ESR Detection of Frictional Heat in Seismic Fault Slip

Electron spin resonance (ESR) is a spectroscopic method to detect paramagnetic defect centers or impurities in materials. Fukuchi (2001) measured ESR spectra of the Nojima pseudotachylyte found along the Nojima fault that caused the 1995 Kobe Earthquake (M 7.2) in Japan. There has been controversy as to whether frictional melting or crushing is more essential for the origin of the Nojima pseudotachylyte. As a result of the ESR measurement, the Nojima pseudotachylyte gave a huge ESR signal of bulky trivalent iron (Fe³⁺) ions derived from ferrimagnetic iron oxides (γ -Fe₂O₃). It has turned out that the bulky Fe³⁺ ion signal is produced by heating the Nojima fault gouge and that the fault gouge changes from an ocherous paramagnetic material to a black ferrimagnetic one with heating. We carry out high-speed frictional experiments using a new rotary-shear high-speed frictional testing machine to confirm that frictional heat in faulting can produce the black material like pseudotachylyte. By shearing under the condition that the axial stress is 0.61 MPa, the equivalent speed is 1.74 m/s and the average displacement is about 15m, the fault gouge changes into the pseudotachylyte-like material with a strong bulky Fe³⁺ signal. From the fault gouge after the shearing, the traces of dehydration are observed along the shear plane. This means that hot fluids dehydrated by frictional heating pass through the fault gouge along the fault plane in natural faulting. Heat and mass transfer may occur with the hot fluid flow. Furthermore, we discuss how to estimate the temperature of frictional heat using the bulky Fe³⁺ signal. If we use the bulky Fe³⁺ signal detected from the Nojima pseudotachylyte, the maximum temperature in faulting is estimated as about 600° C. This result supports that the Nojima pseudotachylyte is a crushing-originated one. >http://www.cc.yamaguchi-u.ac.jp/~fukuchi/index.html</a>