Estimation of Site Effects and Q Factor Using a Reference Event

by Aaron Moya and Kojiro Irikura

Abstract

A method to determine site effects and the Q factor using a reference event is described. Spectral ratios between pairs of earthquakes and a reference one were written as logarithmic summations in order to obtain individual solutions for the numerators and denominators. Given the seismic moment and corner frequency for one of the events following the ω² model, a source model was assumed to constrain the amplitude of the solution. The validity and limitations of this technique were tested on synthetically generated data. We analyzed the variances of the results depending on the different number of stations used, the amount of noise present in the records, and the change in the seismic moment, corner frequency, and source model of the reference event.

Aftershocks of the 2000 Tottori, Japan, earthquake were used to estimate the site effects and Q factor in that region. Two strong motion networks, K-NET and KiK-net, recorded the aftershocks. The latter network contained both surface and borehole receivers at every station. Data were inverted first using only surface records and then using the borehole records from KiK-net stations in order to study amplitude differences. Our results suggest that the study area presents a low Q value and that there is also amplification at borehole sites.

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