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 in the frequency domain. This study utilized high-quality seismic network data from Japan to systematically address the source complexities and radiated energies of M_w 3–7 earthquakes in the time domain. We first determined the apparent moment-rate functions (AMRFs) of the earthquakes using the empirical Green's functions. Some of the AMRFs showed multiple peaks, suggesting complex ruptures at multiple patches. We then estimated the radiated energies (E_R) of 1736 events having more than ten reliable AMRFs. The scaled energy (e_R = E_R/M₀) did not strongly depend on the seismic moment (M₀), focal mechanisms, or depth. The median value of e_R was 3.7 × 10⁻⁵, which is comparable to those of previous studies; however, e_R varied by approximately one order of magnitude among earthquakes. Additionally, we measured the source complexity based on the radiated energy enhancement factor (REEF). The values of REEF differed among earthquakes, implying diverse source complexity. The values of REEF did 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 of source parameters.

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