Robust determination of earthquake source parameters and mantle attenuation

An improved inversion technique is needed to effectively separate the frequency dependence of the source from the intrinsic attenuation of the medium. We developed a cluster-event method (CEM) in which clusters of nearby events, instead of individual events, pair with stations to form the basis for measurements of Q value and corner frequency (f(c)). We assume that the raypaths from one cluster to a station share an identical Q while each event in the same cluster is allowed for only one f(c) in the inversion process. This approach largely reduces the degrees of freedom to achieve a robust inversion. We use an optimization algorithm of simulated annealing to solve the nonlinear inverse problem. The CEM was applied to events at 70-150 km depths in the Japan subduction zone recorded by F-net. We show that the method proposed here leads to better constraints on both source parameters and attenuation. The resultant Q's in the mantle wedge increase from lower than 300 beneath the arc and back-arc to greater than 600 in the fore-arc region. The f(c)'s satisfy a self-similar scaling relationship with seismic moment of M-0 proportional to f(c)(-3) with a best fit stress drop of 21.9 +/- 6.9 MPa in Madariaga's form. This contrasts to the stress drop of 1.4 +/- 1.1 MPa for a global data set composed of prior measurements for crustal events. The results of this study agree with results from previous studies, except with an upward deviation due to higher corner frequencies and stress drops.