Change in anisotropic P-wave velocity observed by seafloor borehole observatory C0002G in the Nankai Trough subduction zone.

Abstract:

P-wave velocity anisotropy from repeat airgun survey data was evaluated around the IODP borehole observatory C0002G, situated above the seismogenic fault of large Tonankai Earthquakes in the Nankai Trough subduction. We intend to conduct repeated circular airgun shooting around the seafloor borehole observatory with an interval of a year, aiming at detection (or evaluation) of temporal change of the P-wave anisotropy due possibly to change in the stress or pore-fluid pressure in the crust around the seismogenic fault preparing for the next large earthquake. So far, three airgun shooting experiments by JAMSTEC R/V Kairei has been conducted in Nov. 2013, Mar. 2015, and Jun. 2015, each with circular shootings 3 km away from the C0002G borehole. From each circular shooting data around the C0002G borehole seismometer, we obtained anisotropic P-wave velocity of the sediment around the borehole. The fast axis direction and magnitude of the anisotropy was more or less consistent and are 60 degrees and 2-3%, but reduction of P-wave velocity of approximately 1 % was identified between the first experiment and the second experiment. During the term between two experiments, an event of significant pore-fluid pressure increase due to nearby drilling by D/V Chikyu IODP Exp. 348, was observed in the borehole. Observed change in P-wave velocity may be associated with possible weakening of the sediment caused by the high fluid pressure during the drilling. The airgun shot data in the third experiment may be used to clarify if the observed change is persistent, but the observed P-wave velocity in the third experiment was significantly influenced by complex ocean sound velocity structure due to strong ocean current (Kuroshio) stayed above the observatory during the third experiment.