Deformation experiments of serpentinite under high pore pressure and hydrothermal conditions

Slow earthquakes (e.g. slow slip events, very low frequency earthquakes and non-volcanic tremors) have been detected in southwest (SW) Japan, whereas such activities are rare in northeast (NE) Japan. In NE Japan, the subducted oceanic crust of the old and cold Pacific plate is characterized by brittle behavior at a deeper depth and inter-plate earthquake activity to 150 km depth. In contrast, SW Japan, the subducted oceanic crust of young and hot Philippine Sea plate behaves ductile manner at a shallower depth and aseismic behavior deeper than 35 km depth. Most slow earthquakes of SW Japan occur with the depth range from 35 to 45 km (Obara, 2002), which depth might be correspond to the brittle-ductile transition. Low velocity anomaly and high Poisson’s ratio have been reported in these depths of SW Japan, suggesting that the corner of mantle wedge are locally serpentinized, where the subducted Philippine Sea plate is dehydrated (Kamiya and Kobayashi, 2000). In addition, the effective normal stress at tremor source region in SW Japan is an order of 100kPa (Nakata et al. 2008), which indicates that tremor source regions in SW Japan maintains high pore fluid pressure. In summary, the region of slow earthquakes activity involves areas of (1) brittle-ductile transition zone, (2) presence of serpentine, and (3) high pore fluid pressure zone. In order to understand the mechanism of slow earthquake, it is important to address effects of these three conditions on the subduction zone rheology. This study focuses on the dynamics behavior of serpentine under high pore pressure and hydrothermal conditions. Experiments were conducted using the gas confining medium apparatus of AIST, and we are also developing gas deformation apparatus at Hiroshima. Stating material was used highly dense and isotopic antigorite serpentinite from Nishisonogi metamorphic belts, western Japan, which porosity is about 0.2 to 0.3 %. We performed preliminary deformation experiments at confining pressure of Pc = 200 MPa, pore pressure of Pp = 150 MPa and temperature of T = 400C. The differential stress increased up to 550 MPa (over limit of apparatus), although the sample shows no brittle failure and ductile deformation. Our mechanical data indicate 22.6 GPa for Young modulus. In another run, the pore pressure is increase up to Pp = 190MPa and shows brittle failure at 340 MPa at axial strain of 0.016. These preliminary data indicates that the high pore pressure plays important role one the failure strength of serpentinites, and we will report more results on the mechanical data of serpentinite under high pore pressure and hydrothermal conditions.