11:00 〜 13:00
[SCG44-P25] Changes in long-term activity patterns of interplate slip from short-term slow slip events in the northern Kii Peninsula, Japan
キーワード：すべり分布、傾斜変化、Episodic Tremor and Slip、モーメント解放履歴、平均すべり速度、フィリピン海プレート
In the southwest Japan subduction zone, short-term slow slip events (S-SSEs) accompanying nonvolcanic tremor have repeatedly been observed. Because the S-SSEs occur on the deeper extension of the anticipated megathrust earthquakes source area, it is important to monitor the S-SSE activity for understanding the process of strain accumulation and release around the plate interface and the stress transfer to the megathrust area.
Long-term moment release history due to the S-SSEs has been discussed (e.g., Sekine et al., 2010; Nishimura et al., 2013), but it is difficult to know the difference in moment release history in local scale because many of these studies assume a rectangular source fault with uniform slip for an S-SSE. Hirose and Kimura (2020) estimated a number of slip distributions for S-SSEs in Shikoku based on National Research Institute for Earth Science and Disaster Resilience (NIED) Hi-net high-sensitivity accelerometer (tiltmeter) data for 18 years (2001-2019). They found the local scale segmentation in the S-SSE slip area in Shikoku and the activity is affected by nearby long-term SSEs. However, these difference of the S-SSE activity in local scale is unknown in the other areas than the Shikoku area.
Here we applied their inversion method (Hirose and Kimura, 2020) to the tiltmeter data in the northern Kii Peninsula area to estimate a slip distribution of the S-SSEs for 18 years (2002-2020) in order to discuss the difference in the S-SSE recurrence behavior in local scale. We also discuss the moment release history and a moving-averaged slip rate history at points on the plate interface.
We detected and analyzed 79 episode of the S-SSEs in the northern Kii Peninsula for the 18 years. The moment magnitude of the S-SSEs ranges from 5.3 to 6.3. We identified two segments of the activity, one is around Ise Bay and the other is around Shima Peninsula. This means that a slip distribution of a particular S-SSE is localized on a single segment for most of the S-SSEs. One notable exception is an increase in the number of S-SSEs that have a wider slip area extending to the multiple segments after 2017. The change in recurrence interval is also observed in the Shima Peninsula area, i.e., one or two months in 2010 or in 2012, whereas approximately four months in 2006 or in 2018. In the Ise Bay area, the recurrence interval is approximately four months, this is nearly constant throughout the study period.
The cumulative seismic moment versus time plot for all of the segments shows a large increase in the moment rate (a slope of the plot) around 2016-2018. In the Shima Peninsula, changes in the moment rate corresponding to the change in recurrence interval are observed.
We also calculated a moving-averaged slip rate history on several points on the plate interface. We found a six-year periodicity in the averaged slip rate on a point in the Shima Peninsula area. On the other hand, in the Ise Bay area, we found a long-term variation in the averaged slip rate whose peaks are different in timing from those in the Shima Peninsula area. We can identify the spatial extent of the variation of about 30-40 km in strike direction and 20-30 km in dip direction in each of the segments. One exception is around 2016-2018: an increase in the averaged slip rate is observed in both of the two segments.
A part of the fluctuation may be due to a nearby long-term SSE around the Shima Peninsula (e.g., Kobayashi and Tsuyuki, 2019), but the causes of the identified fluctuation in the averaged slip rate are unknown. It is important to monitor the slip behavior of the S-SSEs not only for finding the mechanism generating the fluctuation, but also for understanding the interseismic stress buildup process around the megathrust source areas.
Long-term moment release history due to the S-SSEs has been discussed (e.g., Sekine et al., 2010; Nishimura et al., 2013), but it is difficult to know the difference in moment release history in local scale because many of these studies assume a rectangular source fault with uniform slip for an S-SSE. Hirose and Kimura (2020) estimated a number of slip distributions for S-SSEs in Shikoku based on National Research Institute for Earth Science and Disaster Resilience (NIED) Hi-net high-sensitivity accelerometer (tiltmeter) data for 18 years (2001-2019). They found the local scale segmentation in the S-SSE slip area in Shikoku and the activity is affected by nearby long-term SSEs. However, these difference of the S-SSE activity in local scale is unknown in the other areas than the Shikoku area.
Here we applied their inversion method (Hirose and Kimura, 2020) to the tiltmeter data in the northern Kii Peninsula area to estimate a slip distribution of the S-SSEs for 18 years (2002-2020) in order to discuss the difference in the S-SSE recurrence behavior in local scale. We also discuss the moment release history and a moving-averaged slip rate history at points on the plate interface.
We detected and analyzed 79 episode of the S-SSEs in the northern Kii Peninsula for the 18 years. The moment magnitude of the S-SSEs ranges from 5.3 to 6.3. We identified two segments of the activity, one is around Ise Bay and the other is around Shima Peninsula. This means that a slip distribution of a particular S-SSE is localized on a single segment for most of the S-SSEs. One notable exception is an increase in the number of S-SSEs that have a wider slip area extending to the multiple segments after 2017. The change in recurrence interval is also observed in the Shima Peninsula area, i.e., one or two months in 2010 or in 2012, whereas approximately four months in 2006 or in 2018. In the Ise Bay area, the recurrence interval is approximately four months, this is nearly constant throughout the study period.
The cumulative seismic moment versus time plot for all of the segments shows a large increase in the moment rate (a slope of the plot) around 2016-2018. In the Shima Peninsula, changes in the moment rate corresponding to the change in recurrence interval are observed.
We also calculated a moving-averaged slip rate history on several points on the plate interface. We found a six-year periodicity in the averaged slip rate on a point in the Shima Peninsula area. On the other hand, in the Ise Bay area, we found a long-term variation in the averaged slip rate whose peaks are different in timing from those in the Shima Peninsula area. We can identify the spatial extent of the variation of about 30-40 km in strike direction and 20-30 km in dip direction in each of the segments. One exception is around 2016-2018: an increase in the averaged slip rate is observed in both of the two segments.
A part of the fluctuation may be due to a nearby long-term SSE around the Shima Peninsula (e.g., Kobayashi and Tsuyuki, 2019), but the causes of the identified fluctuation in the averaged slip rate are unknown. It is important to monitor the slip behavior of the S-SSEs not only for finding the mechanism generating the fluctuation, but also for understanding the interseismic stress buildup process around the megathrust source areas.