Time-domain source parameter estimation of M_w 3–7 earthquakes in Japan from a large database of moment-rate functions

Abstract

Time-domain analyses of seismic waveforms have revealed diverse source complexity in large earthquakes (M_w > 7). However, source characteristics of small earthquakes have been studied by assuming a simple rupture pattern on the frequency domain. This study utilizes high-quality seismic network data from Japan to systematically address the source complexities and radiated energies of M_w 3–7 earthquakes on the time domain. We first determine the apparent moment-rate functions (AMRFs) of the earthquakes using the empirical Green's functions. Some of the AMRFs show multiple peaks, suggesting complex ruptures at multiple patches. We then estimate the radiated energies (E_R) of 1736 events having more than ten reliable AMRFs. The scaled energy (e_R = E_R/M₀) does not strongly depend on the seismic moment (M₀), focal mechanisms, or depth. The median value of e_R is 3.7 × 10⁻⁵, which is comparable to those of previous studies; however, e_R varies by approximately one order of magnitude among earthquakes. We measure the source complexity based on the radiated energy enhancement factor (REEF). The values of REEF differ among earthquakes, implying diverse source complexity. The values of REEF do not show strong scale dependence for M_w 3–7 earthquakes, suggesting that the source diversity of smaller earthquakes is similar to that of larger earthquakes at their representative spatial scales. Applying a simple spectral model (e.g. the ω²-source model) to complex ruptures may produce substantial estimation errors in source parameters.

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