© ICDP, the International Continental Scientific Drilling Program, 1996-2020

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Drilling along the Nankai Trough

Asia, Japan, Honshu, Kii Peninsula

new workshop-proposal: ICDP-2006/07
for the funding-period starting 2006-01-15
by Takeshi Sagiya, Kelin Wang, Naoji Koizumi
Abstract
This is a proposal of workshop about scientific drilling along the Nankai Trough subduction zone, southwest Japan. The Nankai Trough is one of the world’s best studied seismogenic subduction zone, where large megathrust earthquakes have repeatedly occurred every 90-150 years. The last major earthquakes in this region occurred in 1944 and 1946, and the next earthquake is anticipated in the middle of this century. In order to investigate the seismogenic megathrust, the IODP project NanTroSEIZE was proposed and is going on its authorizing process to drill south off Kii Peninsula, in the central part of the Nankai Trough. The drill holes of NanTroSEIZE are planned to reach the seismogenic fault, and the actual fault material will be sampled and investigated. The NanTroSEIZE project also emphasizes the importance of monitoring, and includes a scope of monitoring of the fault zone using the IODP hole. Here we propose that additional boreholes on the continental side by ICDP drilling will make the monitoring of the plate boundary megathrust much more effective and attractive because we have more flexibility for instrumentation and operation of the boreholes. There is a high sensitivity seismic network as well as a dense continuous GPS monitoring network being operated in Japan. By integrating monitoring at the surface and at the depth, the best monitoring system for the plate boundary megathrust in the world will be completed, and the earthquake sciences will be highly benefited. Candidate items to be monitored at the boreholes include fluid pressure, volumetric strain, 3-component strain, temperature, tilt, seismic waveform, and so on. As a tentative plan, we propose an array of 5 boreholes, 3 or 4 along the Kii Peninsula coast, and 1 or 2 further inland along the IODP transect. The average spacing between boreholes will be about 50 km. In the workshop, we are going to discuss various aspects related to the drilling in order to prepare a ICDP proposal. We will discuss how those monitoring can be done and what else can be done. The workshop topics will also include: scientific targets, candidate locations for drilling, the number of boreholes and their depths, monitoring instrumentation, cooperation with the NanTroSEIZE project, organization for long-term monitoring, the detailed schedule of the whole drilling and monitoring, and so on. We will invite those who have experiences and interests in subduction zone geophysics/geology/geochemistry, borehole instrumentation, drilling techniques, from the international community.
Scientific Objectives
  • This drilling is aimed at comprehensive understanding of physical/chemical processes related to the recurrence of megathrust earthquakes in and around the plate interface. During an earthquake cycle, variations in the interplate coupling strength cause cyclic stress/strain accumulation and release in the surrounding region. Well coordinated monitoring between the borehole array and the seismic as well as geodetic network at the surface will provide the best coverage over the target. On the other hand, physical properties of the fault zone such as permeability and frictional property are also considered to show cyclic behaviors in accordance with the earthquake cycle, which can be investigated mainly by studying IODP core samples. From recent precise monitoring with seismological as well as geodetic techniques, new features of the plate boundary processes have been found. They are episodic slow slip events and non-volcanic low frequency tremors. Both phenomena seem to be occurring around the deeper extension of the seismogenic zone along the subducting plate interface. Seismological features of low frequency tremors indicate crustal fluid is playing an important role. Therefore, one of the important scientific target of this drilling is to monitor a hydrological aspect of the crust. For that purpose, we have an idea to monitor pore pressure changes with an array of deep boreholes both in the ocean bottom and on land. Finally, establishment of basic technologies required for long-term borehole monitoring over an entire earthquake cycle is also highly anticipated.
Keywords
Active faulting, Collision zones, Convergent margins, ICDP-2006/07, IODP, Japan, Kii peninsula, Monitoring, NANKAI, Nankai trough, Pore pressure, Subduction zone
Location
Asia, Japan, Honshu, Kii Peninsula: 34.00000, 136.00000

© ICDP, the International Continental Scientific Drilling Program, 1996-2020

www.icdp-online.org