The 421th Geodynamics Seminar
Technical developments on acoustic emissions
monitoring at high pressures
Dr. Tomohiro Ohuchi (Assistant Professor, GRC)
Date： 7.17.2015 (Fri) 16:30 ~Venu：Meeting Room #486, Science
Research Bldg 1, Ehime Univ.
日時：2015年7月17日(金)16:30〜	 
場所：愛媛大学	 総合研究棟Ⅰ4階共通会議室	 
Abstract
The subduction zone produces a major fraction of the Earth’s seismic activity. Intermediate-depth
earthquakes within the subducting slab form a double seismic zone. The cause of intraslab seismicity
have been attributed to dehydration of hydrous minerals (e.g., Peacock, 2001). Brittle fractureassociating dilatancy is difficult at high pressures (i.e., depths at which intermediate-depth and deepfocus earthquakes occur), although dilatancy prior to failure usually occurs in the case of shallow-depth
earthquakes.
At deeper depths, dehydration embrittlement (i.e., hydrofracturing) is expected to play an importantrole in failure of rocks because the overall volume change of the dehydration reaction is positive andthus pore pressure can be increased (e.g., Raleigh and Paterson, 1965). However, experimental results
on dehydration embrittlement of antigorite are controversial. Dobson et al. (2002) conducted a series ofexperiments on dehydration of antigorite, and they reported that dehydration of antigorite associates
acoustic emission (AE) when the dehydration reaction is positive. Even though the volume change
becomes strongly negative above 2 GPa, Jung et al. (2004) reported that brittle failure of antigorite
occurs at pressures up to 6 GPa. Recently, Gasc et al. (2011) reported that no detectable AEs throughdehydration of antigorite-rich serpentinite. Therefore, the cause of intermediate-depth earthquakes isstill unclear.
In some of subduction zones, a significant activity of deep-focus earthquakes has been reported (e.g.,Kirby et al., 1996). It has been proposed that deep-focus earthquakes are triggered by an instability
faulting caused by olivine phase transformations (Kirby et al., 1991; Green et al., 1992). Schubnel etal. (2013) conducted deformation experiments on germanium olivine (Mg2GeO4) at 2-5 GPa and1000-1250 K, and they observed many AEs generated in the sample. Schubnel et al. (2013) discussed
that fractures nucleated at the onset of the olivine-to-spinel transition.
To investigate the brittle properties of rocks, determination of AE source is critical. In thecommunity of high-pressure rock physics, Green et al. (1992) conducted AE monitoring by using aGriggs apparatus combined with an AE sensor. Dobson et al. (2002, 2004) and Jung et al. (2006)
adopted 2 or 4 AE sensors to a multianvil apparatus. However, the position of AE source has not been
determined in the experiments because of not enough number of sensors used in the experiments. DeRonde et al. (2007) adopted 8 AE sensors to a multianvil apparatus and they succeeded to determine
the position of AE sources. Recently, Gasc et al. (2011) succeeded to develop an experimental setupthat allows determining the position of AE source by using DIA-type multianvil apparatus combinedwith 6 AE sensors. Schubnel et al. (2013) adopted the experimental setup reported by Gasc et al.
(2011) to a D-DIA apparatus installed at a synchrotron facility, and they succeeded to measure strain
and stress of the sample and AE signals. We have developed an experimental setup that is optimized forthe determination of the position of AE source in a synchrotron D-DIA apparatus. We will report somepreliminary experimental results on AE monitoring under the upper mantle conditions.