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[SCG44-P24] Stress changes caused by Boso slow slip events inferred from seismicity data
Keywords:earthquake swarm, Dieterich's seismicity theory, spatiotemporal slip process, earthquake triggering, Philippine Sea plate
In the Kanto area in central Japan, so called the Boso slow slip events (SSEs) have repeatedly occurred every 2-6 years near the Boso Peninsula (e.g., Sagiya et al., 2004; Ozawa et al., 2019). The SSEs occurs on the subducting Philippine Sea plate at approximately 10-30 km depth range. Each slip episode lasts for several weeks accompanied by an earthquake swarm activity. Hirose et al. (2014) pointed out that a stress concentration caused by the slow slip could trigger the earthquake swarm suggested from the space and time evolution of the two Boso SSEs based on GNSS and tiltmeter data, but it is unknown whether the stress concentration actually occurs or not.
Here we estimate the time-dependent stress change during the Boso SSE and earthquake swarm episodes by applying a method proposed by Dieterich et al. (2000) to seismicity data. We used the Japan Meteorological Agency (JMA) Unified Earthquake Catalog to count the number of earthquakes in the range of 10 to 35 km in depth in a rectangular area of about 60 km x 40 km near the east coast of Boso Peninsula, which almost includes the hypocenters of the earthquake swarms during each SSE period from 1998 to 2019. The origin times of the selected earthquakes are then converted to a seismicity rate (the number of earthquakes per unit time) time function that is used to estimate the stress change time-series by applying the Dieterich et al.'s (2000) method.
The estimated stress change time-series shows a stress increase for all of the SSEs (in 2002, 2007, 2011, 2014 and 2018). Moreover, the comparison of the stress change with a time-dependent slow slip process for each SSE (Fukuda, 2018) indicates that the timings of rapid stress increases during an episode correspond to the stages of slip accelerations and slip area expansion to the westward for the 2002, 2007, 2011 and 2014 episodes. This is independent evidence of stress increase in the earthquake swarm area during an SSE episode, and supports the above hypothesis that the increased stress induced by slow slip triggers an earthquake swarm activity.
Acknowledgments:
We are grateful to Professor Shinji Toda for providing us a stress computation code based on a method proposed by Dieterich et al. (2000). The earthquake hypocenter catalog was provided by the Japan Meteorological Agency. This research was supported by JSPS KAKENHI Grant Numbers JP16H06474, JP21H05206, 21K03702.
Here we estimate the time-dependent stress change during the Boso SSE and earthquake swarm episodes by applying a method proposed by Dieterich et al. (2000) to seismicity data. We used the Japan Meteorological Agency (JMA) Unified Earthquake Catalog to count the number of earthquakes in the range of 10 to 35 km in depth in a rectangular area of about 60 km x 40 km near the east coast of Boso Peninsula, which almost includes the hypocenters of the earthquake swarms during each SSE period from 1998 to 2019. The origin times of the selected earthquakes are then converted to a seismicity rate (the number of earthquakes per unit time) time function that is used to estimate the stress change time-series by applying the Dieterich et al.'s (2000) method.
The estimated stress change time-series shows a stress increase for all of the SSEs (in 2002, 2007, 2011, 2014 and 2018). Moreover, the comparison of the stress change with a time-dependent slow slip process for each SSE (Fukuda, 2018) indicates that the timings of rapid stress increases during an episode correspond to the stages of slip accelerations and slip area expansion to the westward for the 2002, 2007, 2011 and 2014 episodes. This is independent evidence of stress increase in the earthquake swarm area during an SSE episode, and supports the above hypothesis that the increased stress induced by slow slip triggers an earthquake swarm activity.
Acknowledgments:
We are grateful to Professor Shinji Toda for providing us a stress computation code based on a method proposed by Dieterich et al. (2000). The earthquake hypocenter catalog was provided by the Japan Meteorological Agency. This research was supported by JSPS KAKENHI Grant Numbers JP16H06474, JP21H05206, 21K03702.