4/13/01

Mark Shwartz, News Service (650) 723-9296; e-mail: mshwartz@stanford.edu

Stanford co-hosts conference commemorating 1906 earthquake

The April 18, 1906 San Francisco earthquake may be the single most important seismic event in American history. The "Big One" left a wealth of research on the causes and effects of earthquakes in its wake, and also spurred the formation of the Seismological Society of America (SSA) -- a group whose mission is to promote the scientific study of earthquakes and to increase public safety and awareness.

Ninety-five years after the quake, its effects are still being felt. To commemorate this historic event, the annual meeting of the SSA will be held in San Francisco this year from April 18 to 20 at the Cathedral Hill Hotel. The meeting is sponsored by Stanford and the U.S. Geological Survey's Menlo Park campus.

Several sessions will focus on past and future seismic activity in the San Francisco Bay Area. Others will deal with more general areas of research, current controversies and recent events such as the Jan. 26 earthquake in Gujrat, India.

Scientists from around the world hope that sharing the results of their research at the meeting will lead to a greater understanding of the enigmatic San Andreas fault and its most spectacular display to date \-- the 1906 earthquake.

To commemorate the exact moment of the event, several scientists will join earthquake survivors and their friends at 5:12 a.m. PDT on Wednesday, April 18. Every year survivors gather for libations in San Francisco at the corner of Market and Geary streets at Lotta's Fountain, which served as a message board and meeting place in the aftermath of the quake.

Several Stanford scientists will present current research on the San Andreas fault and other topics. John Townend, a graduate student in the Department of Geophysics, is working with Professor Mark Zoback to better understand the San Andreas and similar faults. "The San Andreas is very enigmatic," Townend said. "It's the biggest and best-studied fault around, and yet its mechanics are less well understood than those of smaller faults."

At the meeting, Townend will discuss the San Jacinto fault, a large fault running parallel to the San Andreas in southern California. Since it last ruptured in 1918, one section of the San Jacinto has experienced a paucity of earthquakes relative to adjacent segments of the fault. By studying the stresses associated with thousands of recent earthquakes, Townend has determined that this section is stronger than other segments and seems to be withstanding greater stresses. The San Jacinto and San Andreas faults are both strike-slip faults that involve the ground on one side of the fault moving horizontally past the other side of the fault. "Hopefully if we understand more about how this section of the fault re-accumulates stress, we'll ultimately be able to understand how the San Andreas fault operates and generates big earthquakes," he said.

Geophysics postdoctoral fellow Jeff McGuire will present a new technique to measure the slow movement of some faults that does not result in shaking ­ and consequently isn't recorded by seismic stations. McGuire and geophysics Professor Paul Segall collaborated with Shin'ichi Miyazaki of the Geographical Survey Institute of Japan to use Global Positioning System (GPS) stations that employ satellites to measure slow movements of the Earth's crust. The researchers have used almost 100 GPS stations in Japan to record a "slow earthquake." The ground moved in six months as far as an ordinary magnitude 6.7 tremor would cause it to move in five seconds.

"This technique is important for calculating seismic hazard in areas where slow earthquakes occur," McGuire said, adding that the risk of large earthquakes is actually less than expected in areas where some stress is released slowly and without shaking.

Geophysics Professor Gregory Beroza and graduate student Eva Zanzerkia will present research on whether fluids trigger earthquakes by lubricating faults.

By studying the frequency of aftershocks following the 1992 earthquake near the town of Landers, Calif., in the Mojave Desert, they found a clue. A section of the fault that was more likely to be affected by the presence of water had a much higher and more consistent rate of aftershocks for three-and-a-half years following the earthquake. Fluids resist the stresses that push the two sides of a fault together and may reduce the friction that keeps faults from moving. "If fluids are present at depth, it might explain why some faults slip more easily than expected," Beroza said.

David Schaff, another geophysics graduate student, has been working with Beroza on the San Andreas and Calaveras faults. Schaff has studied the signature of hundreds of tiny earthquakes recorded by seismic stations in Northern California. Their work shows that damage to the Earth's crust caused during an earthquake actually may reduce the strength of the shaking. Stronger rocks transmit waves generated by an earthquake much better than weaker rocks. During an earthquake, rock in the vicinity of the fault may be weakened by cracks, reducing amplitude of strong shaking.

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By Betsy Mason