17:15 〜 19:15
[SCG45-P38] Investigation of Fast-Earthquake Activation Associated with Tectonic Tremors in the Iwate-Oki Region of the Japan Trench Subduction Zone
キーワード：日本海溝、テクトニック微動、日本海溝海底地震津波観測網S-net、スロースリップイベント
Understanding the relationship between slow and fast earthquakes is a crucial topic in seismology. A fast-earthquake activation phenomenon associated with slow earthquakes, specifically shallow tectonic tremors, has been reported in the Japan Trench subduction zone (e.g., Obana et al., 2021; Nishikawa et al., 2023). These observations suggest a linked activity between earthquakes and tectonic tremors. Understanding and modeling this phenomenon is essential for elucidating the physics of seismicity and improving earthquake forecasts along the Japan Trench. Particularly, in the Iwate-Oki Region of the Japan Trench subduction zone, tectonic tremors and seismic activity are highly active, and the above-mentioned phenomena frequently have been reported (Nishikawa et al., 2023).
In this study, we investigated the relationship between tectonic tremors and earthquake clusters in more detail. We implemented the envelope correlation method (Obara, 2002) following Ide (2010) and detected tectonic tremors from August 15, 2016, to December 31, 2024. Based on Takagi et al. (2019), we corrected the sensor tilt orientations of the seafloor observation network for earthquakes and tsunamis along the Japan Trench (S-net) and used data from 16 stations. We used two horizontal components for the analysis. Next, we investigated the spatiotemporal relationship between tectonic tremors and earthquake clusters. We used the Japan Meteorological Agency catalog for the same period as the tectonic tremor detections and used earthquakes with a magnitude of two or larger. Earthquake clusters were identified using the space-time epidemic-type aftershock-sequence model (Zhuang et al., 2002; Nishikawa & Ide, 2017). We then examined the temporal relationship between tectonic tremor bursts and earthquake clusters and the correlation between the number of events comprising them.
As a result, we found that earthquake clusters tend to occur after tectonic tremors rather than before them; however, there is no correlation between the number of preceding tectonic tremors and the number of subsequent earthquakes. In typical foreshock-mainshock-aftershock activity, aftershocks tend to occur more frequently than foreshocks. However, in the relationship between tectonic tremors and earthquake clusters in the Iwate-Oki region of the Japan Trench subduction zone, tectonic tremors occurred more frequently before the earthquake clusters than after. This suggests that the mechanism linking tectonic tremors and earthquake clusters differs from the typical foreshock-mainshock-aftershock mechanism. In other words, it is unlikely that tectonic tremors occur as “aftershocks” of earthquake clusters (fault slip phenomena induced by earthquake clusters). However, earthquake clusters may occur as “aftershocks” of tectonic tremor bursts.
Another possibility is the involvement of a third phenomenon, other than earthquakes and tectonic tremors. We suggest that slow slip events (SSEs) can be a potential candidate. If SSEs and tremors occur in the slow-earthquake-generating regions and subsequently trigger earthquake clusters, the observations in this study can be explained. The lack of correlation between the number of preceding tectonic tremors and subsequent earthquakes supports the hypothesis that tectonic tremors do not directly induce fast earthquakes but rather by SSEs. Future modeling of seismic activity in the Japan Trench subduction zone will require formulations consistent with the observational facts presented in this study.
In this study, we investigated the relationship between tectonic tremors and earthquake clusters in more detail. We implemented the envelope correlation method (Obara, 2002) following Ide (2010) and detected tectonic tremors from August 15, 2016, to December 31, 2024. Based on Takagi et al. (2019), we corrected the sensor tilt orientations of the seafloor observation network for earthquakes and tsunamis along the Japan Trench (S-net) and used data from 16 stations. We used two horizontal components for the analysis. Next, we investigated the spatiotemporal relationship between tectonic tremors and earthquake clusters. We used the Japan Meteorological Agency catalog for the same period as the tectonic tremor detections and used earthquakes with a magnitude of two or larger. Earthquake clusters were identified using the space-time epidemic-type aftershock-sequence model (Zhuang et al., 2002; Nishikawa & Ide, 2017). We then examined the temporal relationship between tectonic tremor bursts and earthquake clusters and the correlation between the number of events comprising them.
As a result, we found that earthquake clusters tend to occur after tectonic tremors rather than before them; however, there is no correlation between the number of preceding tectonic tremors and the number of subsequent earthquakes. In typical foreshock-mainshock-aftershock activity, aftershocks tend to occur more frequently than foreshocks. However, in the relationship between tectonic tremors and earthquake clusters in the Iwate-Oki region of the Japan Trench subduction zone, tectonic tremors occurred more frequently before the earthquake clusters than after. This suggests that the mechanism linking tectonic tremors and earthquake clusters differs from the typical foreshock-mainshock-aftershock mechanism. In other words, it is unlikely that tectonic tremors occur as “aftershocks” of earthquake clusters (fault slip phenomena induced by earthquake clusters). However, earthquake clusters may occur as “aftershocks” of tectonic tremor bursts.
Another possibility is the involvement of a third phenomenon, other than earthquakes and tectonic tremors. We suggest that slow slip events (SSEs) can be a potential candidate. If SSEs and tremors occur in the slow-earthquake-generating regions and subsequently trigger earthquake clusters, the observations in this study can be explained. The lack of correlation between the number of preceding tectonic tremors and subsequent earthquakes supports the hypothesis that tectonic tremors do not directly induce fast earthquakes but rather by SSEs. Future modeling of seismic activity in the Japan Trench subduction zone will require formulations consistent with the observational facts presented in this study.