Near-Fault Strong Ground Motions Recorded during the Morelia Normal-Fault Earthquakes of May 2006 in Mexicali Valley, B. C., Mexico

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By the end of May 2006, a series of earthquakes had occurred in the Mexicali Valley, Baja California, Mexico. This activity, which included an M w 5.4 event, developed in the vicinity of the Cerro Prieto geothermal field. The earthquakes occurred on the Morelia fault, one of the east-dipping normal faults in the Mexicali Valley. Cracks and fissures visible on the ground after the stronger earthquake defined two parallel fault segments of 4 and 2 km length, the shorter segment being 1 km to the east of the other one. Down-dip displacements of up to 25–30 cm were measured at some places along this surface break zone. The ten larger events of the series were recorded by strong-motion stations that operate in the epicenter area, so that accurate hypocenter locations were obtained on the basis of P -wave arrival times from the strong-motion recordings. The estimated shallow depths, combined with the earthquake magnitude, explain the strong shaking felt by residents and some damage observed on the Cerro Prieto geothermal plant. The M w 5.4 event produced peak ground accelerations that go from 0.002 g at CUC (on rock) to 0.5 g at GEO (on sediments), at 13 and 1.7 km from the epicenter, respectively. The station GEO recorded closer to the epicenters and on the hanging-wall side of the fault. Static ground displacements and a predominance of the strike-normal over the strike-parallel velocity components were determined from the acceleration records of this station. These and other ground-motion characteristics are also seen on pseudovelocity and absolute acceleration response spectra calculated from data of the larger event. Altogether, the observed ground-motion characteristics provide useful insights into the levels of ground shaking that near-fault structures in the Mexicali Valley should be designed to withstand.