Seafloor Borehole Observation Network in the Nankai to Observe Slow Slip Events and Slow Earthquakes

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Shallow part of the Nankai subducting plate interface is under the sea. Shore based observation is not sensitive to slow slip events there. Observation in IODP seafloor borehole has revealed existence of shallow slow slip events which repeats at relatively short time intervals (Araki et al., 2017). GPS/A observation by Japan Coast Guard has also detected shallow slow slip event in 2017-2018 in other part of the Nankai Trough, which may be larger in magnitude and longer in slip duration than those observed by the borehole observation. Currently, distribution of areas of such slow slip events, duration and recurrence interval of larger slow slip are not clear. Interval of few times a year with GPS/A observation is infrequent to capture slow slip events. Borehole observation is much more sensitive than GPA/A, but currently distributed in dense linear array, and is not suitable to watch wide area. As we have already infrastructure, seafloor cabled observation network (DONET) that can distribute and connect new observatories in wide area, we are working to add borehole observatories which has enough sensitivity to observe slow slip events as expected from past observations.

In addition to three working IODP deep borehole observatories (C0002, C0010, C0006), we plan to install a shallow hole observatory with a tiltmeter in 2019 to form SSE observation areal network in the east of the borehole array. The new borehole is shallower than 20m but expected to provide quiet enough observation environment to capture known magnitude SSEs. Another shallow hole observatory with similar configuration is planned near the area of the SSE observation by GPS/A in 2019-2020. Further deep borehole observatory installation is also considered, in which cost is reduced by simpler observation system with fiber-optic broadband strainmeter array and pore-fluid pressure sensor.

Presenting Author: Eiichiro Araki

Authors