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

Abstract

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 ∼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 (γ‐Fe2O3). The FMR signal is produced by the thermal dehydration of antiferromagnetic iron oxides (γ‐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 ∼390 m depth may have attained ∼390°C during the 1995 Kobe earthquake. The temperature in the fault plane may have returned to its initial state after ∼1 year. This result suggests that a heat flow anomaly generated by faulting may be difficult to detect.

Magnetic properties of fault pseudotachylytes in granites

“…A number of reactions, driven by frictional heating, may account for the observed increases of magnetic susceptibility in fault rocks. Two examples of such reactions are: (R 1 ) siderite ⇒ pyrite or pyrrhotite ⇒ magnetite, under low oxygen fugacity [ f O 2 ] (Chelungpu Fault [Tanikawa et al, 2008]); (R 2 ) goethite ⇒ hematite, under higher f O 2 (Nojima Fault [Fukuchi et al, 2005] Fukuchi, 2003;Petrík et al, 2003]. However, other factors such as host rock composition and degree of melting also affect f O 2 and thus the magnetic properties of pseudotachylytes [Henkel and Reimold, 2002;Nakamura and Iyeda, 2005;Zechmeister, 2005;Hirono et al, 2006].…”

“…These mid-Proterozoic and Caledonian veins, from a few millimeters up to several meters thick, reveal a strong remanent magnetization (Q ratios = 10 to 100), carried by single domain (SD) to pseudo-single domain (PSD) magnetite grains. Fukuchi [2003] and Fukuchi et al [2005] studied pseudotachylytes from the Nojima Fault, Japan, responsible for the devastating 1995 Kobe earthquake (M = 7.2). These veins originated from cataclasis and not from frictional melting [Lin, 2001].…”

“…[4] Nevertheless, recent investigations on borehole cores through the Nojima Fault, Japan, and the Chelungpu Fault, Taiwan [Fukuchi, 2003;Fukuchi et al, 2005;Hirono et al, 2006] revealed that magnetization acquisition processes could be more complex than a simple TRM and could also include a chemical remanent magnetization (CRM) as a major component of the NRM. Although TRM acquisition is inevitably coseismic due to the quasi instantaneous cooling of the pseudotachylyte melt [Nakamura et al, 2002;Ferré et al, 2005], the timing of CRM acquisition is less well understood.…”

ESR Techniques for the Detection of Seismic Frictional Heat

“…Since ferrimagnetic minerals commonly show huge ESR absorption due to their spontaneous magnetization, we can detect them as FMR (ferrimagnetic resonance) signals using the ESR technique. Detailed ESR analyses showed that the growth process of FMR signals during heating may fundamentally follow the zero-order reaction kinetics (Fukuchi, 2003;Fukuchi et al, 2005). Therefore, we can use FMR signals as an effective index of frictional heat.…”

“…Thus, we had required any new technique to directly estimate frictional heat from fault rocks. Recent ESR (electron spin resonance) and magnetic studies of fault zones revealed that fault rocks may have been magnetized due to the thermal decomposition of iron-bearing paramagnetic or antiferromagentic minerals included in host rocks into ferrimagnetic ones by frictional heating (Fukuchi, 2003;Fukuchi et al, 2005Fukuchi et al, , 2007Ferré et al, 2005;Han et al, 2007). Since ferrimagnetic minerals commonly show huge ESR absorption due to their spontaneous magnetization, we can detect them as FMR (ferrimagnetic resonance) signals using the ESR technique.…”