Seismic noise can yield maps of Earth's crust.

One way in which researchers can garner clues about Earth's inner structure is to analyze intense ground motions from earthquakes or test explosions. Now, scientists are realizing that the small, random, and nearly constant seismic waves that travel in all directions through Earth's crust also carry useful information.

Any seismic waves that travel between one point and another provide information about the intervening rocks, says Peter Gerstoft of the Scripps Institution of Oceanography in La Jolla, Calif. To prove that point, he and his colleagues looked at data on the minuscule ground motions in the 50-millihertz to 400-mHz frequency range that had been gathered by a 148-instrument network of seismometers in southern California. Most of those vibrations were probably caused by the motion of tides and pounding.

In the first step of their analysis, the researchers added up the ground motions that each seismometer collected over the course of a month, thereby converting the seismic noise into signals that were large enough to analyze. Then, the researchers used a mathematical technique to isolate the seismic waves that traveled directly between any two particular seismometers. By repeating this second step for each of the more than 10,000 paths linking the 148 seismometers, the scientists assembled an ultrasoundlike image of Earth's crust in the region.

The image clearly distinguishes between broad, sediment-rich areas where the seismic waves traveled at slower-than-average speeds, such as the Los Angeles Basin, and rockier regions, such as mountains, where the waves traveled more quickly. The San Andreas fault shows up plainly because porous sedimentary rocks lie on one side of that rupture and dense rocks lie on the other.

The new technique should enable geologists to collect data more cheaply and efficiently because they won't have to set off series of explosions or wait for a large earthquake to provide seismic signals, says Gerstoft.--S.P.