Abstract
The shear (SH)-wave transfer function and the horizontal-to-vertical (HV) spectral ratio are essential to estimate the S-wave velocity pro- le and thickness of surface layers overlying a bedrock on the basis of resonance frequencies. In practice, it is the second method the most used. In this work, we propose a full-wave numerical method, based on a pseudospectral spatial differentiation, to simulate SH and P-S waves generated by random sources distributed spatially and temporally (ambient noise). The modeling allows us to implement seis- mic attenuation, surface waves and causal source radiation patterns, based on random values of the angles of the moment tensor at each source location. We focus on the location of the resonance peaks, since this property is strictly related to the thickness of the layers. First, we analyze Lamb’s problem for which an analytical P-S solution exists. The modeling algorithm is veri ed for a Ricker time history, but the analy- sis can be performed by using spikes as sources. The experiments based on ambient noise are compared to those of a coherent line source as a reference spectrum (e.g., an earthquake event far away from the receivers). SH-wave resonance frequencies can be identi ed in the spectra only when the random sources are located below the bedrock. In the case of P-S waves, the SH-wave transfer function is a good approximation to the HV spectrum, mainly when the noise is generated in the bedrock. Finally, we have assumed a square basin and found that coherent (e.g., earthquake-type) sources may yield identi able peaks but ambient noise gives unreliable results.
Highlights
There is nowadays a growing consensus that the most significant source of ambient seismic noise in the Earth is produced by wind-generated ocean gravity waves and their interactions, the ensuing storms and the coupling with the solid earth (Ardhuin et al, 2011)
We propose a modeling algorithm based on the Fourier-Chebyshev pseudospectral method to compute wavefields in the presence of simulated ambient noise
Ambient noise can be simulated by randomly distributed point sources in space and time as a continuous emission of energy
Summary
There is nowadays a growing consensus that the most significant source of ambient seismic noise in the Earth is produced by wind-generated ocean gravity waves and their interactions, the ensuing storms and the coupling with the solid earth (Ardhuin et al, 2011). In an attempt to model ambient noise and compare techniques to assess siteresponse, within a 2D setting, Coutel and Mora (1998) generate synthetic seismograms with a Chebyshev pseudospectral method for diverse configurations, and test four estimation techniques They consider incident SV plane waves (earthquake) and micro tremors (randomly oriented surface sources, i.e., noise) combined with the HV and the HH sediment-to-bedrock ratio. The HV ratio predicts the resonance frequency of the 1D transfer function corresponding to a vertically incident SH wave Shear-wave motion measurements of the ETF is somehow closely related to the S-wave transfer function These authors show that the amplitude discrepancy is primarily due to two factors, the vertical site response and the HVSR at the bedrock. Sources can be body forces or momenttensor components with random properties
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