PROPOSED SITES/OBSERVATORIES

Site JT-1C

Proposed Site JT-1C is located on the deep-sea terrace for observatory installation within a seismically active zone (Figs. 3, 6, 7). This zone is known to be capable of generating from micro sized to large (M > 7) earthquakes. Other objectives are to (1) recover a record of the volcanic ash stratigraphy spanning the past 3 m.y.; (2) use the sedimentology, biostratigraphy, and structural fabric of cores to obtain a better understanding of the subsidence history of the forearc since the early Miocene; and (3) determine the physical properties for geologic studies and for characterizing the borehole. Time permitting, efforts will be made to reach the Cretaceous basement to determine its nature and to extend our knowledge of the subsidence and deformation history.

Site JT-2G

Proposed Site JT-2G is located on the deep-sea terrace south of Site JT-1C for observatory installation within a seismically inactive zone (Figs. 3, 6). Slip within this zone has not been accompanied by detectable earthquakes for more than a decade. No clear historical record is available that indicates seismic slips in this zone except possibly in 1678 or 1915. Other primary objectives are to (1) use the sedimentology, biostratigraphy, and structural fabric of cores to obtain a better understanding of the subsidence history of the forearc since the early Miocene and (2) determine the physical properties for geologic studies and for characterizing the borehole. Time permitting, efforts will be made to reach the Cretaceous basement to determine its nature and to extend our knowledge of the subsidence and deformation history.

Alternate Sites JT-3, 4, and DSDP-439

These are alternate sites located near the primary sites on the deep-sea terrace (Figs. 2, 6). Proposed Site JT-3 is located ~4 km north of Site JT-1C and is a contingency site should there be problems at Site JT-1C.

Proposed Site JT-4 is ~11 km south of JT-1C and is a contingency site for studying arc volcanism should there be additional time after drilling the primary sites. It will be APC/XCB cored to recover undisturbed sediments to perform a detailed study of Pleistocene/Pliocene volcanic ash, sedimentology, biostratigraphy, and subsidence history.

Site 439 was drilled and cored during DSDP Leg 57 down to the Cretaceous basement. This site is proposed as an alternate to Site JT-2G. Because this site has already been cored, it provides an alternate site in which an instrumented borehole could be completed more rapidly than the primary site. Such a contingency site could become important should difficulties arise at the primary sites.

Observatory Design

All the instruments will be third-party tools. Both sites are to be equipped with the following sensors near the bottom of the drilled holes: (1) a high-resolution volumetric strainmeter (Carnegie Institution of Washington/University of Tokyo joint development), (2) a broadband seismometer (Guralp CMG-1) and back-up sensor (PMD2023), (3) a tiltmeter (AG510), and (4) a temperature sensor. Any heat-generating instrument will be separated from the strainmeter. A pressure gauge will be placed on the seafloor to monitor pressure changes at a resolution equivalent to an ~10-cm water column.

1. Strainmeter
   The Sacks-Evertson borehole volumetric strainmeter has proven to have resolution of better than 10^-11 at various locations on land, including San Andreas, California, Iceland, and Japan (Sacks et al., 1978; Linde et al., 1988, 1993, 1996). The instrument must be buried and cemented in solid contact within a competent rock section. The deepest installation so far has been at ~500 m. Because of the high dynamic range and very broad frequency response (up to 20 Hertz) of the borehole strainmeter, an ocean-bottom installation will provide valuable data for subduction zone earthquakes. For example, on-land borehole strainmeters have proven to be effective in detecting the slow initial stage of large earthquakes.
2. Seismic Sensor
   In September 1989, a feedback-type accelerometer capsule was installed in Hole 794D in the Japan Sea during Leg 128 (Ingle et al., 1990; Suyehiro et al., 1992, 1995). The instrument recorded a teleseismic event (Mb 5.4 at ~4000-km epicentral distance) clearly showing a surface wave dispersion train (Kanazawa et al., 1992). In May 1992, a comparison of seafloor and borehole (Hole 396B) sensors was made using a deep-sea submersible for installation and recovery (Montagner et al., 1994). Although, at this stage, there is no apparent conclusion as to how we should establish seafloor seismic observatories, it is becoming clear that oceans can provide low-noise environments. In this particular case, where seismic sensors are to be installed as near to the source as possible, borehole installation should give better constraints on hypocenter depths. It is imperative that no fluid motion occur around the sensor; therefore, the seismometer and the strainmeter must be cemented at the same location in the same operation.
3. Tiltmeter
   Biaxial borehole tiltmeters (Applied Geomechanics Model 510) will be included to measure crustal deformation at a resolution of 10^-8 rad with a dynamic range of 44 dB.
4. Temperature
   Temperature changes inside the strainmeter will be measured to compensate for the effects of temperature variation. Temperature on the seafloor will also be recorded at 5x10^-4 degree sensitivity.

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