LOS ANGELES, Jan. 1 — A new picture of the ground beneath Southern California shows one of its most populous areas could be at greater risk of earthquake damage than previously realized. A basin of soft sediment beneath the San Gabriel Valley, just east of downtown Los Angeles, is about 3 miles deep — 1½ times deeper than previously estimated.

‘This was completely unexpected. We had no good idea of what the machinery down there driving earthquakes looks like.’ — GARY FUIS USGS seismologist. “I don’t think anybody is going to run out and build a stronger building because of this paper, but it will help an earthquake engineer do better modeling,” he said.

THAT COULD lead to more surface shaking during an earthquake, according to a study appearing in January’s issue of Geology, the journal of the Geological Society of America.

Such basins shake “like big bowls of Jello,” said Gary Fuis, a seismologist with the U.S. Geological Survey in Menlo Park who was the study’s lead author.

“The potential damage threat is greater because we’ve found that the basin is deeper,” but specifics will have to await computer modeling of the new data, Fuis said.

The study used data from a series of test explosions in 1994 which created shock waves that were picked up by seismographs as they bounced through underground rock and soil. The area mapped, to a depth of about 12 feet, stretches about 100 miles from Seal Beach through the San Gabriel Valley and across the San Andreas Fault to the Mojave Desert.

Egill Hauksson, a seismologist at the California Institute of Technology in Pasadena, said having a detailed picture of the subsurface is important for understanding earthquake hazards.

The study also suggests the presence of an immense, ramplike crack stretching under the San Gabriel Mountains that could be the deep generator of earthquakes along several Southern California faults.

The fault, called a decollement, rises from 12 miles deep near the San Andreas Fault to about 8 miles as it moves south. It connects the faults that caused the Whittier and Sierra Madre earthquakes of the late 1980s and early 1990s.

If the discovery is confirmed, it could help scientists make better guesses about where future quakes might occur in the region, based on how pressure from the San Andreas Fault is channeled.

“This was completely unexpected. We had no good idea of what the machinery down there driving earthquakes looks like,” Fuis said. “Now we think we have a picture of how it all works.”

He said scientists weren’t sure if the large crack could generate earthquakes or if it transferred its deformation to shallower faults. “We’ll start looking for deformations — wrinkles — that might reflect movement,” Fuis said.

Southern California has long been considered among the places most at risk for earthquakes. Its densely packed cities were built above an area where two plates of the Earth’s crust grind together, and it is lined with earthquake faults, many of them buried.

The San Gabriel Valley and the San Fernando Valley, just north of downtown, both sit on basins of sediment laid down by ancient seas and rivers.

The San Gabriel Valley was the center of the 1987 Whittier Narrows and 1991 Sierra Madre quakes, which together caused eight deaths and $700 million in damage. The San Fernando Valley was the site of the 1994 Northridge earthquake that killed dozens and caused $15.3 billion in insured losses.

What are earthquakes? What is a fault? What are the basic types of earthquakes? Are earthquakes really on the increase? What is the hypocenter? What is the epicenter? How many earthquakes are reported yearly? Earthquake frequency worldwide: What is the Richter Scale? What is the Modified Mercalli Scale? How can I prepare for an earthquake? Damage of recent earthquakes in the U.S.:

An earthquake is caused by a sudden slip on a fault and it occurs when plates grind and scrape against each other. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.

A fault is a thin zone of crushed rock between two blocks of rock, and can be any length, from centimeters to thousands of kilometers. It is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Most faults are the result of repeated displacements over a long period of time.

The two basic types:

A strike-slip earthquake occurs on an approximately vertical fault plane as the rock on one side of the fault slides horizontally past the other.

In a dip-slip earthquake the fault is at an angle to the surface of the earth and the movement of the rock is up or down. Not necessarily. Earthquakes of magnitude 7.0 or greater have remained fairly constant throughout this century and according to records have actually seemed to decrease in recent years. In the last 20 years more earthquakes are noticed yearly because of the increase in the number of seismograph stations in the world and improved global communications. This increase has helped seismological centers to locate many small earthquakes which were undetected in earlier years.

The hypocenter is the point where the earthquake rupture begins, usually deep down on the fault.

The epicenter is the point on the surface directly above the hypocenter.

The National Earthquake Information Center now locates about 12,000 to 14,000 earthquakes worldwide each year, or 35 a day on average.

Frequency of Earthquakes Worldwide
Description Magnitude Annual Average

Great 8 or higher --- 1
Major 7 - 7.9 --- 18
Strong 6 - 6.9 --- 120
Moderate 5 - 5.9 --- 800
Light 4 - 4.9 --- 6,200
Minor 3 - 3.9 --- 49,000
Very Minor 1 - 3 --- 9,000 (daily)

The Richter Scale is used to measure the magnitude of earthquakes, as determined by seismograph measurements of the height of ground oscillations during an earthquake. The Modified Mercalli Scale measures the intensity of an earthquake; in other words, the effect of the earthquake on the Earth's surface. This scale is composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, and is designated by Roman numerals. The scale does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.