Strong ferrimagnetic resonance signal and magnetic susceptibility of the Nojima pseudotachylyte in Japan and their implication for coseismic electromagnetic changes

Abstract

Pseudotachylyte is a glassy fault rock providing certain evidence of ancient seismic fault movement. I examine the Nojima pseudotachylyte found along the Nojima fault, which caused the 1995 Kobe earthquake (M = 7.2) in Japan, using the electron spin resonance technique. The Nojima pseudotachylyte has a strong ferrimagnetic resonance (FMR) signal derived from bulky trivalent iron ions in ferrimagnetic iron oxides. This FMR signal appears by heating the surrounding fault gouge that is the source material of the Nojima pseudotachylyte. Furthermore, the magnetic susceptibilities of the Nojima pseudotachylyte are 102–103 times larger than those of the surrounding fault gouge and are proportional to the FMR signal intensities. Here I reconstruct the magnetization process of fault gouge during ancient seismic fault slip using the FMR signal. The results indicate that the instantaneous magnetization of the Nojima fault gouge may have induced instantaneous geomagnetic changes and that the instantaneous geomagnetic changes may have further induced geoelectric changes by electromagnetic induction.