'94 QUAKE THEORY PUBLISHED; SANTA MONICA ROCK FORMATIONS CITED.

Byline: David R. Baker Daily News Staff Writer

Wedges of rock buried beneath Santa Monica may help explain why the city suffered so much damage during the Northridge Earthquake despite its distance from the epicenter, according to a scientific paper published today.

The report stated that the immense rock structures, separating the Santa Monica Mountains from the Los Angeles Basin, amplified shaking from the Jan. 17, 1994, quake and battered buildings more than scientists could easily explain.

Similar underground structures may lie hidden in the San Fernando Valley, Seattle and Salt Lake City, and might have added to the deadly force of the 1995 earthquake in Kobe, Japan, said the study's author, Robert Graves.

``We can map out where these areas are and do more detailed investigations,'' said Graves, a research seismologist for the Woodward-Clyde Federal Services consulting firm in Pasadena. ``That would give us a much better handle on the seismic danger.''

Some researchers previously proposed that a bowl-shaped formation beneath Santa Monica may have acted as a lens, focusing waves traveling through the rocks. But Graves' theory, published today in the Bulletin of the Seismological Society of America, suggests a different, shelflike structure shaping the earthquake's energy in another way.

Graves and his co-authors suggest that waves of ground motion from the earthquake entered Santa Monica through two directions. Some were bent as they passed through the fault and ended up traveling roughly parallel to the surface. Others approached the surface from below.

The two converged just south of the fault line, where most of the damage occurred.

``You have two kinds of waves hitting at the same time, and that increases the shaking,'' Graves said.

Paul Davis, a geophysicist at the University of California, Los Angeles, who worked on the previous lensing theory, noted that he and Graves had examined different frequencies of ground motion. He also said a study planned for next year would take a closer look at the structure of the earth below Santa Monica.

``The questions are what, exactly, breaks the buildings, and how do these geological shapes increase the shaking?'' he said.

Graves also wants to study other areas that may harbor underground structures similar to the one he suggests lies below Santa Monica. He said he would like to focus on the northern San Fernando Valley, with its complex system of faults, but did not know when he would be able to begin such a study.

``That area needs to be looked at,'' he said. ``It's certainly seismically active.''