[S-SS06] 地震活動とその物理

[SSS06-11] 南カリフォルニアにおける動的誘発地震の高分解能調査

*宮澤 理稔¹、Brodsky Emily²、Guo Huiyun² (1.京都大学防災研究所、2.カリフォルニア大学サンタクルズ校)

キーワード：動的誘発、遅れ破壊、等価マグニチュード、誘発度

Earthquake triggering by seismic waves has been recognized as a phenomenon for nearly thirty years. However, our ability to study dynamic triggering has been limited by our ability to capture the triggering stresses accurately and record the resultant earthquakes. Here we use full waveforms from a dense seismic network and a modern, high-resolution seismic catalog to measure triggering in Southern California from 2008 to 2017 based on interevent time ratios. We find that the fractional seismicity rate change, which we term triggering intensity or triggerability, as a function of peak strain change for the period of ~20 s due to distant earthquakes is monotonically increasing and compatible with earlier measurements made with a disjoint dataset from 1984 to 2008. A triggering strain of 1 microstrain is equivalent to the local productivity generated by an M1.8 earthquake. This result implies that a prediction of seismicity rate changes can be made based on recorded ground shaking using the same formalism as currently used for aftershock prediction. For a teleseismic event, this small level of triggering occurs throughout the region and thus aggregates to a regional effect. We find that the triggering rate decays after the triggerer follows an Omori-Utsu law, whereas at a much slower rate than a typical aftershock sequence. The slow decay rate suggests that an ancillary process such as creep or fluid flow must be part of dynamic triggering. The prevalence of triggering in areas of creep or fluid involvement reinforces this inference. A triggering cascade of secondary earthquakes is insufficient to explain the data.