10:00 AM - 10:15 AM
[SSS10-09] Understanding the stress state based on temporal changes in the activity of small to mid-sized earthquakes with DEFS
Keywords:stress, elastic strain energy, the 2016 Kumamoto earthquake, temporal changes in seismicity
Recently, we investigated the deviatoric stress magnitude of the background stress fields before the 2016 Kumamoto earthquake sequence, modelling the stress fields before and after the mainshock and examining temporal changes in elastic strain energies and stress orientations caused by the mainshock (Terakawa et al., 2025). The results of the previous study showed that the stress orientations in the source region for the post-seismic period depend on the deviatoric stress magnitude. This suggests that the focal mechanisms of events that followed the mainshock and temporal changes in seismic activity of small to mid-sized events depend on the physical quantity. In other words, such information obtained from observed seismic data can provide clues for understanding the stress state in the Earth’s crust.
In this study we tried to understand the stress state in the crust, based on temporal changes in seismic activity of events between the pre- and post-seismic period of the Kumamoto earthquake. Firstly, we computed temporal changes in Energetics-based Failure Stress (DEFS) caused by the mainshock, under the stress models with different effective friction coefficients (m’), which were obtained in the previous study (Terakawa et al., 2020, 2025). Unlike DCFS, the spatial distribution of DEFS depends on the assumed effective friction coefficient, or the deviatoric stress magnitude of stress fields. The values of DEFS are controlled by the deviatoric coseismic stress magnitude to that of the background stress field. Therefore, the dependence became remarkable especially within the source region. We expect the increase in seismic activity in the source region in the stress model with small effective friction coefficients (m’< 0.1). Conversely, we expect the decrease in seismic activity in the stress model with larger effective friction coefficients. On the other hand, the model dependence became less remarkable in the regions farther from the source region.
Next, we evaluated temporal changes in seismicity between the pre- and post-seismic periods, using data listed in the JMA catalog (1997 to 2024). We should pay attention to the fact that earthquakes are not only controlled by stress but also by pore-fluid pressure. In this study we aimed to evaluate the effects of stress on temporal changes in seismicity. On this purpose, events triggered by the increase in pore-fluid pressures may cause biased results unless we correctly evaluate changes in pore-fluid pressure for the post-seismic period to incorporate the information into DEFS. Therefore, we tried to remove such events, using the technique of cluster analysis (Zaliapin and Ben-Zion, 2013). We classified events into single events and cluster events, and further classified cluster events into mainshocks, foreshocks and aftershocks where the event with the largest magnitude in the family is mainshock, events that occurred before the mainshock are foreshock and those that followed the mainshock are aftershocks. Since foreshocks and aftershocks are often reported to be triggered by fluids, we targeted only single events and mainshocks for the analysis. Comparing the theoretical and observed temporal changes in seismicity, we will discuss the stress state.
In this study we tried to understand the stress state in the crust, based on temporal changes in seismic activity of events between the pre- and post-seismic period of the Kumamoto earthquake. Firstly, we computed temporal changes in Energetics-based Failure Stress (DEFS) caused by the mainshock, under the stress models with different effective friction coefficients (m’), which were obtained in the previous study (Terakawa et al., 2020, 2025). Unlike DCFS, the spatial distribution of DEFS depends on the assumed effective friction coefficient, or the deviatoric stress magnitude of stress fields. The values of DEFS are controlled by the deviatoric coseismic stress magnitude to that of the background stress field. Therefore, the dependence became remarkable especially within the source region. We expect the increase in seismic activity in the source region in the stress model with small effective friction coefficients (m’< 0.1). Conversely, we expect the decrease in seismic activity in the stress model with larger effective friction coefficients. On the other hand, the model dependence became less remarkable in the regions farther from the source region.
Next, we evaluated temporal changes in seismicity between the pre- and post-seismic periods, using data listed in the JMA catalog (1997 to 2024). We should pay attention to the fact that earthquakes are not only controlled by stress but also by pore-fluid pressure. In this study we aimed to evaluate the effects of stress on temporal changes in seismicity. On this purpose, events triggered by the increase in pore-fluid pressures may cause biased results unless we correctly evaluate changes in pore-fluid pressure for the post-seismic period to incorporate the information into DEFS. Therefore, we tried to remove such events, using the technique of cluster analysis (Zaliapin and Ben-Zion, 2013). We classified events into single events and cluster events, and further classified cluster events into mainshocks, foreshocks and aftershocks where the event with the largest magnitude in the family is mainshock, events that occurred before the mainshock are foreshock and those that followed the mainshock are aftershocks. Since foreshocks and aftershocks are often reported to be triggered by fluids, we targeted only single events and mainshocks for the analysis. Comparing the theoretical and observed temporal changes in seismicity, we will discuss the stress state.