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

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 0.002g at CUC (on rock) to 0.5g 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.