IMAGING THE FAULT ZONES OF THE 2000 WESTERN TOTTORI EARTHQUAKE BY A NEW INVERSION METHOD TO ESTIMATE THREE-DIMENSIONAL DISTRIBUTION OF THE SCATTERING COEFFICIENT

Abstract

We developed an inversion method to estimate the spatial variation of scattering coefficient from observed envelopes of S coda waves. Synthetic envelopes are calculated on the basis of the single scattering theory, assuming double-couple sources and a one-dimensional depth-dependent velocity structure. In our algorithm, we evaluated scattering attenuation taking spatial variation of scattering coefficient into consideration in order to model energy attenuation by passing through the localized anomalous zones with high scattering coefficients. The inversion method was applied to the seismograms in and around the focal area of the 2000 western Tottori earthquake with M 7.3. Obtained results show that there exist several zones with large scattering coefficient zones (LSZs). One of the most predominant LSZ is located along and around the fault zone of the M 7.3 earthquake in the upper crust. This LSZ suggests the existence of damaged zones in the main shock fault. Within this LSZ, an exceptionally small scattering coefficient area was detected. This area corresponds to an asperity with a large amount of coseismic slip of the main shock. Other prominent LSZs are detected in the mid to lower crust below the northwestern part of the main shock fault. In addition, LSZs are distributed in a seismic low-velocity zone, near Quaternary volcanoes, and near hypocenters of deep low-frequency microearthquakes. Some of the imaged LSZs are probably related to local concentration of fluid in the crust.