We analyzed the strong-motion records of two Mw 4.8 normal-fault earthquakes that occurred in June and September 1999 in the sedimentary environment of the Mexicali Valley. Peak ground accelerations of up to 0.452g were recorded for these events at distances between 2 and 37 km. We found that peak amplitudes recorded at the shorter distances tend to match the attenuation functions for southern California. However, as the distance from the epicenter increases, the Cerro Prieto peak amplitudes decay at a significantly higher rate. We also show that acceleration data from other small-to-moderate size earthquakes of the area decay with the same attenuation pattern. Thus, as the sediments of the Mexicali Valley rapidly attenuate the intensity of the ground motion, the potential of the earthquakes to cause damage at distance is reduced too. Although the higher accelerations were recorded at the station GEO, located in the yard of the Cerro Prieto Geothermal Plant, the damage caused to the plant was minimal.
In this study, we also explored the possibility of nonlinear behavior of the sediments beneath the station GEO in the Cerro Prieto earthquakes. For this, a set of GEO acceleration records from smaller-magnitude earthquakes of the area was studied first to gain an understanding of the response of sediments to less intense ground motions. Horizontal- to vertical-component ratios of shear-wave spectra indicated very similar ground amplifications for 17 earthquakes with magnitudes between 2.2 and 4.0. This result suggested that the sediments at GEO behave linearly during earthquakes in this range of magnitude. The mean site amplification from the smaller-magnitude earthquakes was then compared to the ground amplifications from the two stronger events. Relative to this weak-motion amplification, only a light nonlinear response of the sediments for the June event was inferred. For the September event, however, the comparison showed a clear reduction in the site amplification. Such deamplification of motion suggests nonlinear behavior of the sediments during the more intense ground shaking of this event. Furthermore, a well-defined peak of resonance in the weak-motion amplification was clearly absent in the response of the two stronger events, as has been reported in other studies of nonlinear response of sediments.