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

我们用电子自旋共振技术研究了1995年日本神户7.2级地震中沿野岛断裂发现的野岛假玄武岩。Nojima假黄铁矿具有很强的FMR(铁磁共振)信号，来源于铁磁性铁氧化物(a^^~-Fe_2O_3：磁铁矿)中的大体积三价铁离子。这种铁磁共振信号是通过加热周围的断层泥而出现的，断层泥是野岛假硅质岩的来源物质。此外，Nojima假玄武岩的磁化率比周围断层泥的磁化率大10^2~10^3倍，且与FMR信号强度成正比。利用铁磁共振信号重建了古地震断层滑动过程中断层泥的磁化过程。因此，Nojima断层泥的最大瞬时磁化强度可能已达到约10-20nT，并可能进一步通过电磁感应引起约120 mV/km2的最大地电势变化。F…此外，我们还利用干断层泥进行了高速剪切试验。结果表明，干断层泥中的摩擦热温度保持在600℃左右，即使摩擦热释放出吸附水和层间水，使摩擦系数骤然降低。干燥的断层泥在高速剪切作用下转变为黑色亚铁磁性物质(a^^~-Fe_2O_3：磁赤铁矿)。黑色亚铁磁性材料经高速剪切后的矫顽力比在电炉中加热后的要小得多。这表明，我们可以通过估计矫顽力的大小来确定它是脉动加热还是长期加热。另一方面，我们研究了长石光激发发光(OSL)的物理基础。从野岛断层泥中的长石中检测到的蓝峰在600℃的加热下完全衰变，同时出现另一个宽阔的黄色峰。这个黄色的峰也会通过加热到870℃而完全衰变。这些结果表明，这些OSL峰适用于地震摩擦热的检测。以野岛假玄武岩中的长石为例，蓝峰和红峰的减少幅度要大得多。这表明这些长石受到了古代地震摩擦加热的高温影响。较少

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