Spatial Characteristics of High Frequency Ground Motion Along the Chilean Subduction Zone

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The Chilean Subduction Zone is known for large megathrust earthquakes originating at the interface of the subducting Nazca Plate beneath the South American Plate. We characterize high frequency ground motion along the Chile trench using earthquakes shallower than 40 km with magnitudes of 2.5-6.3. We analyze broadband three-component waveforms recorded by multiple networks (C1, CX, XY, ZE, XS) in the frequency range of 0.25-20 Hz and epicentral distances of 40-500 km. The northern Chile (NCH) data set includes 810 earthquakes recorded at 35 stations while the central Chile (CCH) data set uses 1174 earthquakes at 138 stations. The use of broadband data provides >> 10,000 observations at each frequency. We perform regressions on bandpass filtered ground velocities and Fourier velocity spectra to construct source-attenuation models. Comparison of the distance functions shows discernible differences in distance scaling between the data sets with a more rapid attenuation of seismic amplitude for the CCH paths compared to those of the NCH. At hypocentral distances greater than 250 km, the contribution from paths through deep structure at high frequencies (>8Hz) shows that a simple propagation model fails to fit all frequencies. We use two-corner frequency spectral source models to fit the low-frequency excitations for large-magnitude earthquakes. Given the source spectral and attenuation models and empirical durations of the ground motion, Random Vibration Theory is used to model the excitations using 28 and 48 earthquakes with known Mw in the NCH and CCH, respectively. While the high-frequency roll-off controlled by kappa doesn't show significant spatial dependence, the shape of excitations in NCH significantly differs from those in CCH. For large magnitudes, the excitation levels in the NCH are higher at low frequencies and are lower at high frequencies compared to the CCH . Estimated spectral accelerations and maximum accelerations with distance using source-attenuation models are consistent with results of other studies but show some spatial variations.

Presenting Author: Asiye Aziz Zanjani

Authors