CDEX Web Magazine 地球発見

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Graphic Guide:Sensors 8,000 Meters below the Sea Level

Plans are now afoot to explore the seismological mechanisms that triggered the Great East Japan Earthquake on March 2011 by placing heat sensors and other equipment deep beneath the seafloor. The Center for Deep Earth Exploration’s experience in placing long-term borehole monitoring equipment in the Nankai Trough should pay off, but the new project will present new challenges because of the differences in the submarine environment. What sorts of innovations are being developed to overcome these challenges?
(Published in April 2012)

Masanori Kyo Interviewee:
Masanori Kyo
Group Leader
Technology Development Group
Center for Deep Earth Exploration

Building on Breakthrough Achievements

 “Frictional heat is created when a piece of bedrock rubs against another,” explains CDEX Technology Development Group Leader, Masanori Kyo. “The temperature of the surrounding rocks also rises before it gradually cools over time. The greater the energy from the slip, the hotter the rocks get, so by measuring temperatures along a fault, you can estimate the size of the slip.”

 Kyo is in charge of placing the monitoring equipment inside the boreholes being drilled as part of the IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST).

 This project follows a similar undertaking along the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE)—another earthquake-prone area, located along the southeastern margin of Japan. For the NanTroSEIZE project, seven types of sensors (including strain indicators and thermometers) were placed into a borehole installed casing pipes. This is the first time in scientific history for so many measuring devices to be placed in a single undersea borehole for such long-term observations.

 “The seas along the Nankai Trough flow rapidly due to the Kuroshio Current, which is great underwater movement,” says Kyo, who was also the manager of the Long-Term Borehole Monitoring System (LTBMS) of NanTroSEIZE. “High-precision sensors are vulnerable to shock, so their placement required great care. We conducted preliminary trials in the actual ocean areas to ensure success.”

 The installation of the first set of equipment was completed in December, 2010. This was just three months before the Great East Japan Earthquake.

 Soon afterwards, consideration was given to moving the borehole observatory near the epicenter. Preparations to initiate the monitoring system of the NanTroSEIZE took two years, but an effort was made to begin the observations as soon as possible because the area around the epicenter was undergoing rapid changes. Full deliberations on the submission of a research proposal to the Integrated Ocean Drilling Program (IODP) were underway by May 2011. And while the evaluation of such proposals generally takes several years, this time, intensive deliberations led to a go-ahead for the emergency drilling proposal in August.

 “I think our research has great significance both scientifically and socially,” Kyo points out. “The March 2011 disaster attracted tremendous international attention, and there was an unexpectedly large response to a call for applications for the position of onboard scientists. Given the heightened expectations, we have to make sure we succeed.”

 Kyo believes that measuring heat will be pivotal to success. Heat sensors are simpler and easier to use than other types of equipment, and the efforts made to overcome the challenges at Nankai related to the Kuroshio Current will be a big plus.