Installation of a High Sensitivity Ocean Borehole Strainmeter in the Nankai Trough Under Severe Sea Current Conditions

摘要

A high-sensitivity volumetric strainmeter has been installed into the C0010 bore-hole observatory using the drilling vessel (D/V) Chikyu during the Expedition 365 cruise in the Nankai Trough, Japan. At this location, crustal deformation occurs in association with large interplate earthquakes. However, strong Kuroshio ocean currents cause vortex-induced vibrations (VIVs) in the region, which can cause fatal damage to the strainmeter. Therefore, laboratory vibration tests were performed prior to installation to confirm that the antivibration mechanism inside the strainmeter was functional against the severe vibrations during installation. VIV was measured prior to installing the strainmeter into the C0010A borehole using accelerometers at the installation site. The results indicated that the VIV were within the specification of the antivibration mechanism. This meant that installation of the strainmeter into the borehole was possible. To maximize sensor sensitivity, it is extremely important to ensure mechanical coupling of the strainmeter with the borehole wall by cementing operation after installation. The cementing process was confirmed using a pressure recording device incorporated within the strainmeter. Pressure data clearly showed that seawater had been displaced with cement slurry. Data from the strainmeter clearly showed tidal waveforms, which are comparable to those of pressure data recorded by a borehole pressure sensor installed at approximately the same depth. Accuracies of the strain data were validated through the procedure. They suggest that the first installation of the ocean borehole strainmeter in the Nankai Trough was successful, and therefore, highly sensitive strain measurement is now possible in a seismically active area.