Fluidization and melting of fault gouge during seismic slip: Identification in the Nojima fault zone and implications for focal earthquake mechanisms

Abstract

The record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active Nojima fault in Japan. The fault rocks formed at about 3 km depth, and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite. Each layer is thinner than a few millimeters, and corresponds to one seismic slip event. The very thin slip zone width suggests that some mechanisms of slip weakening operated, and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important. Fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method. It is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains. Once fluidization occurs, the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state. When fault gouge is melted, frictional resistance is governed by the viscosity of melt which depends mainly on temperature, chemical composition, H2O concentration and the volume fraction of unmelted grains. For each pseudotachylyte layer, we estimated the temperature of melt using various temperature indices, and measured volume fraction of unmelted grains. We synthesized these data to reconstruct the change in viscosity during seismic slip events. The melt viscosity is very high (107–9 Pas) during initial melting due to the combined effect of low temperature (750°–800°C) and large volume fraction of solid grains. Thus seismic slip is inevitably restrained instantaneously. Once this mechanical barrier is overcome, the viscosity reduces continuously and dramatically as slip increases. At 1000°C, the viscosity reduces to 104Pas and eventually to 10Pas at 1280°C. Thus stress drops almost completely and rupturing tends to run away.

Apatite fission‐track dating of fault‐related rocks along the Nojima and Kusumoto faults and its tectonic significance

“…These faults, including the Nojima fault, were activated mainly by Quaternary east-west compressional stress [e.g., Oike and Huzita, 1988;Toda et al, 1998;Yokokura et al, 1998]. Fault-related rocks, such as pseudotachylyte and cataclasite, have also been observed both from drill cores [Boullier et al, 2001;Tanaka et al, 2001] and outcrops Fukuchi, 2003;Otsuki et al, 2003] from the Nojima fault. The existence of pseudotachylyte at the surface implies that significant uplift of $15 km may have occurred [Boullier et al, 2001], whereas geophysical and geomorphological data suggest the total uplift along the Nojima and adjacent faults is less than $3 km [Yokokura et al, 1998;Murata et al, 2001].…”

“…The northwestern and northeastern sides of the island are bounded by two active faults, the Nojima and Kusumoto faults, respectively, and the axial part of the island has been uplifted since the Pliocene [Otsuki et al, 2003] or Pleistocene . The total uplift by the Nojima fault was estimated at $500 m since 1.2 Ma by measuring displacement across the fault ] and a reflection survey across the Osaka Bay, east of Awaji Island, revealed that the fault-bounded block forming Awaji Island has been uplifted $3 km relative to the subsiding Osaka Bay [Yokokura et al, 1998].…”

“…[2] The Nojima fault (Figure 1), whose movement during the 1995 Kobe earthquake (M7.2 or M w 6.9) caused serious damages to the urban Kobe area, southwest Japan, has become one of the most intensively studied active faults [e.g., Ohtani et al, 2000;Tanaka et al, 2001;Fukuchi, 2003;Otsuki et al, 2003]. It is situated in the inner zone of southwest intraplate Japan (Figure 1a), where numerous active faults with $10-km length exist (Figure 1b).…”

ESR Techniques for the Detection of Seismic Frictional Heat

“…After the Kobe earthquake, the black fault rock with high magnetic susceptibility was found in the Nojima fault zone (Otsuki et al, 2003;Fukuchi, 2003). This fault rock consists of multiple sheets of a few mm wide veins, each of which was produced from granitic fault gouge during ancient earthquakes (Fig.…”

Rock-Fluid Interaction Along Seismogenic Faults Infered from Clay Minerals in Okitsu Mélange, the Cretaceous Shimanto Belt, SW Japan

“…Various mechanisms for seismogenesis have been proposed, such as frictional melting (Sibson, 1975;Spray, 1992), thermal pressurization (Mase and Smith, 1987;Melosh, 1979;O'Hara et al, 2006;Sibson, 1977;Wibberley and Shimamoto, 2005), acoustic fluidization (Melosh, 1979;Otsuki et al, 2003), elastohydrodynamic lubrication (Brodsky and Kanamori, 2001), and silica gel lubrication (Di Toro et al, 2006). Some of these are related to frictional heating.…”