Double distance dependence in high-frequency ground motion along the plate boundary in Northern Chile

Abstract

In subduction zones, the forearc crust transitions from a highly fractured wedge near the trench to a less fractured basement near the volcanic arc. Here we study the role of wedge integrity on the frequency content of strong ground motion produced by subduction earthquakes in Northern Chile. Our data includes aftershocks from the 2014 Mw 8.1 Iquique earthquake and intermediate-depth seismicity, including the 2005 Mw 7.6 Tarapacá earthquake. We focus on the S-wave spectral decay parameter κ obtained from strong ground motion in the frequency band 2−40Hz, paying particular attention to how κ varies with hypocentral distance. We report two distinct trends of κ versus hypocentral distance. One trend applies to events near the trench, where we observe a rapid increase of κ with hypocentral distance. Another trend applies to events near the bottom of the megathrust and greater depths, where we observe a relatively slow increase of κ with hypocentral distance. We interpret this difference as resulting from a heterogeneous tectonic structure with lateral variations in anelastic attenuation (i.e., attenuation seems stronger in the fractured wedge near the trench). Our results improve our understanding of the role of high-frequency ground motion in seismic hazard analysis near the Chilean coastline. In addition, our results stress the need for a better characterization of anelastic attenuation in the Chilean subduction zone, specially close to the highly fractured wedge near the trench.