Exploring anisotropic seismic property of the seismogenic plate boundary in the Nankai Trough using a seafloor borehole observatory

Abstract:

Stress state in the vicinity of a seismogenic fault would be a key parameter governing its fault dynamics. Stress analysis in a borehole such as breakout may give stress information, but drilling seismogenic fault at depth is still challenging and it is even more difficult to perform repeated stress measurements for temporal evolution of stress state. Here we consider applying seismic anisotropy as an index of stress state and by observing its temporal change to identify change of stress around the seismogenic fault. In this study, we explored techniques to assess seismic anisotropy in the Nankai Trough accretionary prism, using a borehole seismometer deployed in IODP borehole C0002G, which is located just above the Tonankai earthquake fault. The borehole seismometer is situated at about 900 m below 1966 m deep seafloor, and is operational since January 2013 when the observatory was connected to DONET seafloor cable network.

We developed a technique to analyze seismic anisotropy on converted S-wave from microseismic noise records and applied the technique on the borehole seismometer records, by which we expect to evaluate temporal change of anisotropy continuously. We obtained anisotropy of a few percent. We further evaluated depth dependency of anisotropy direction and obtained the difference between the uppermost sedimentary basin and accretionary prism near the plate boundary.

We also performed airgun array shooting around the borehole in November 2013 to check validity of the anisotropy result. We applied two different analysis on the airgun records, the one was P-wave seismic anisotropy from the travel time, and the other was S-wave anisotropy using converted S-wave from airgun P-wave. Preliminary results from these analysis were consistent with the microseismic noise analysis. Repeated airgun shooting is planned at the interval of a year or so to evaluate our ability to detect its temporal change.