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[SSS07-P17] Stress Drops of Small Earthquakes in Southeastern Kanto Region
Keywords:Stress Drop, Southeastern Kanto Region, Cluster
1. Introduction
The Pacific and Philippine Sea Plates subduct beneath the southeastern Kanto region from east and south, respectively. This causes complex seismicity and several clusters can be observed in the region, including beneath Chiba, Asahi and Koga cities (Figure 1a).
In this study, we analyzed stress drops of 311 small earthquakes with 4.0 <= Mw <= 5.0 that occurred from January 1, 2003 to December 31, 2020 in the southeastern Kanto region and investigated the difference of stress drops in individual clusters as well as the relationship with the focal depth.
2. Methods
We used velocity waveforms observed at Hi-net stations of the National Research Institute for Earth Science and Disaster Prevention (NIED) and followed the method of Yamada et al. (2021). First we selected an earthquake with Mw 3.5 closest to each earthquake of interest and made pairs of analyzed and smaller earthquakes. We used waveforms of smaller earthquakes as Empirical Green’s Functions (EGFs) and calculated their spectra. Next, we divided the spectra of waveforms of analyzed earthquakes with those of the EGF ones, which provided the ratios of source characteristics of individual earthquake pairs because all the effects of path, site and characteristic of seismometer can be removed. We then estimated the corner frequencies by assuming that source spectra follow the omega-squared model (Boatwright, 1978). Finally, we calculated values of stress drop from a model of circular fault (Madariaga, 1976).
3. Results and Discussions
Figure 1(b) shows the spatial distribution of stress drop analyzed in this study.
First, we divided earthquakes into six groups as shown in Fig.1(b) and investigated average values of stress drop for individual groups. We found that the cluster in red rectangle in Fig. 1(b) had a larger average of stress drop than that in dark blue. The difference was statistically significant. The former includes the earthquakes that took place on Kanto Fragment (Toda et al., 2008) and the Philippine Sea Plate and the latter consists of the ones on the subduction surface of the Pacific Plate. As stress drop can be used as a proxy of the shear strength (Yamada et al., 2021), our results suggest that the shear strength on the Philippine Sea Plate would be higher than that on the Pacific Plate in the study area. No notable depth dependence was observed for all the groups.
We also investigated the spatial pattern on stress drop within each cluster. The cluster represented in red rectangle in Fig.1(b) had larger values of stress drop around north and south tips. Similarly, earthquakes close to east and west tips in orange rectangle had larger stress drops. The spatial pattern of stress drop would reflect a finer scale heterogeneity such that the central part of a cluster has a lower shear strength. However, it should also be verified that the shapes of spectra were really consistent with the omega-squared model, especially for earthquakes with extremely high and low stress drops. In addition, we will examine that the analyzed waveforms had good signal-to-noise ratio and investigate the reliability of results in near future.
References
・Boatwright (1978), https://doi.org/10.1785/BSSA0680041117
・Madariaga (1976), https://doi.org/10.1785/BSSA0660030639
・Toda et al. (2008), https://doi.org/10.1038/ngeo318
・Yamada et al. (2021), https://doi.org/10.1186/s40623-020-01326-8
The Pacific and Philippine Sea Plates subduct beneath the southeastern Kanto region from east and south, respectively. This causes complex seismicity and several clusters can be observed in the region, including beneath Chiba, Asahi and Koga cities (Figure 1a).
In this study, we analyzed stress drops of 311 small earthquakes with 4.0 <= Mw <= 5.0 that occurred from January 1, 2003 to December 31, 2020 in the southeastern Kanto region and investigated the difference of stress drops in individual clusters as well as the relationship with the focal depth.
2. Methods
We used velocity waveforms observed at Hi-net stations of the National Research Institute for Earth Science and Disaster Prevention (NIED) and followed the method of Yamada et al. (2021). First we selected an earthquake with Mw 3.5 closest to each earthquake of interest and made pairs of analyzed and smaller earthquakes. We used waveforms of smaller earthquakes as Empirical Green’s Functions (EGFs) and calculated their spectra. Next, we divided the spectra of waveforms of analyzed earthquakes with those of the EGF ones, which provided the ratios of source characteristics of individual earthquake pairs because all the effects of path, site and characteristic of seismometer can be removed. We then estimated the corner frequencies by assuming that source spectra follow the omega-squared model (Boatwright, 1978). Finally, we calculated values of stress drop from a model of circular fault (Madariaga, 1976).
3. Results and Discussions
Figure 1(b) shows the spatial distribution of stress drop analyzed in this study.
First, we divided earthquakes into six groups as shown in Fig.1(b) and investigated average values of stress drop for individual groups. We found that the cluster in red rectangle in Fig. 1(b) had a larger average of stress drop than that in dark blue. The difference was statistically significant. The former includes the earthquakes that took place on Kanto Fragment (Toda et al., 2008) and the Philippine Sea Plate and the latter consists of the ones on the subduction surface of the Pacific Plate. As stress drop can be used as a proxy of the shear strength (Yamada et al., 2021), our results suggest that the shear strength on the Philippine Sea Plate would be higher than that on the Pacific Plate in the study area. No notable depth dependence was observed for all the groups.
We also investigated the spatial pattern on stress drop within each cluster. The cluster represented in red rectangle in Fig.1(b) had larger values of stress drop around north and south tips. Similarly, earthquakes close to east and west tips in orange rectangle had larger stress drops. The spatial pattern of stress drop would reflect a finer scale heterogeneity such that the central part of a cluster has a lower shear strength. However, it should also be verified that the shapes of spectra were really consistent with the omega-squared model, especially for earthquakes with extremely high and low stress drops. In addition, we will examine that the analyzed waveforms had good signal-to-noise ratio and investigate the reliability of results in near future.
References
・Boatwright (1978), https://doi.org/10.1785/BSSA0680041117
・Madariaga (1976), https://doi.org/10.1785/BSSA0660030639
・Toda et al. (2008), https://doi.org/10.1038/ngeo318
・Yamada et al. (2021), https://doi.org/10.1186/s40623-020-01326-8