LONG BEACH, Calif. (AP) _ It's another day at the prehistoric beach for Heidi Spenner as she scrapes up a layer of sand with a metal spatula, pinches some of the dark gray muck and rolls it between her fingers.

The U.S. Geological Survey scientist is examining a patch of ancient shoreline, sandy dirt from 400 feet below the surface. It's one of many layers of soil that fill the Los Angeles basin and affects how violently the ground moves in an earthquake.

Researchers like Spenner say data from a 1,000-foot continuous core sample extracted from the border of Long Beach and Los Alamitos could give them a clearer picture of how the ground shakes.

Researchers plan to use the information to update seismic hazard maps, which can help emergency officials identify areas most prone to damage in a quake. Water managers also can use the data to understand how seawater intrudes into underground freshwater supplies and trace pollutants.

For years, water agencies and oil companies drilled into the basin, but they generally were not interested in the details of the first 1,000 feet. Oil is generally found at least 3,000 feet below the surface.

``Amazingly, the upper thousand feet of the basin is probably the least well understood,'' said Dan Ponti, a USGS geologist who is leading the project. ``What we're trying to is ... redefine our understanding of the overall framework of the basin.''

After strong earthquakes like Northridge in 1994, it became clear that damage is not evenly spread out from the epicenter _ that the soil between snapped rocks and the surface plays a role in magnifying or focusing seismic waves.

Researchers hope the Long Beach coring project and several others planned for southern California can help them understand what is happening beneath the feet of the millions of people who live in the geologically active area.

``It's like not having all of the pieces to a large jigsaw puzzle,'' he said. ``We need to find a few critical ones to be able to put the puzzle together and see the whole picture.''

Much of metropolitan Los Angeles sits on the basin, a giant bowl surrounded by mountains and ocean. Researchers once believed that in an earthquake the whole region shook uniformly like a cup of pudding.

But layers of soil affect earthquake waves differently. Loose sand _ like Spenner's sample _ amplifies shaking. Clays, on the other hand, tend to mute the movement.

The sediments were deposited by erosion and the rise and fall of the ocean, whose levels have changed drastically at least 20 times in the last million years during ice ages and warming trends.

The USGS teamed up with the Water Replenishment District of Southern California for the coring project. The hole was drilled on the agency's land, and a nearby trailer serves as a laboratory for the researchers.

The core samples are quickly moved to the trailer for analysis at a series of stations. At the first stop, a machine scans the soil and measures its density and how fast sound waves can travel through it.

The core is then sliced in half. The exposed side of one of the halves is photographed, packaged and shipped to storage. The other half moves to the other stations in the trailer.

At Spenner's station a core sample is hand-drawn on the conveyor belt. By touching the sand, Spenner determines that the sample is from a time when the area was a beach about 300,000 years ago.

``What we want to do is document what we see,'' she said.

Jack Hillhouse, a USGS geophysicist, works at another station about 5 feet away studying magnetic properties of the sample _ an important tool in determining age.

After the hole was drilled, a series of groundwater monitoring wells and earthquake measuring instruments were installed and will be used for at least 10 years.

``No one has done this kind of detailed and integrated work in the shallow subsurface of the L.A. basin before,'' said USGS hydrologist Eric Reichard. ``When all of the analyses are complete, this will be the best described site in the region.''