EERI Newsletter, March 2008 Volume 42, Number 3 very minor damage that doesn’t phone customers lost service. The impact its servicability. Earthquake Swarm in earthquakes were felt as far north ShakeMap intensities were only up as and to the east in to V/VI, and the “Did you feel it?” Yuma, Arizona. Caltran geologist Martha Merriam intensities were V/VI, so the ground According to Francisco Suarez Vidal contributed this report. shaking was not high. Snow on the of Centro de Investigacion Cientifi- ground made it difficult to document A swarm of earthquakes as large as ca y de Educacion Superior de En- ground deformations associated M5.1+ began on February 9, 2008, senada (CICESE), there was no with geotechnical phenomena. in Baja California north of the Cerro ground breakage along the geothermal field (the second Prieto fault. He did find cracks in This earthquake could provide in- largest geothermal facility for power concrete walls, and reported that sights in areas such as (1) possible production in the world) and are con- merchandise fell off shelves in sev- surface fault rupture associated tinuing at press time.These earth- eral markets and stores in that area. with a M6 event on a normal fault quakes have right-lateral strike-slip A special report issued by the system that is common in this tec- focal mechanism solutions with a Southern California Seismic Net- tonic region, (2) ground motion re- small normal component (about work (SCSN) noted that if seismicity cordings with implications for proj- 30%), suggesting tectonic movement migrates north, there is cause for ects such as Yucca Mountain, (3) on the Cerro Prieto fault and some concern in California because of landsliding, basin edge effects and extension in the step-over between potential movement along related differential ground deformation the Cerro Prieto and Imperial faults. associated with alluvium settling, faults, such as the San Jacinto fault and (4) earthquake-induced damage No major damage or injuries have roughly along the trend of the Cerro to well casings. There is also a po- been reported; however, factories Prieta fault. However, such a migra- tential issue in terms of a significant and schools were closed in , tion has not been reported in the earthquake occurring on a fault that a city of 750,000 located on the US/ Imperial or Mexicali valleys. The has a low slip rate, which may not border. In Mexicali, the ratio of large to small earthquakes have moved for a long period. In general hospital building sustained between the first few events and addition, the use of remote sensing cracks in walls and ceilings, and two events a week later has increased. is being explored as a reconnais- bridges with 2-inch to 3-inch cracks More earthquakes in the M5 range sance tool for this event. were closed until engineers could could be expected in the next few determine they were safe. Four hun- weeks, possibly spaced further For more information, including dred thousand people were left with- apart in time. photos and maps, visit the clearing- out power, and 1.2 million cellular house established by the Nevada Bureau of Mines and Geology at http://www.nbmg.unr.edu/ WellsEQ/.

Circles show M4 and greater (largest M5.4) earthquakes 1967 to 2007; stars show February 2008 M4.7 and three of the four M5+ earthquakes. Earth- quake locations by CICESE (RESNOM) using Baja stations were north and Epicenter of the Wells earthquake. west of those calculated by the USGS.

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