Site response study based on seismic data recorded in sediments of the Mexicali valley, Baja California, Mexico

The severe damages caused by strong earthquakes that occur in the Valley of Mexicali have a great negative impact on the industrial and agricultural activities of the region. This fact motivated the present study, which aims to understand how the ground responds during small and large earthquakes. A primary objective was to investigate the level of acceleration at which the site response starts to become non-linear. To do so, we used the reference-station spectral ratio and single-station spectral ratio (HVSR) methods with acceleration data collected in the valley. We first estimated the average weak-motion HVSRs for 10 sites using earthquakes that produced maximum accelerations of 10-100 cm/s2 at epicentral distances mainly between 2 and 30 km. These mean HVSRs show amplitudes from 2 to 9, with the highest factors (4-9) at frequencies between 0.5 and 6 Hz. Next, we searched for non-linear ground effects by comparing HVSRs of strong and weak motions. Strong-motion HVSRs, calculated using acceleration data from magnitude 6.3, 6.5, and 7.2 earthquakes, showed clear reductions in amplitude relative to weak-motion ones. These reductions, which start from frequencies below 3 Hz, were attributed to the non-linear behavior of soils subjected to high levels of deformation. The intense ground liquefaction induced by these three strong earthquakes and the nonlinearity effects identified here are highly consistent. We investigated the degree of nonlinear response using DNL, a parameter computed by integrating the logarithm of the ratio of the strong-and weak-motion HVSRs, with respect to frequency. For accelerations below 120 cm/s2, the absolute values of DNL calculated at 20 Hz fluctuated between 1 and 4 around an average value of 2.5. Such DNL behavior at this low level of acceleration was considered as evidence of a linear response of the sites in small earthquakes. Nonlinear effects, on the other hand, became clearly evident at higher accel-eration levels, where DNL at 20 Hz reached values close to 15. Finally, we hope that the results presented here will be useful for realistic strong-motion predictions and seismic design in liquefaction-prone soils in the Valley of Mexicali.