Dynamic characterization of the Mejillones Basin in northern Chile, using combined geophysical field measurements

Abstract

The Mejillones Peninsula, located in northern Chile, represents a site of interest for seismic hazards assessment due to ~ 100-year seismic gap, the potentially large site effects, and the presence of the most important port in the region. In addition, the western edge of the Pampa Mejillones is limited by the Mejillones Fault, which controls the development of the Mejillones Basin to the east. In this research, we created a dynamic characterization of the northern portion of the basin using an integrated modeling of geophysics observations. These observations were: gravimetric method, magneto-telluric method (MT) and multichannel analysis of surface waves. The results of the geophysical campaign show soil thicknesses up to 750 m on the deeper zone with steeper slopes to the west and lower slopes to the east, in agreement with the normal-fault half-graben basin geometry. The sedimentary stratum is characterized by shear velocities between 300 and 700 m/s, extremely low electrical resistivities (below 1 Ω-m) and densities from 1.4 to 1.8 g/cm3. The integrated interpretation allowed us to generate a spatial and petrophysical model of the Mejillones Basin, from which we studied the seismic behavior of the zone. Preliminary results of the dynamic site amplification (using the Spectral Element Method) show peak ground accelerations (PGA) of almost 100% higher than the solicitation and stronger basin-edge effects. In this work, we demonstrate that the presented methodology (integrated geophysical characterization and numerical simulation) can be applied to characterize any deep forearc basin to quantify their seismic response in order to assess their seismic hazard.