14:30 〜 14:45
[SSS10-13] Spatial variability of the response of the Nagano seismicity to the 2011 Tohoku-oki earthquake, in Japan
キーワード：地震、ETAS、東北地方太平洋沖地震
We investigated seismicity in and around Nagano Prefecture, in central Japan, over a 23-year period (January 2000-January 2023), focusing on activated seismicity following the 2011 Tohoku-oki earthquake. Nagano Prefecture is located several hundred kilometers away.
We considered ten regions covering areas where seismicity was relatively active and examined, for each region, the timeseries of earthquake occurrences for the 23-year period. Although eight regions showed seismic activation when the Tohoku-oki earthquake occurred, two did not. The seismically active regions showed two common features, namely that large earthquakes frequently occurred immediately after the Tohoku-oki earthquake, and that the rate of occurrence of earthquakes increased at the moment of the Tohoku earthquake. For the two regions where seismicity was not activated, no such features were observed.
To capture essential aspects of how seismicity in and around Nagao Prefecture revealed activation after the Tohoku-oki earthquake, we introduced the Epidemic Type Aftershock Sequence (ETAS) model (Ogata, 1988). The ETAS model was fitted to the target interval from 2000 until immediately before the Tohoku-oki earthquake and then extrapolated until the most recent time (January 2023). The observed occurrence rates of the post-Tohoku-oki sequence were compared with the rates of extrapolation from fitting with the ETAS model. Based on this comparison, three out of the eight regions showed that, despite an increase in the rate of occurrence of earthquakes, these did not return to pre-Tohoku-oki levels: Two of the three regions were located across the border of Nagao, Gifu, and Fukui Prefectures, where earthquake swarms occurred in 1998 and 2020, while the rest was located near Suwa Lake where no strong (magnitude 5 or larger) earthquakes occurred during the entire period (2000-2023). For the remaining five regions, activated seismicity eventually subsided.
We are now tackling the issue of how observed spatial variability of seismicity pattern is associated with properties such as seismogenic volume. A recent study revealed that source mechanisms of some earthquakes in both the 1998 and 2020 swarms at the border of Nagao, Gifu, and Fukui Prefectures have compensate linear vector dipole (CLVD) components (Kumazawa and Ogata, 2021), suggesting the possibility of a rise in fluid. An analysis of spatiotemporal variation of CLVD values available by F-net (Full Range Seismograph Network of Japan)-derived focal mechanism solutions estimated by NIED (National Research Institute for Earth Science and Disaster Resilience) is underway. Additionally, we are seeking consistency between results from seismicity analysis and crustal deformation detected by data from GEONET (GNSS Earth Observation Network System).
Acknowledgements
We used the earthquake catalog maintained by the Japan Meteorological Agency, the Preliminary Catalog by the Hi-net Automatic System (NIED), and the F-net earthquake mechanism information (NIED). This study was partially supported by the MEXT of Japan, under the STAR-E (Seismology TowArd Research innovation with data of Earthquake) Program (Grant Number JPJ010217) (K.Z.N.), JSPS KAKENHI (Grant Number JP 20K05050) (K.Z.N., J.I., M.K.) and Chubu Electric Power's research based on selected proposals (K.Z.N., M.K.), and the Collaboration Research Program of IDEAS, Chubu University IDEAS202211 (J.I., K.Z.N., M.K.).
We considered ten regions covering areas where seismicity was relatively active and examined, for each region, the timeseries of earthquake occurrences for the 23-year period. Although eight regions showed seismic activation when the Tohoku-oki earthquake occurred, two did not. The seismically active regions showed two common features, namely that large earthquakes frequently occurred immediately after the Tohoku-oki earthquake, and that the rate of occurrence of earthquakes increased at the moment of the Tohoku earthquake. For the two regions where seismicity was not activated, no such features were observed.
To capture essential aspects of how seismicity in and around Nagao Prefecture revealed activation after the Tohoku-oki earthquake, we introduced the Epidemic Type Aftershock Sequence (ETAS) model (Ogata, 1988). The ETAS model was fitted to the target interval from 2000 until immediately before the Tohoku-oki earthquake and then extrapolated until the most recent time (January 2023). The observed occurrence rates of the post-Tohoku-oki sequence were compared with the rates of extrapolation from fitting with the ETAS model. Based on this comparison, three out of the eight regions showed that, despite an increase in the rate of occurrence of earthquakes, these did not return to pre-Tohoku-oki levels: Two of the three regions were located across the border of Nagao, Gifu, and Fukui Prefectures, where earthquake swarms occurred in 1998 and 2020, while the rest was located near Suwa Lake where no strong (magnitude 5 or larger) earthquakes occurred during the entire period (2000-2023). For the remaining five regions, activated seismicity eventually subsided.
We are now tackling the issue of how observed spatial variability of seismicity pattern is associated with properties such as seismogenic volume. A recent study revealed that source mechanisms of some earthquakes in both the 1998 and 2020 swarms at the border of Nagao, Gifu, and Fukui Prefectures have compensate linear vector dipole (CLVD) components (Kumazawa and Ogata, 2021), suggesting the possibility of a rise in fluid. An analysis of spatiotemporal variation of CLVD values available by F-net (Full Range Seismograph Network of Japan)-derived focal mechanism solutions estimated by NIED (National Research Institute for Earth Science and Disaster Resilience) is underway. Additionally, we are seeking consistency between results from seismicity analysis and crustal deformation detected by data from GEONET (GNSS Earth Observation Network System).
Acknowledgements
We used the earthquake catalog maintained by the Japan Meteorological Agency, the Preliminary Catalog by the Hi-net Automatic System (NIED), and the F-net earthquake mechanism information (NIED). This study was partially supported by the MEXT of Japan, under the STAR-E (Seismology TowArd Research innovation with data of Earthquake) Program (Grant Number JPJ010217) (K.Z.N.), JSPS KAKENHI (Grant Number JP 20K05050) (K.Z.N., J.I., M.K.) and Chubu Electric Power's research based on selected proposals (K.Z.N., M.K.), and the Collaboration Research Program of IDEAS, Chubu University IDEAS202211 (J.I., K.Z.N., M.K.).