Determination of corner frequencies of source spectra for subduction earthquakes in Avacha Gulf (Kamchatka)

Abstract

Abstract The source spectra of M = 4.0–6.5 subduction earthquakes of 2011–2014 in Kamchatka are studied. The dataset comprises 1272 source spectra recovered from S waves of 372 earthquakes recorded by six digital rock-ground stations. The structure of the spectra is examined on the basis of a spectral model with three corner frequencies fc1, fc2, and fc3. It was assumed that the spectra behave as f–2 between fc2 and fc3, where fc3 denotes “source-controlled fmax” after Aki and Gusev. To determine the corner frequencies, we extracted the source spectra from S-wave spectra using a previously developed attenuation model for the study area. The spectra were first reduced to the reference hard-rock station, employing a specially determined set of spectral amplifications of stations. We approximated the recovered source spectrum by a piecewise power-law function, estimated fc1, fc2, and fc3, and examined their dependence on the seismic moment M0 (i.e., scaling). The dependence fc1(M0) does not contradict the hypothesis of source similarity when one expects fc1 ∝ M0− 1/3. For fc2 and fc3, the scaling is close to fc2 ∝ M0− 0.23 and fc3 ∝ M0− 0.13, respectively, indicating a clear violation of the similarity, especially prominent for fc3. Systematic identification of the frequency fc3, its determination, and analysis of its scaling are the main results of the study, important for understanding the physics of earthquake source processes. The use of fc3 as a source parameter in strong ground motion simulations will eliminate biases in estimating attenuation parameters, in particular, the spectral decay parameter “kappa”.