More Earth Rumblings

Within a day of a major Easter Sunday quake, Scripps Institution of Oceanography at UC San Diego scientists Yuri Fialko and David Sandwell headed to the U.S.-Mexico border city of Calexico on a rapid response research mission to deploy several continuous GPS instruments near the rupture site.

“Most of the deformation occurs in the first day or two after the earthquake, so it is important to get out there soon after the event,” said Sandwell, who used satellite image data obtained from the GPS instruments to put together a picture of the ground movements.

The 7.2-magnitude earthquake rattled Baja California residents at 3:40 p.m. and was felt as far away as Las Vegas. Its epicenter was approximately 51 kilometers (32 miles) southeast of Mexicali in Baja California, Mexico.

Scripps researchers monitored the temblor, one in a string of recent high-profile quakes worldwide, and more than 2,000 aftershocks that occurred on the Laguna Salada Fault. The instruments operated for several weeks and monitored the very subtle changes in the ground that occurred after the major earthquake event.

“This is a fairly typical aftershock response,” says Frank Vernon, Scripps research geophysicist and director of the ANZA seismic network that captured real-time seismic data of the Easter event and the aftershocks.

Sandwell and Fialko worked with colleagues at the Center for Scientific Research and Higher Education of Ensenada (CICESE) in Ensenada, Mexico, and UC Riverside to develop a slip model from the surface deformation data that shows the distance and direction of fault movement. With the data, the team created an interferogram picture produced from satellite images, revealing the two types of movements that occurred along the fault.  At the north end of the rupture zone a right-lateral strike slip motion occurred, which results when the tectonic plate on the right side slides parallel past the left without producing vertical motion. The eastern side of the fault showed evidence of up-and-down motion.

The results from a radar interferometry image derived from the satellite data also revealed the boundaries of a 20-kilometer wide and 60-kilometer-long liquefaction zone. The agricultural area southwest of the ruptured fault produced sand volcanoes where the earthquake-induced stress liquefied the soil.

The California Real-Time Network, a prototype early warning system, which uses GPS satellites to detect the deformation triggered by large earthquakes, also detected the earthquake.

“Our early warning system picked up the earthquake accurately, as we had hoped,” said Yehuda Bock, Scripps geophysicist and director of the California Real-Time Network.

Prior to this temblor, the largest earthquake to shake Southern California occurred in the Mojave Desert in 1992. Known as the Landers earthquake, it reached magnitude 7.3. According to Vernon, Southern California residents can expect to feel aftershocks from this recent earthquake for a few months.

Scripps researchers will continue to monitor the Baja region to better understand the faults and stress that triggered the Easter tremor.

-- Annie Reisewitz

Related Links (4):

1. Scripps ANZA Seismic Network eqinfo@ucsd.edu

2. California Real-Time Network http://geoapp03.ucsd.edu/gridsphere/gridsphere?cid=Sierra+El+Mayor+Earthquake

3. Satellite Geodesy Lab        http://topex.ucsd.edu/

4. IGPP www.igpp.ucsd.edu