rather than east-west, Hauksson said.

But seismic waves from aftershocks are migrating westward toward Ventura. They just don’t travel eastward the same way.

One possible explanation, Caltech experts say, is that soil liquefaction usually occurs where the water table is high and the soil is loose or sandy--as it is in the Redondo Beach area. Also, thick accumulations of sediment, such as in downtown Los Angeles, amplify the ground shaking. But the San Gabriel Valley’s more dense, rocky soil is less prone to liquefaction or shaking.

The last earthquake to affect the San Gabriel Valley directly was a 5.8 temblor in 1991 along the Sierra Madre Fault, which extends along the San Gabriel Mountains for about 75 miles and which some experts consider to be the most dangerous fault in the Los Angeles Basin.