ODP Leg 195 abstract

Leg 195

West Pacific Seismic Network

Plate consuming boundaries are concentrated in the Western Pacific area. It is most suitable region to study the dynamics of plates undergoing subduction, formation and evolution of island arcs and marginal seas, and the relation of these processes to mantle convection. Over the past years a dense regional geophysical network has been established on land. The network in Japan is one of the densest arrays of seismic stations in the world, and good coverage extends throughout eastern Asia. However, the land network must be supplemented by stations to provide data from the mid-ocean floor and from the plate subduction boundary. Development of the ocean seismic network is proceeding using ODP boreholes that are outfitted as long-term geophysical observatories. They provide unique seismic data hitherto unavailable. These data help quantify the dynamics of subducting plates from their entry into the mantle to their destruction in the deep mantle.

The proposal for the Western Pacific Geophysical Network called for two sites which were endorsed by the International Ocean Network (ION). The long-term ocean seismic observatory network was included as an initiative in the ODP Long Range Plan (LRP) as a contribution to the Global Seismic Network. The GSN has been successful is resolving the earth’ss interior from land and island based seismic installations, but still lacks coverage in large areas of the oceans. Two Western Pacific sites are designed to aid study of earthquake dynamics, the dynamics of the subducting plates, the formation of island arcs, and the relation of these processes to mantle convection. The first of the sites is scheduled for drilling during FY 00 and the second is scheduled here for drilling in 2001. Long-term seismic observatories will be installed at both sites. Both observatories will be connected to nearby telecommunications cables, in future.

A quiet and stable borehole will be drilled for installation of the long-term seismic observatory.

A logging program is planned to measure the quality of the borehole The logging program will measure physical properties, anisotropy, and hole shape, similar to what was done at pilot site OSN-1 during Leg 136. Logs must be acquired prior to installation of any downhole instrumentation. The laterolog will measure resistivity in basement intervals. Standard geophysical logs will measure physical properties; hole volume can be estimated with high accuracy using the BHTV log in the basement intervals. This will improve grouting procedures for the strain sensors and emplacement for the seismometers. High-resolution temperature logs will be used to identify permeable zones and in-flow/out-flow from both drilling-induced and natural fractures in the holes.

Drilling Plan

A quiet and stable borehole is required for installation of the long-term seismic observatory.

The logging program will measure physical properties, anisotropy, and hole shape, similar to what was done at pilot site OSN-1 during Leg 136. Logs must be acquired prior to installation of any downhole instrumentation. The laterolog will measure resistivity in basement intervals. Standard geophysical logs will measure physical properties; hole volume can be estimated with high accuracy using the BHTV log in the basement intervals. This will improve grouting procedures for the strain sensors and emplacement for the seismometers. High-resolution temperature logs will be used to identify permeable zones and in-flow/out-flow from both drilling-induced and natural fractures in the holes.

Leg Prop # Description Site Prop Lat. Prop Long. Prop Water Depth Prop. Total Depth Prop

195 431 ION Sites WP1 WP-1A 19°30'N 135°0.06'E 5715 470

rev WP-2A 42°0'N 160°0'E 5700 400

505 Marianas MAF-4B 13°46.99'N 146°0.17'E 2930 400

Add MAF-8A 13°53.45'N 146°57.32'E 4180 700

MAF-9A 16°32.25'N 147°13.25'E 2250 399

Staff Scientist: Carl Richter

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