Performance evaluation of the newly developed volumetric strainmeter for the ocean borehole observatory in Nankai seismogenic zone

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Author(s): Kitada, K.; Araki, E.; Kimura, T.; Kinoshita, M.
Author Affiliation(s): Primary:
JAMSTEC, Institute for Frontier Research on Earth Evolution, Yokosuka, Japan
Volume Title: AGU 2011 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 fall meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Long term, in-situ monitoring of seismic activity, slow slip event, pore fluid behavior and strain accumulation around mega earthquake zone is a key for understanding the process of earthquake generation. During the IODP Expedition 332 in last December, we have successfully installed the borehole observatory including our new volumetric strainmeter in the Kumano forearc basin of the Nankai Trough. In the KY11-09 cruise by R/V Kaiyo from this July to August 2011, the performance test on the strainmeter installed into the Site C0002 in Nankai Trough was conducted for the connection to the data recorder in order to achieve the long term borehole monitoring. The observatory will be connected to submarine cabled observation network called Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) for the real time monitoring, which were constructed in and around the drilling target area during the KY11-09 cruise. Assessing the strain response based on the several externally applied stresses is a crucial step toward evaluating and interpreting the strain data collected in the ocean borehole. Especially, in order to detect strain change based on the regional stress field, it is important to verify the performance by comparing with the theory model after removed the effect of the environmental factors. In this study, we have installed the borehole volumetric strainmeter which is the same type as installed in Nankai Trough, into the 216 mm outside diameter borehole with depth of about 21 m in Kamioka mine (Hida, Japan) last December and the evaluation test of the long term performance was started. The collected strain data showed the drift rate of about 400-500 nstrain/day which can be explained by the temperature change of silicone oil inside the sensing part of the strainmeter and/or the other effect. The drift corrected data clearly showed the earth tidal strain change and corresponds with areal strain change predicted by the earth tidal model (GOTIC2). 0.2-0.4 Hz microseisms (amplitude 0.15 nstrain) and earthquake with magnitude 5.3 (amplitude 0.38 nstrain) were recorded in the strain data, corresponding with microseisms (amplitude 140 nrad.(X), 180 nrad.(Y)) and earthquake (amplitude 650 nrad.(X), 1350 nrad.(Y)) recorded in tiltmeter installed next to the strainmeter. Further, in order to evaluate strain change associated with pore pressure change, we have conducted the pressure test by pressurizing the bottom section of the borehole. As a result, the strain value decreased (dilatation) after the pressurizing and then increased (compression) with gradual pressure decay, which may be caused by the opening effect of the borehole wall around the bottom. However, the additional tests are needed to explain the relation between the strain data and the pore pressure change. In this presentation, we will introduce the newly constructed observatory in Kamioka mine for the development of long term borehole monitoring system and show the results of the performance evaluation of the borehole volume strainmeter.
Year of Publication: 2011
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 19 Geophysics, Seismology; Boreholes; Design; Drilling; Engineering geology; Expedition 314; Expedition 315; Expedition 332; IODP Site C0002; Instruments; Integrated Ocean Drilling Program; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; Pacific Ocean; Seismicity; West Pacific
Coordinates: N331800 N331800 E1363800 E1363800
Record ID: 2012039973
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