Background and History

1. Earthquakes and Tsunami in the Nankai Trough

The Nankai Trough, Japan is one of most active subduction zone in the world. In this area, great mega-thrust earthquakes repeatedly occurred every 100 to 150 years. These earthquakes are caused by interactions between two plates, oceanic Philippine Sea plate and continental Eurasian plate. Most recent earthquakes occurred in 1944 and 1946, and both of which generated not only strong motion but also huge tsunamis destroying coastal area in the south part of Honsyu and Shikoku islands, Japan.

2. Deployment of DONET seafloor cabled observation network

To elucidate earthquake preparation and generation process, and to evaluate their risks, it is very important to monitor in-situ geophysical and geodetic signals around the source fault region. Therefore we have developed real-time observatories in the seafloor and in seafloor borehole. We first developed seafloor observatories, called DONET (Dense Oceanfloor Network Systems for Earthquake and Tsunamis), which is development program of submarine cabled real-time seafloor observatory network to monitor seismic and tsunami observation by distributed 20 stations in the Nankai Trough Kumano nada area. Each DONET station has seismometers and pressure gauges to monitor seismic ant tsunami activity with very wide dynamic ranges. Technical development and installation phase of DONET began in 2006, and the installation of the DONET was completed in 2011. Now we are deploying a similar seafloor observation network, consists of 31 stations, in off-Kii suido area since 2014. These systems are called DONET-1 (Kumano nada area) and DONET-2 (Kii suido) (Fig 1).

Fig. 1. DONET-1 and DONET-2 seafloor cabled observation network in the Nankai Trough.

3. Long-term borehole monitoring system (LTBMS) installation

Borehole observation system called Long-term borehole monitoring system (LTBMS) was also developed for seismic, geodetic, and hydrological observation in the seafloor targeting on the seismogenic faults in the Nankai Trough. In this system, we may install various sensors in the same seafloor borehole such as a broadband seismometer, a tiltmeter, a strainmeter, geophones and accelerometer, thermometer array as well as pressure ports for pore-fluid pressure monitoring (Fig. 2). We also monitor seafloor pressure and temperature as a part of pressure monitoring package. The signal from the sensors in the borehole is digitized and transmitted through cables to the seafloor where we connect data recorders or a DONET interface to make the observatory as a part of DONET seafloor network.

Fig. 2. Cross sectional view of the Long-term borehole monitoring system (of IODP hole C0002G)

We have a plan to establish the long-term borehole monitoring system in a few selected locations (Fig. 3.) in the Nankai Trough as a part of NantroSEIZE project in the Integrated Ocean Drilling Program (IODP).

Fig.3. Planned locations (C0002, C0010, C0006) of IODP borehole observatories in the Nankai Trough.

The first borehole observatory system was installed into the C0002G borehole site during IODP (Integrated Ocean Drilling program) Exp. 332 in December, 2010 in the DONET1 area.

The long-term borehole monitoring system consisted from borehole sensors, a km long tubing and cables and the ROV platform, was assembled on the D/V Chikyu (Fig. 4) and installed in the C0002G borehole under strong Kuroshio current. Installation in such strong ocean current was very tricky but we were successful by excellent D/V Chikyu operation (Fig. 5).

Fig. 4. The borehole sensors being prepared on the D/V Chikyu in IODP Exp 332. CMG3T broadband, tiltmeter, and geophone/accelerometer housed in a sensor carrier.

Fig. 5. Deployment of borehole broadband seismometer in IODP borehole C0002G.

The C0002G site is located above mega splay fault, which should be source faults of recent earthquakes. Borehole sensors consisted of a volumetric strainmeter, tiltmeter, pore fluid pressure gauge, seismometers, and thermometer array was installed into the C0002G site.

4. Connection to DONET

After the installation in IODP Exp 332, we tried to visit the borehole observatory in the seafloor by ROV for eventual connection to the DONET network for making the observatory on-line. In KY11-09 cruise by the JAMSTEC R/V Kaiyo and the NT12-E01 cruise by the JAMSTEC R/V Natsushima, we confirmed the proper function of the borehole instruments installed during the IODP Expedition 332 and collected good quality of the temporal data through the ROV direct connection. In Jan. 2013, the C0002G long-term borehole observatory was connected to the DONET cable for real-time monitoring during the KY13-02 cruise by the JAMSTEC R/V Kaiyo and ROV Hyper Dolphin (Fig. 6).

The connection operation was conducted by the following steps (Fig. 7):

1) Deployment of the DONET interface unit on the ROV platform of C0002G observatory. All borehole sensors were connected to the connection I/F unit via jumper cables.

2) 10km Extension cable was connected to DONET1 NODE-D, and the extension cable was spread by ROV Hyper-Dolphin between NODE-D and C0002G with length of around 8 km.

3) Connection the extension cable to the DONET interface unit on the ROV platform. After the connection, real-time observed data of borehole sensors have been transmitted via DONET-1 seafloor cable network (Fig. 8).

Fig.6 ROV Hyper-Dolphin before dive for I/F deployment.

Fig.7 Schematic of the DONET interface unit to connect the long-term borehole monitoring system to the DONET seafloor cable network.

Fig. 8. Long-term borehole monitoring system (C0002G) connected to the DONET network

References related with the long-term borehole monitoring system