Earthquake News From The AGU Meeting

December 09, 1997

Geologist Roland Burgmann of the University of California, Davis, will discuss several new studies of fault activity in the San Francisco Bay Area.

* Hidden Hayward Fault: Its urban setting and seismic history make the Hayward the most worrisome fault in the Bay Area. To better understand its earthquake potential, scientists are trying to create a detailed picture of the subterranean shape of the fault. Burgmann will review new clues to that shape, including evidence that:

-- Not all movement on the fault is sideways. At the fault's southern end, the eastern side of the fault appears to be temporarily rising relative to the western side. This uncharacteristic motion seems to indicate that some portions of the fault below and to the southeast of this segment locked up during the past few years.

-- The Hayward's characteristic creeping may be occurring only at the surface, leaving quake-producing stress unrelieved at greater depths.

Time and place: "Transient slip along the Hayward Fault," Thursday, 4:05 p.m., MC 300.

* South Bay Stepover: Studies of 25 years of seismicity and recent geodetic measurements in the South Bay are yielding more information about the complex junction just north of San Jose where the Calaveras, Hayward and Mission faults meet. Burgmann, Davis student David Manaker and Andy Michael of the U.S. Geological Survey have found that while the southern Calaveras creeps along at 0.27 inches to 0.66 inches per year, the northern end of the fault slips only 0.12 inches -- an indication of unrelieved strain. Also: A three-dimensional model of the region's subsurface architecture to about 9 miles seems to confirm suggestions that blind thrust faults underlie the area.

Time and place: "Earthquake hazards associated with the Mission Fault stepover," Thursday, 8:30 a.m., MC Hall D.

* Rise of Loma Prieta: When Loma Prieta subsided about 4 inches in the 1989 San Francisco Bay Area earthquake, it was an important indication that this section of the San Andreas Fault moves vertically as well as horizontally. Using a tool called apatite fission track analysis, Burgmann and Stanford University researcher Trevor Dumitru found that the Loma Prieta segment rose about 0.03 inches annually for the past 3 million years -- a relatively fast pace. This means that faults along the western edge of Silicon Valley are rapidly pushing up the Loma Prieta range. What's not clear is whether that rapid movement makes these thrust faults more or less likely to produce a big earthquake.

Time and place: "Constraints on post-Miocene uplift in the San Francisco Bay Area," presented earlier this week.

* Afterslip Report: With a new analytical program devised by Stanford geophysicist Paul Segall, Roland Burgmann and Segall have produced a detailed picture of the surprising movement on local faults in the five years after the 1989 earthquake. The program makes highly precise measurements of ground movement using the Global Positioning System. Along the rupture of the Loma Prieta segment of the San Andreas, the ground shifted about 2.6 inches in just the first year after the quake, and moved a total of about 4 inches by the time it stopped shifting 1994. On the Foothills Fault Zone, which runs along the eastern base of the Santa Cruz Mountains, the ground moved about 3.3 inches in the first year after the quake and stopped moving by 1992. This harmless creep relieved a lot of post-Loma Prieta deformation on the faults. Still unknown is whether that also reduced the faults' earthquake risk.

Time and place: "Triggered time-dependent slip," Thursday, 2:45 p.m., MC 300.

* That Sinking Feeling: Recharging the aquifer has stopped ground subsidence in the Santa Clara Valley, which had caused flooding in some bayshore communities. Burgmann and JPL/Cal Tech colleagues say that although ground-water pumping after 1915 lowered the valley floor by a whopping 9 feet, new observations show that a natural pattern has returned. Now the valley rises about 1 inch in the winter and subsides the same amount in the summer.

Time and place: "Distinguishing ground subsidence," presented earlier this week.

* The Shake Felt Round the World: A series of enormous earthquakes in the 1950s and '60s along the Aleutian arc sent a stress wave rolling beneath the Arctic and Pacific ocean basins at a speed of about 125 miles per year, report UC Davis post-doctoral researcher Fred Pollitz, Burgmann, and a UC Berkeley colleague. When the wave reached the San Andreas Fault system in the late 1970s, there was a marked increase in the number of earthquakes.

Time and place: "Postseismic stress diffusion," Wednesday, 2:30 p.m., MC 121.

Media contacts:
-- Roland Burgmann will be at the AGU meeting through Dec. 12. He will see e-mail in the evenings, burgmann@geology.ucdavis.edu. On campus from Dec. 15, his office number is (530) 752-6808.

-- Sylvia Wright, News Service, (530) 752-7704, swright@ucdavis.edu




University of California - Davis

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