The 446th Geodynamics Seminar
Intermediate-depth earthquakes can be caused by localized
heating in dehydrated antigorite shear zones
Dr. Tomohiro Ohuchi (Assistant Professor, GRC)

Abstract
The subduction zone produces a major fraction of the Earth’s seismic activity. The mechanisms ofintermediate-depth (> 40 km depth) and deep-focus (> 300 km) earthquakes are fundamentallydifferent from those of shallow (≤ 40 km) earthquakes. This is because the frictional strength ofsilicate rocks is proportional to the confining pressure and it exceeds the upper limit of the stress levelin the upper mantle (< 300 MPa: Obata and Karato, 1995) at pressures higher than 1 GPa (~30 kmdepth). Furthermore, brittle fracture associating dilatancy is difficult at high pressures. The fracture
strength of silicate rocks is much higher than 300 MPa at upper mantle pressures due to the positive
pressure dependence of the strength (Masuda et al., 1987). Therefore, the cause of intraslab seismicity
at intermediate depths have been attributed to dehydration of serpentinite (i.e., the dehydration
embrittlement model: e.g., Peakock, 2001) because the water released during dehydration reaction ofserpentinite reduces the effective confining pressure. The dehydration embrittlement model is now
widely accepted, because the location of the double seismic zone in the subducting Pacific slab
corresponds to the main dehydration field in the pressure-temperature diagram of the hydrous
peridotite (Omori et al., 2002). However, a recent experimental study using the techniques of acoustic
emission (AE) monitoring and in-situ x-ray diffraction showed that antigorite-rich serpentinite
samples produced no detectable AEs in the samples in the course of their dehydration. Anotherexplanation for the origin of intermediate-depth earthquakes is the hypothesis of a periodic shearheating mechanism (Kelemen & Hirth, 2007). The occurrence of ultramafic pseudotachylite in naturalperidotite shear zones supports the validity of the shear-heating mechanism. The hypothesis of aperiodic shear-heating mechanism explains the origin of seismicity in the dry upper mantle.
To investigate the origin of intraslab earthquakes at intermediate depths, I conducted uniaxial
deformation experiments on anhydrous dunite and wet harzburgite at pressures 1-3 GPa andtemperatures 600-1100 degC with a constant displacement rate using a deformation-DIA apparatus.
Pressure, stress, and strain were measured in situ by using x-ray diffraction patterns and radiographies.
AEs were also recorded continuously on six sensors, and three-dimensional AE source location weredetermined. At temperatures lower than 950 degC, samples tend to develop throughgoing faults. Flowstrength was higher than 1 GPa, and a sudden stress drop (1-2 GPa) associated with faulting wasobserved. AEs were recorded during sampled deformation at strains higher than 1E-4 s-1 and attemperatures below 1000 degC. The b-value was usually higher than 1 at the primary phase and it
decreased to < 1 just before a mainshock. At temperatures higher than 1100 degC, AEs were hardly
recorded (i.e., ductile flow). I will report the details of the localized heating, which is the cause of
faulting in deforming samples at high pressures.