HR: 1330h
AN: V52B-0434    [PDF]
TI: Unzen Scientific Drilling Project, Japan: Studies of Stress, Fractures and Physical Properties in Flank Wells
AU: * Ikeda, R
EM: ikeryu@ep.sci.hokudai.ac.jp
AF: Division of Earth and Planetary Sciences, Hokkaido University,, N-10, W-8, Kita-ku,, Sapporo, 060-0810 Japan
AU: Omura, K
EM: omura@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai,, Tsukuba, 305-0006 Japan
AU: Hickman, S H
EM: hickman@usgs.gov
AF: Earthquake Hazards Team, U.S. Geological Survey, 345 Middlefield Road,, Menlo Park, CA 94025 United States
AU: Kueck, J
EM: jkueck@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg A34, Potsdam, 14473 Germany
AU: de Wall, H
EM: dewall@geologie.uni-wuerzburg.de
AF: Universitaet Wuerzburg, Sanderring 2,, Wuerzburg, 97070 Germany
AU: Matsuda, T
EM: mtatsuo@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai,, Tsukuba, 305-0006 Japan
AB: We have been conducting an integrated studies of in-situ stresses, fracture geometry and permeability, and rock physical properties in the flank drilling of Unzen volcano. These measurements are indispensable for the next phase of the drilling project into the magma conduit to better understanding the dynamics of magma emplacement and degassing, the physics of volcanic seismicity, and mechanical controls on the orientation and magnitude of fracture permeability. Two boreholes were drilled in the flank of the volcano: one at Minami-Senbongi (USDP-1, 752 m deep) and the other at Ohnokoba (USDP-2, 1,462 m deep). We conducted well logging, in-situ stress measurements and an injection test in these boreholes. Similar variations in physical properties were observed in each well: electrical resistivity, 10 - 1000 Ohm-mœ¯P-wave velocity, 2.5 - 4.0 km/sec; density, 2.2 - 2.3 g/cm3; gamma-ray intensity, 70 - 100 API; and bottom-hole temperature, 47cªC (in USDP-1) and 33cªC (in USDP-2). These variations correlate with small-scale changes in lithology (alternating between mud flows, lava blocks, and pumice flows) over depth intervals ranging from 1 - 20 m. In the USDP-1 well, a fluid injection and temperature recovery test was conducted to constrain the thermal and permeability structure of the volcanic flank. It was revealed that three layers exist: a relatively impermeable mud flow layer, an unconfined and highly permeable block-and-ash flow aquifer, and a thick and relatively impermeable sedimentary layer. In the USDP-1 well, the magnitudes of the maximum and minimum horizontal compressive stresses, SHmax and Shmin, were determined using the hydraulic fracturing method. The magnitude of SHmax is close to the vertical stress, indicating a transitional normal to strike-slip faulting regime. In addition, borehole televiewer logs of the hydraulic fracture indicate an SHmax azimuth azimuth determined in USDP-1 is almost parallel to the Beppu-Shimabara graben, in which the Unzen volcano resides, and coincides with the SHmax azimuth estimated from focal mechanisms of shallow earthquakes. Thus, the current orientation of Shmin is nearly optimal for driving north-south extension across the Beppu-Shimabara graben. In addition, since this Shmin direction is nearly perpendicular to the inferred orientation of the feeder dike for 1990-95 Unzen eruption, it appears that the current in-situ stress field plays an important role in controlling the dynamics of magma ascent beneath Unzen.
DE: 8424 Hydrothermal systems (8135)
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting