BY DAWN LEVY
Some people's hearts beat fast with fear at thoughts of "The Big One." But the hearts of seismologists and geophysicists race for a different reason: Major quakes provide a wealth of data to analyze and debate. Now Stanford geophysicists are finding that "little ones" -- earthquakes less than 5.0 in magnitude -- offer information just as important as that provided by their scarier seismic sisters.
Smaller earthquakes provide scientists with a unique means of determining the detailed structure of faults more accurately, says Gregory Beroza, associate professor of geophysics, who presented his work at the December meeting of the American Geophysical Union, a research society with more than 35,000 members in 130 countries.
"Dramatically improved earthquake locations will allow us to study earthquakes in fundamentally new ways," Beroza says.
Recently Beroza and his colleagues at Stanford and the U.S. Geological Survey have employed a method -- waveform-based earthquake location -- to pinpoint the whereabouts of small earthquakes. The technique precisely measures the arrival time of seismic waves by comparing seismic waveforms of related, small quakes. Waveforms are the wiggle in a seismogram that represents ground motion. Since locating a quake depends critically on these arrival-time measurements, more precise measurements lead directly to more precise locations.
"Small earthquakes have simple signatures relative to large earthquakes," Beroza says. "We use waveforms to get more accurate arrival-time measurements, which translates to more precise locations. The time of the earthquake origin is regarded to be the time at which the fault first slips. We can't measure this directly but must infer it from the arrival times of seismic waves, which we can measure."
Gregory Beroza, associate professor of geophysics, and his colleagues have compared the waveforms from thousands of small quakes to get a more precise determination of their arrival times, and hence their locations. The arrows show arrival times as determined by standard methods. These are subject to considerable uncertainty, which can be greatly reduced by alignment of entire waveforms to measure arrival times consistently.
When the ground shakes, a network of seismometers records data, and an expert at reading seismograms determines when the first wave of seismic energy arrived.
Beroza compares the improvement in the resolution of the seismic data to the enhanced image resolution astronomers enjoy with the Hubble telescope versus land-based telescopes. "It completely transforms our view of faults and brings seismicity info focus," he says.
On seismic maps, circles of increasing size indicate earthquakes of increasing magnitude. Maps of seismically active regions appear as a blur of circles.
With pinpoint detection, the blurs resolve into lines. "What we see is kind of bizarre," Beroza says. "We see identical earthquakes that occur over and over and over again on the same part of a fault. We see streaks of earthquakes -- locations that are spread out horizontally. And we see holes in the seismicity -- regions where there are few earthquakes. We don't yet know why these streaks occur. We don't fully understand the holes."
The biggest earthquakes to which Beroza and his colleagues have applied the method are 5.4 in magnitude. The technique works "really well" for quakes of magnitude 4.0 and smaller, Beroza says. Most instruments go off scale for larger quakes, he says, and further studies are needed to determine to what extent the method can be applied to larger earthquakes.
The technique is simple: Seismologists look at waveforms created by earthquakes and estimate where the wave starts to deviate from the baseline. This, they say, is the arrival time of the seismic wave. Earthquake waves travel at about 6 kilometers per second, so a quarter-second error in determination of the earthquake's start can result in mapping errors of 1.5 kilometers, or about a mile.
Next the seismologists look at waveforms from earthquakes that occur close to one another. For such earthquakes, the seismic waves recorded at the Earth's surface will be quite similar, even if they are not closely spaced in time (they may have occurred in different years). Their approach takes advantage of the similarity in waveforms to reduce dramatically the errors in arrival-time measurements.
Though the technique has been used since the late 1970s on a handful of earthquakes, in the last three years Beroza's group and others have applied the technique to much larger numbers of earthquakes. It was employed on a few thousand aftershocks of the June 1992 Landers, Calif., earthquake in the Mojave Desert to map fault structures with enhanced precision. It also has been used to pinpoint the locations of more than 7,000 seismic events along California's Calaveras faultline near Morgan Hill.
Stanford graduate student Eva Zanzerkia used the technique to show that some aftershocks of the 1992 Landers earthquake occurred along well-defined but previously unidentified structures and that an observed progression of earthquakes along these structures may be triggered by water deep in the Earth's crust.
Beroza's collaborators in this research include visiting Professor Goetz Bokelmann from the University of Bochum in Germany, Stanford graduate students David Schaff and Zanzerkia, and scientists Bill Ellsworth, Felix Waldhauser and Alex Cole at the U.S. Geological Survey in Menlo Park, Calif.
Caption 1: Gregory Beroza, associate professor of geophysics, and his colleagues have compared the waveforms from thousands of small quakes to get a more precise determination of their arrival times, and hence their locations. The arrows show arrival times as determined by standard methods. These are subject to considerable uncertainty, which can be greatly reduced by alignment of entire waveforms to measure arrival times consistently.
Caption 2: Top: A blur of circles of different sizes denotes aftershocks of different magnitudes along California's Calaveras fault. Bottom: Improved locations result in sharper mapping of faultlines. SR