Ferrimagnetic resonance signal produced by frictional heating: A new indicator of paleoseismicity

[1] High-speed fault slips during earthquakes may generate sufficient frictional heat to produce fused fault rocks such as pseudotachylyte. We have carried out high-speed slip tests using natural fault gouge to judge whether or not frictional heating universally occurs during seismic fault slips. In our shearing tests, natural fault gouge is put between two cylindrical silica glasses and sheared under a fixed axial stress of 0.61 MPa. Despite such a low stress near the Earth's surface, a darkened cohesive material resembling pseudotachylyte is made from the fault gouge along the edge of a circular shear plane when shearing at a high speed of 1500 rpm ( the maximum slip rate reaches similar to 1.96 m/s at the edge). Electron spin resonance measurements reveal that the darkened cohesive material has a strong ferrimagnetic resonance (FMR) signal, which is derived from bulky trivalent iron ions in ferrimagnetic iron oxides (gamma-Fe2O3). The FMR signal is produced by the thermal dehydration of antiferromagnetic iron oxides (gamma-FeOOH) in the fault gouge. This may be applicable to the detection of past heating during seismic fault slip. We thus attempt to reconstruct the temperature of frictional heat generated on the Nojima fault plane in the 1995 Kobe earthquake ( M = 7.3) by inversion using the FMR signal. The computer simulation indicates that the frictional heat generated on the Nojima fault plane at similar to 390 m depth may have attained similar to 390 degrees C during the 1995 Kobe earthquake. The temperature in the fault plane may have returned to its initial state after similar to 1 year. This result suggests that a heat flow anomaly generated by faulting may be difficult to detect.