Site 1150 is in a seismically active zone (Fig. F76). Recent significant interplate thrust earthquakes in this zone (39°-40°N) occurred in 1968, 1989, and 1992. The 1968 event (magnitude [M] = 7.2) is considered an aftershock of the 1968 Tokachi-oki earthquake (moment magnitude [Mw] = 8.2). The 1992 events are suggested to have been accompanied by a slow earthquake releasing seismic moment of Mw 7.3-7.7 (Kawasaki et al., 1995). Both the 1989 (five M = >6 events during 27 October-7 November) and the 1992 (six M = >6 events during 16-29 July) activities were of swarm type. The ocean-bottom seismometer aftershock observations delineated the seismically active zones as a number of distinct clusters (Nishizawa et al., 1990; Hino et al., 1996). These swarm activities were confined between 143° and 143.8°E.
We installed the geophysical observatory at Site 1150 to physically relate these earthquakes, plate motion, and seismic structures to earthquake cycles and dynamics. This site is ideal for testing how stable and unstable plate slips (regular to slow earthquakes to creep) exist in space and time and how this affects the surrounding stress/strain conditions.
The installation of the geophysical observatory at Hole 1150D proceeded as shown in Table T23. Based on core descriptions and logging results from Hole 1150B, the strainmeter was emplaced at 1120 mbsf (Fig. F77) in a relatively uniform and higher density (~1.7 g/cm3) and higher P-wave velocity (~1960 m/s) section.
Attaching four cables to
the casing pipe proceeded much faster than planned (see Step 7 in Table T23).
Centralizers (9
-in outside and 3-in height) were attached ~3 m from each side of every joint
(Fig. F78). The
cables were attached to the casing pipe by tie wraps covered with duct tape.
Joining the 4
-in casing
pipes (API-J55-STC, 10.5 lb/ft, ~11.7-m length each) took longer than normal
because the "iron neck" could not be used.
As a result, ~48 hr elapsed between the final drilling of Hole 1150D and the initial entry of the instrument assembly into the hole. Hole penetration resistance increased to the extent that it was possible to achieve entry only to a depth of 23 m less than required. Since it was necessary to redrill the open-hole section, the seafloor package (hanger/riser with MEG) with attached 1.1-km in-hole assembly was brought back to the moonpool. At this point, we checked the system and found all sensors to be operational. In effect, four unarmored cables withstood the pressure and the rubbing against the hole walls and the VIT guide sleeve. Instead of pulling out completely, the string below the hanger/riser was hung from the moonpool ceiling, keeping the cabled casing pipe link intact (see "Operations") so that reinstallation could be much quicker than the initial attempt.
The 95.2-m-long, 9
-in
open-hole section was redrilled on 25 July in 5.5 hr (17.3 m/hr). The hole was
then washed with water to clear cuttings but was not stabilized with heavy mud.
Installation of the instrument string then proceeded successfully. The bottom of
the hole was filled with cement with 1.9 g/cm3 density and 10.8 m3
volume (Step 21 in Table T23).
This volume can fill 213 m height of 10-in-diameter hole or 150 m of 0.7-m2
hole, assuming an elliptical shape based on caliper measurements of a
corresponding depth range at Hole 1150B. This cementing method is required for
the strainmeter operation, and the scouring action of the cement flowing past
the 1-in clearance around the strainmeter probably ensured a clean hole. For
other sensors, the cementing likely provides the best coupling to the
surrounding rocks compared to other methods that utilize mechanical arms, pads,
or sand. Unlike other methods, cementing prevents water motion and temperature
fluctuation from becoming noise sources.
After successfully cementing the instruments, the power access terminal (PAT) battery frame was made up in the moonpool. The PAT was hung at three points by three cables from the dual acoustic releases (Fig. F79). The drill pipe passed through the PAT center and the guide sleeve of the VIT. The actual lowering started after waiting for daylight since the operation was complicated, involving many weight transfers while the ship's heave was a few meters. The frame was lowered successfully onto the reentry cone (see Step 24 in Table T23) by using the logging cable, which allowed precise depth measurements and good heave compensation. After the PAT landed on the reentry cone, the acoustic release system was commanded by a transducer hung from the port side of the ship. Upon release, three small glass sphere buoys attached to the three cables pulled the cables up.
The installation was completed after successful disconnection of the J-tool to decouple the drill string from the observatory, although that operation took longer than anticipated. The electrical connection between the MEG and the PAT was made in September by the remotely operated vehicle (ROV).
The array of emplaced instruments (Fig. F80) from bottom to top in Hole 1150D consists of the three-component strainmeter, three-component broadband PMD seismometer, two-component tiltmeter, and three-component CMG broadband seismometer (see "Borehole Instruments" in the "Borehole Instrument Package" chapter). We chose to emplace the three-component strainmeter at this site rather than at Site 1151 because of the tectonic setting as described in "Tectonic and Seismic Setting" in the "Leg Summary" chapter. A 3-m-long stinger pipe with centralizers was attached to the strainmeter bottom. This length was chosen for optimal safety upon reentry. The sensors other than the strainmeter were attached to the coupling tube that was bolted to the strainmeter (Fig. F81).
Each sensor was checked three times through the MEG: before installation, after retrieval, and just before reinstallation. All sensors performed consistently with no problem.
As described in "Installation
Techniques" in the "Borehole
Instrument Package" chapter, the cable link between the hole bottom
and the seafloor is supported by the 4-in
casing pipe. This way the cables can be protected, the installation depth is
precisely predetermined, and the casing string does not heave inside the hole
because the hanger/riser is coupled to the reentry cone. The centralizers
further protect the cables. Figure F78
shows how the four cables were attached to the casing pipe string. Four cable
reels were placed on the foreside of the moonpool and fed through sheaves
(600-mm diameter × 84-mm groove width) hung from the ceiling.
A total of 95 joints of casing pipe hangs the instruments. The cables were cut at the moonpool 60 cm longer than the length required to attach the underwater mateable connectors (UMCs) to the MEG bottom stab plate, in case the need for reheading occurred. The cable termination took ~13 hr with six people working on two cables in parallel.
All the components were successfully emplaced. These are the power supply (seawater battery [SWB]), data recorder (storage acquisition module [SAM]), and a unit to merge and digitize data and control the observatory (MEG) (see "Seafloor Instruments" in the "Borehole Instrument Package" chapter). The MEG was slid into its holder attached to the hanger/riser pipe connected to the four cables on its bottom side. To avoid plug contamination, an ROV dummy receptacle is on the top. Zinc anodes were attached wherever necessary to prevent corrosion of important components, such as at the stainless-steel stab plates attached to the bottoms of the stainless-steel MEG and the titanium SAM.
The PAT consists of three units of SWBs (see "Seafloor Instruments" in the "Borehole Instrument Package" chapter). The SAM data recorder package is placed near the center of PAT on which the oil-filled 8-conductor cable is placed (Fig. F82). An ROV must disconnect one end of the cable from the parking position and connect it to the top of the MEG after removing the dummy receptacle. From the VIT camera check, the relative position of the MEG and the UMC parking position was found to be farthest apart, 180° around the center (Fig. F6). Table T24 depicts the tasks to be accomplished by an ROV.
The establishment of the first seafloor borehole geophysical observatory ('Neath Seafloor Equipment for Recording Earth's Internal Deformation [NEREID-1]) was successful. All the tasks planned for Leg 186 at this site were completed. Although the borehole sensors are physically inaccessible, the seafloor components are designed to be replaceable by an ROV. The MEG can be replaced. The SAM must be replaced as the data disks are filled after ~1 yr of recording (72 GB). Batteries can be revived by replacing the magnesium anodes. The system status can be checked by the back from ocean bottom (BOB) module by an infrared communication link to the SAM. The site is only a few kilometers from the termination point of the fiber-optic cable observatory system installed in 1996 by the University of Tokyo. Our plan is to connect our observatory to this cable once we prove that the borehole observatory is producing reliable data (Fig. F83). Then the power can be supplied directly from land and the data disseminated in real time.
