10:45 AM - 12:15 PM
[SSS12-P02] Slip behavior of the shallow plate interface off-Iwate based on acoustic ranging survey
Keywords:acoustic distance measurement, slow slip event, off-Iwate, shallow plate interface
Detection of slow slip events (SSEs) along the Japan trench based on land-based geodetic survey is relatively difficult due to the farther distance from the shallow plate boundary to the land area than that in the Nankai trough. However, the deployment of a seafloor cable system, S-net, has illuminated clusters of tremors and very-low-frequency events (VLFEs) surrounding the off-Miyagi main rupture area of the 2011 Tohoku earthquake. These activities are suggested to be temporally correlated with relatively large earthquakes in that region. In addition, by using the slip amount of small repeating earthquakes, the quasi-periodical occurrence of episodic SSE is pointed out. However, the temporal evolution of the offshore SSEs have not been observed yet by GNSS-acoustic survey, except for a certain site in the Nankai trough, where extremely frequent surveys had been conducted. To overcome the shortcoming of GNSS-A, seafloor acoustic distance measurement (ADM), that can provide precise and nearly continuous data, is available and is much more feasible to detect such time-varying phenomena. Then, we applied an ADM survey at the shallowest plate interface off-Iwate, where periodical SSEs are reported.
The basic concept of ADM is to monitor slight changes in baseline length between a pair of transponders installed on seafloor from the round-trip time of acoustic signal. The survey started with four transponders across the trench axis in July 2019 in the KS19-12 cruise. Then an additional sub-surface moored type transponder was installed in November 2019 just on the trench axis by Ryofu-maru (JMA) until June 2020. One transponder out of the four was found to be failed and recovered in October 2020. Finally, the remaining three transponders were recovered in April 2022 by Ryofu-maru. In this study, we analyzed data in two across-trench baselines and one on the incoming Pacific plate consisting of three well-operated transponders for nearly two years since July 2019. Sound speed along each baseline is carefully corrected for concurrently measured seafloor temperature and pressure, as well as the change in the inclination of individual transponders. After these corrections, linear regression was applied to the time series of each baseline length. The resulting average change rates in the baseline length are 0.5 cm/yr for all the baseline, which is much smaller than the typical convergence rate of ~8 cm/yr expected from plate motion model. On the other hand, long-term undulations over a year reaching to ~2 cm (peak to peak) in the baseline length have remained. However, these long-term undulations were nearly coherent among baselines, even including the one on the incoming plate, which is expected to be invariant. Therefore, the long-term undulations were considered as an error due to insufficient sound speed correction rather than tectonic displacement. Considering the uncertainty of ~2 cm for two years, the significance of reliability of the rate of the change in baseline can be 1 cm/yr in this survey. As the results, we concluded that no obvious contraction or extension over 1 cm/yr was observed across the trench during the survey period.
An SSE of ~27 cm slip in total was reported in 2015 on the shallow plate interface off-Iwate based on the activity of repeating earthquakes. A less significant repeater activity that probably show minor SSE was also reported during our survey period. Based on our ADM measurement, the magnitude of the possible SSE was considered too small or the up-dip limit of the slip area of the SSE did not reach the trench axis. A typical recurrence time of SSE in this region is considered roughly three years. Therefore, if an SSE of magnitude comparable to that in 2015 would happen during the future ADM survey, it can be a chance to measure the slip amount of the SSE directly and to reveal the up-dip limit of such an event.
The research was supported by Kakenhi (19H05596).
The basic concept of ADM is to monitor slight changes in baseline length between a pair of transponders installed on seafloor from the round-trip time of acoustic signal. The survey started with four transponders across the trench axis in July 2019 in the KS19-12 cruise. Then an additional sub-surface moored type transponder was installed in November 2019 just on the trench axis by Ryofu-maru (JMA) until June 2020. One transponder out of the four was found to be failed and recovered in October 2020. Finally, the remaining three transponders were recovered in April 2022 by Ryofu-maru. In this study, we analyzed data in two across-trench baselines and one on the incoming Pacific plate consisting of three well-operated transponders for nearly two years since July 2019. Sound speed along each baseline is carefully corrected for concurrently measured seafloor temperature and pressure, as well as the change in the inclination of individual transponders. After these corrections, linear regression was applied to the time series of each baseline length. The resulting average change rates in the baseline length are 0.5 cm/yr for all the baseline, which is much smaller than the typical convergence rate of ~8 cm/yr expected from plate motion model. On the other hand, long-term undulations over a year reaching to ~2 cm (peak to peak) in the baseline length have remained. However, these long-term undulations were nearly coherent among baselines, even including the one on the incoming plate, which is expected to be invariant. Therefore, the long-term undulations were considered as an error due to insufficient sound speed correction rather than tectonic displacement. Considering the uncertainty of ~2 cm for two years, the significance of reliability of the rate of the change in baseline can be 1 cm/yr in this survey. As the results, we concluded that no obvious contraction or extension over 1 cm/yr was observed across the trench during the survey period.
An SSE of ~27 cm slip in total was reported in 2015 on the shallow plate interface off-Iwate based on the activity of repeating earthquakes. A less significant repeater activity that probably show minor SSE was also reported during our survey period. Based on our ADM measurement, the magnitude of the possible SSE was considered too small or the up-dip limit of the slip area of the SSE did not reach the trench axis. A typical recurrence time of SSE in this region is considered roughly three years. Therefore, if an SSE of magnitude comparable to that in 2015 would happen during the future ADM survey, it can be a chance to measure the slip amount of the SSE directly and to reveal the up-dip limit of such an event.
The research was supported by Kakenhi (19H05596).