Geologists ponder the depth of earthquakes.

When a section of the Pacific plate scrapes North America, seismologists expect to see a whole lot of shakin' goin' on. In Southern California, where this clash is ongoing, experts puzzle over the limited size and number of quakes over the past century.

Geologists Harold Magistrale of San Diego State University and Hua-wei Zhou of the University of Houston help resolve the mystery in the Aug. 2 Science. They report that a hidden layer of soft schist rock helps to buffer Southern California from the most violent shaking.

They also say their data can improve predictions of the biggest quake likely to strike specific sites.

Zhou used earthquake data from 1981 through 1994 to determine hypocenters- the location and depth-of 37,000 Southern California earthquakes. They found differences of 4 to 10 kilometers in depth between some quakes that were geographically close together, indicating that Earth's brittle, quake-prone outer layer has well-defined "steps" where its thickness changes dramatically.

Where composed of schist, the layer is thinner.

An earthquake's magnitude is proportional to the length and depth of the rupture created by slipping plates. Because schist softens under less temperature and pressure than other rock in the outer layer, it does not rupture as far down, thus limiting a quake's magnitude.

While schist outcrops on the surface are widely scattered in Southern California, the new data show that a broad region on either side of the San Andreas fault rests on schist, as does the area west and offshore of the Newport-Inglewood fault, which runs under densely populated zones. Rock distribution can explain much, but improved predictions of quake magnitude will have to await a more thorough grasp of heat flow under the region, says Hiroo Kanamori, a geophysicist at the California Institute of Technology. Temperatures deep underground can vary 50#161#C or more at sites less than 150 km apart, he notes, regardless of rock distribution.