Robust determination of earthquake source parameters and mantle attenuation

Abstract

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 ofQ value and corner frequency (fc). 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 fcin 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 resultantQ'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. Thefc's satisfy a self‐similar scaling relationship with seismic moment ofM0 ∝ fc−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.

Multiscale Seismic Tomography

“…(2.39), the corner frequency f ci is coupled with the wholepath attenuation operator t * , and so trade-off may occur between them during the spectra-fitting procedure estimating them simultaneously (e.g., Scherbaum 1990;Ko et al 2012;. As shown in Eq.…”

Upper mantle Q structure beneath old seafloor in the western Pacific

“…Although there is a nominal minimum of the misfit curve in this case, the small differences in misfit illustrate that all reasonable values of f c fit the spectrum nearly as well when t à 0 and ln Ω are also allowed to vary. This example is our largest event and best constrained minimum, yet it still is inadequate to independently determine f c .The main cause of this problem is the trade-off between corner frequency and the attenuation operator [Ko et al, 2012] given the relatively small range of traveltimes for each event. Both the corner frequency and attenuation operator can have similar effects in bending the spectrum.…”

Three-dimensional P- and S-wave attenuation structures around the source region of the 2016 Kumamoto earthquakes

“…Since there is a trade-off between the f ci included in S i f and t * ij (e.g., Scherbaum 1990; Ko et al 2012), before evaluating t * ij , we independently estimate f ci using a procedure exploited by Somei et al (2014), which is based on the S-wave coda spectral ratio method (e.g., Aki and Chouet 1975) (see Additional file 1: Section 1 and Figure S1 for the description and example of the f c estimation).…”