Spatial Dependence of High-Frequency Ground Motion Along the Chilean Subduction Zone From Aftershock Sequences of the Mw 8.8 Maule and Mw 8.2 Iquique Earthquakes

Abstract

The Chilean subduction zone is a highly active seismic region, producing megathrust earthquakes approximately every decade. We apply a stochastic approach to assess spatial variations in seismic wave characteristics along the Chilean trench using weak motion data. We analyze >10,000 weak motion records from 1,984 aftershocks ($M_l$ 2.5–6.3, with maximum depth and distance of 40 and 500 km, respectively) of the 2010 $M_w$ 8.8 Maule earthquake in central Chile (CCH) and the 2014 $M_w$ 8.2 Iquique earthquake in northern Chile (NCH). Source-attenuation models for the Iquique, Valparaíso, and Maule sub-regions were developed using Random Vibration Theory and regressions on filtered ground velocities and Fourier spectral amplitudes (0.25–20 Hz). The results reveal that geometrical spreading and frequency-dependent attenuation vary spatially. The source spectra differ mostly at intermediate frequencies using a two-corner-frequency source model. Differences in distance scaling and attenuation of seismic amplitude indicate faster energy loss along CCH paths, providing independent validation for the existing Chilean non-ergodic ground motion model. Seismic energy propagates efficiently through the colder, drier crust of NCH, while the weaker and warmer crust of CCH attenuates it faster. The frequency dependence of attenuation $\left(\eta \right)$ ranges from 0.30 to 0.35, but high–frequency decay $\left({\kappa }_o\right)$ does not show significant spatial dependence. Separating contributions from different seismic phases becomes critical for stochastic modeling at higher frequencies and distances >250 km. The properties of the crustal medium, such as anelastic attenuation and geometrical spreading, can be reliably constrained using moderate earthquakes and scaled to larger events for regional ground motion simulations.