Direct Evidence for Diverse Source Complexity in Small Earthquakes (Mw 3.3–5.0) Obtained from Near-Source Borehole Seismic Data

Keisuke Yoshida
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Abstract

Small earthquakes (Mw <5) may have a similar degree of complexity as large earthquakes. However, their seismic waveforms are strongly distorted during wave propagation, making their complexity challenging to resolve. In many cases, the source parameters of small events are determined based on models that assume their source patterns are simple. In this study, to directly examine the source complexities in small events, we examined high-quality near-source (<8 km) seismic waveforms recorded by two excellent downhole sensors in Japan. The results show that the P waveforms of microearthquakes (Mw <2) are always simple at the sensors and agree well with the synthetic waveforms based on a 1D structure up to 20 Hz. The microearthquake waveforms in this frequency band essentially represent path effects besides the static source effect, suggesting that the contribution of structural inhomogeneity to the observed waveforms is small. Taking advantage of this, we inferred the moment rate functions of 164 Mw 3.3–5.0 events from the shapes of the direct P waves. They showed diversity in their complexity, and even conservatively estimated, 25% of the events had multiple subevents. The results suggest that methods that account for complexity, rather than those that assume a simple source pattern, are required to characterize even small events.
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从近源钻孔地震数据中获得的小型地震(Mw 3.3-5.0)震源复杂性多样性的直接证据
小地震(Mw <5)的复杂程度可能与大地震类似。然而,它们的地震波形在波形传播过程中会发生强烈扭曲,使其复杂性难以解决。在许多情况下,小型地震的震源参数是根据假定其震源模式简单的模型确定的。在本研究中,为了直接研究小型事件的震源复杂性,我们研究了日本两个优秀的井下传感器记录的高质量近震源(<8 千米)地震波形。结果表明,微地震(Mw 小于 2)的 P 波形在传感器处始终很简单,并且与基于一维结构的合成波形非常吻合,最高可达 20 Hz。该频段的微震波形基本上代表了静源效应之外的路径效应,表明结构不均匀性对观测波形的影响很小。利用这一点,我们从直接 P 波的形状推断出了 164 个 Mw 3.3-5.0 事件的矩率函数。这些事件的复杂性表现出多样性,即使保守估计,也有 25% 的事件具有多个子事件。这些结果表明,即使是小规模的事件,也需要采用考虑复杂性的方法,而不是假设简单震源模式的方法。
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