[J07-2] Tracking the sea floor in motion II

[J07-2-02] Initial characteristics of LTBMS borehole sensors installed in the Nankai Trough, Japan

^○Toshinori Kimura, Eiichiro Araki, Yuya Machida (JAMSTEC, Yokosuka, Japan)

In the Nankai Trouhg area, the subduction of the plate has caused repeating great earthquakes along mega-thrusts faults between plate interfaces. We have planned to install long-term borehole monitoring system (LTBMS) in the Nankai Trough to monitor seismic activity for elucidating the generation process of the mega-thrust earthquake in the subduction zone. As first LTBMS site in this area, the C0002G observatory was activated on January 2013. In April and June 2016, the second LTBMS observatory, C0010A was successfully installed in the Nankai Trough, and was connected to DONET seafloor cable, respectively. The LTBMS observatory has a sensor suite comprising a broadband seismometer, geophone, accelerometer, volumetric strainmeter, tiltmeter, pressure gauge, and thermometer. These sensors were designed to collect weak and broadband signals that cannot be captured by land and/or seafloor observatories. After the installation and connection, realtime multi-borehole data via DONET seafloor cable network are available. In this presentation, we report initial characteristics of LTBMS sensors after the cable connection. The results includes: 1) Power spectral density analysis calculated from short-term records. 2) Running spectrum analysis using continuous long-term data. 3) Estimation of sensor orientation. 4) Initial drift and long-term stability of geodetic sensors. We compared PSD plots calculated from ambient noise records observed by C0002G, C0010A and DONET seismometers. Results confirmed that borehole seismometers are now functional to detect very weak seismic signal, such as VLF events. We also summarized initial response of tiltmeter and volumetric strainmeter, which can be used to detect weak geodetic signals as co-seismic events. Now auto evaluation processes are running on the data storage server not only to monitor sensor characteristics, but also to analyze very small seismic and geodetic events that are captured only by the LTBMS sensors.