ESR & OSL analyses of seismic frictional heating events recorded in fault gouge and their application to deep drill core samples

点击翻译按钮获取中文摘要

We 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 FMR (ferrimagnetic resonance) signal derived from bulky trivalent iron ions in ferrimagnetic iron oxides (a^^~-Fe_2O_3: maghemite). 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 10^2〜10^3 times larger than those of the surrounding fault gouge and are proportional to the FMR signal intensities. We reconstructed the magnetization process of fault gouge during ancient seismic fault slip using the FMR signal. As a result, the maximum instantaneous magnetization of the Nojima fault gouge may have reached about 10-20nT and may have further induced a maximum geoelectric potential change of about 120mV/km^2 by electromagnetic induction. F … More urthermore, we carried out high-speed shearing tests using the dry fault gouge. As a result, we have found that the frictional heat temperature in a dry fault gouge zone remains a high temperature such as 600℃ even though the frictional coefficient suddenly decreases due to thermal pressurization with the release of absorptive and interlayer waters by frictional heating. The dry fault gouge changes into a black ferrimagnetic material (a^^~-Fe_2O_3: maghemite) by high-speed shearing. The coercive force obtained from the black ferrimagnetic material after high-speed shearing is much smaller than that after heating in an electric furnace. This indicates that we can determine whether it is pulsated heating or long-term heating by estimating the magnitude of coercive force. On the other hand, we studied physical basis of optically stimulated luminescence (OSL) emitted from feldspars. The blue peak detected from feldspars in the Nojima fault gouge by UV-stimulations completely decays by heating at 600℃ and at the same time another broad yellow peak appears. This yellow peak also completely decays by heating until 870℃. These results indicate that these OSL peaks are applicable to the detection of seismic frictional heat. In case of feldspars in the Nojima pseudotachylyte, the blue and red peaks have been much more decreased. This suggests that the feldspars have been subjected to high temperature by ancient seismic frictional heating. Less