CDEX Web Magazine 地球発見

CDEX
Special topic 1:Start of real-time monitoring with DONET connecting the long-term borehole observatory installed by CHIKYU

Off the Kii Peninsula the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) is under construction on the seafloor. In January 2013, the long-term borehole observatory, which the CHIKYU installed in IODP expedition 332 to a depth of approx. 980m beneath the seafloor, was added to the seafloor network that is already in operation. How will this project at the forefront of science get a handle on the phenomenon of the massive earthquake?
(Published online February 2013)

Interviewee
Dr. Yoshiyuki Kaneda
Principal Research Scientist, Pro ject Leader, Earthquake and Tsunami Research Project for Disaster Prevention, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

Connecting the long-term observatory to DONET

 On January 24, 2013, JAMSTEC’s remotely operated vehicle HYPER-DOLPHIN made a dive to the seafloor at approximately 80 km south-east of Owase city, Japan. Its aim was the long-term borehole observatory installed by Deep Sea Scientific Drilling Vessel CHIKYU in December 2010 during IODP Expedition 332, NanTroSEIZE – Riserless Observatory Installation.

The long-term borehole observatory is installed over a length to 980 m beneath the sea floor, which itself is at a depth of 1,937.5 m, and includes a seismometer, tilt meter, strain gauge and temperature sensors. The HYPER-DOLPHIN's aim is to connect this observatory with cables to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) off the Kii Peninsula. This connection will enable observation data from deep underground to be sent in real time to the shore station.

 On January 24 the arrival of the first data from the long-term borehole observatory was confirmed at the shore station in Owase city. We have acquired yet another tool to help us understand giant earthquakes in the Nankai Trough.

The DONET network in the Nankai Trough Long-term borehole observatory
The DONET network in the Nankai Trough Long-term borehole observatory

The real time observation network built on the Pacific floor of the Western Japan

 What is DONET?

 Twenty observation stations are currently deployed on the seafloor south-east off the Kii Peninsula. They are titanium cases containing seismometers and water-pressure gauges, buried several meters beneath the seafloor. These observation stations, connected with cables to the shore station, form a real time observation network for future Tonankai earthquakes.

 Says Project Leader Yoshiyuki Kaneda of the Earthquake and Tsunami Research Project for Disaster Prevention, “The important feature of DONET is its use of nodes.” The nodes are used in a so-called outlet system. The cable connected to the shore station in Owase city is laid out in a big circle, and nodes, like power outlet taps, are connected to the end of cables branching from this cable. There are a total of five nodes. Each node has eight sockets, and all observation stations are set up with cables extending from the sockets. Four observation stations each to five nodes makes 20 stations, together forming the DONET system.

 Says Kaneda, “DONET has both network redundancy and expandability.”

 The DONET redundancy means that it uses cabling loops from the shore station and can receive data either clockwise or counter-clockwise, so that even if a cable is disconnected somewhere continued data transmission is possible. Its expandability means that the use of nodes enables easy replacement or new installation of observatories. The long-term borehole observatory has now been connected precisely in this way.