Jun Muto

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Effect of stress state on slow rupture propagation in synthetic fault gouges

Earth, Planets and Space, 2015

Slow slip events (SSEs) in subduction zones are known to proceed so sluggishly that the associate... more Slow slip events (SSEs) in subduction zones are known to proceed so sluggishly that the associated slow ruptures do not generate any detectable radiating seismic waves. Moreover, they propagate at speeds at least four orders of magnitude slower than regular earthquakes. However, the underlying physics of slow slip generation has yet to be understood. Here, we carry out laboratory studies of unstable slip along simulated fault zones of lizardite/chrysotile (liz/ctl) and antigorite (i.e., low-and high-temperature serpentine phases, respectively) and olivine, under varying conditions of normal stress, with the aim of better understanding the influence of stress state on the process of slow rupture along the plate interface. During a single unstable slip, we clearly observe a slow rupture phase that is often followed by an unstable, high-speed rupture. We find that lower fault-zone friction coefficients (μ values from 0.7 down to 0.5) lead to increasing degree of the slow rupture mode, and also that the slow rupture velocities (V r = 0.07 to 5.43 m/s) are largely consistent with those of short-term SSEs observed in nature. Our findings suggest that the generation of SSEs is facilitated by conditions of low normal stress and low fault-zone strength along the plate interface, which may be weakened by metamorphic reactions that result in the production of hydrous phases (e.g., serpentine) and/or the direct involvement of fluid itself, leading to a reduction in effective normal stress.

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Frictional discharge plasma and seismo-electromagnetic phenomena

Physics of the Earth and Planetary Interiors, 2008

Although several mechanisms of precursory electromagnetic emissions of earthquakes have been prop... more Although several mechanisms of precursory electromagnetic emissions of earthquakes have been proposed, there has been no in situ observation of electromagnetic phenomena occurring at the frictional contacts during rock frictions. From an experiment simulating the motion of an asperity on a fault surface, we report the generation of photon emissions around frictional contacts between natural rock minerals. Spectroscopic analysis clarified that the photon emissions are caused by dielectric breakdown of ambient gases (electric-discharge plasma) due to frictional electrification. The plasmas were found to occur under normal stress of 4MPa and sliding speed of 10−2m/s. This indicates that plasma generations do not require high-normal stresses needed to fracture rocks and sliding speeds as high as seismic rates (∼1m/s). Thus, frictional discharges could occur at microscopic asperities at the onset of the slips of earthquakes, and might be one of the sources of the seismo-electromagnetic emission.

Frictional Discharge from Sheared Asperities: Implication for Fractal Seismo-electromagnetic Radiation

IEEJ Transactions on Fundamentals and Materials, 2006

To clarify the fractal natures of electromagnetic radiations prior to earthquakes, we conducted a... more To clarify the fractal natures of electromagnetic radiations prior to earthquakes, we conducted a friction experiment simulating the essential elements of the motion of an asperity on a fault plane, and observed the generation of photon emissions around frictional contacts due to the dielectric breakdown of ambient gases by frictional electrification. According to the fractal distribution of fault asperities, we show that the frictional discharge (plasma generation) at asperities on the fault plane can be one of origins of the fractal seismo-electromagnetic radiation prior to earthquakes.

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Skin Depth of Electromagnetic Wave through Fractal Crustal Rocks

IEEJ Transactions on Fundamentals and Materials, 2010

Skin depth of electromagnetic (EM) wave depends on frequency of EM wave ν and electrical properti... more Skin depth of electromagnetic (EM) wave depends on frequency of EM wave ν and electrical properties of rocks and minerals. Previous studies have theoretically assumed that the skin depth L α (ν) can be expressed as a function of frequency ν by L α (ν) ∝ ν -φ and φ = 1 at high frequency or φ = 1/2 at low frequency. Based on fractal theory of rocks, we point out that the frequency exponent φ reflects internal fractal structures (i.e., occupancy, distribution and connectivity) of dielectric/conductive matrices of rocks such as pores, cracks, grain boundaries, inclusions and various fluids. Laboratory measurements of dielectric constant and conductivity of granite and previous studies on various rocks as a function of frequency show that φ is an exponent ranging from 1/4 to 1. By extrapolation of the skin depth by laboratory measurements at a given frequency into at other frequencies, the skin depth with variation in φ becomes longer or shorter than that by previous studies. Moreover, at a given frequency, the skin depth decreases with increasing a fractal dimension of fracture systems (decreasing φ). Thus, the skin depth of EM wave through the crust for detecting seismo-EM radiations and through rock salt domes for detecting ultra-high energy neutrinos depends on fractal structures of dielectric/conductive matrices in heterogeneous crust.

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Electric potential changes associated with nucleation of stick-slip of simulated gouges

Tectonophysics, 2011

Our friction experiments using simulated gouges detected premonitory changes in electric potentia... more Our friction experiments using simulated gouges detected premonitory changes in electric potential before stick-slip events. These precursor electric signals have been detected both in piezoelectric quartz and nonpiezoelectric gabbroic gouges. The changes in the potential were locally detected by electrodes set very close to the fault surface associated with slow slip prior to stick-slip events. The magnitude of electrification is proportional to fault slip associated with gradual stress releases, indicating that the electrification is slipdependent process. From the detailed microstructural analyses on a representative sample, Riedel (R 1 ) shears known to be formed during stable sliding were the most densely developed around the electrode pair which detected the precursory voltage changes. This indicates that local increases in the voltages were likely caused by frictional electrification due to slow slip on R 1 shears in nucleation phases of stick-slip events. Our experimental results imply that natural faults with thicker gouge zones would require greater precursory slips, resulting in larger electric signals in the nucleation phase of earthquakes.

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Frictional discharge plasma from natural semiconductor/insulator junctions: Origin of seismo-electromagnetic radiation

Physics and Chemistry of the Earth, Parts A/B/C, 2006

By a pin-on-disk frictional experiment simulating the motion of an asperity on a fault plane, we ... more By a pin-on-disk frictional experiment simulating the motion of an asperity on a fault plane, we observed photon emissions from the gap between the quartz disk and pins made of natural semiconductor or insulator minerals during friction. The patterns and intensities of photon emissions depended significantly on the gas pressure, on the normal stress and on the shape and material of the pins. The frictional contacts made of different grain sets or ore veins can be regarded as natural semiconductor/insulator junctions arising from the difference in electron states or work functions between the contacts. During seismic activities, such semiconductor/insulator junctions in natural fault zones would generate discharge plasmas that could be one of origins of seismo-electromagnetic phenomena. Finally, in addition to its effect on the geoelectromagnetic perturbations, the significances of the discovery of discharge plasmas on geochemical reactions on a fault zone arising from tectonic activities are discussed.

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Electric-discharge Luminescence between Frictional Natural Quartz by Pin-on-disk Method: Possible Origin of Seismo-electromagnetic Radiation

To elucidate mechanisms of electromagnetic radiations before and during earthquakes, electric-dis... more To elucidate mechanisms of electromagnetic radiations before and during earthquakes, electric-discharge luminescence around a frictional quartz interface was investigated using a vacuum tribometer with a pin-on-disk arrangement. A pin with a quartz tip slides on a quartz disk under various normal forces and sliding velocities. The disk and the pin were made from a single crystal of Brazilian quartz. The quartz disk was cut normal to c-axis and polished on both sides. The thickness and the radius of the disk were 1 mm and 17.5 mm, respectively. An optical microscope and a charge coupled device (CCD) video camera were used to take luminescence images through the transparent quartz disk. The frictional experiments can be conducted under sliding velocities ranging from 10 mm/s (0.1 rps) to 102 mm/s (1 rps) and normal forces ranging from 50 mN to 150 mN. From the total area of contact points, normal stress can be estimated ranging from 4 MPa to 11 MPa. All experiments were performed unde...

Self-affinities of landforms and folds in the Northeast Honshu Arc, Japan

Acta Geophysica, 2013

ABSTRACT A method to analyze self-affinities is introduced and applied to the large scale fold ge... more ABSTRACT A method to analyze self-affinities is introduced and applied to the large scale fold geometries of Quaternary and Tertiary sediments or geographical topographies in the inner belt of the Northeast Honshu Arc, Japan. Based on this analysis, their geometries are self-affine and can be differently scaled in different directions. We recognize a crossover from local to global altitude (vertical) variation of the geometries of folds and topographies. The characteristic length for the crossover of topographies (landforms) is about 25 km and is related to the half wavelength of the crustal buckling folds or possible maximum magnitude of inland earthquakes in the Northeast Honshu Arc. Moreover, self-affinity of the folds and topographies can be connected with the b-value in Gutenberg-Richter℉s law. We obtain two average Hurst exponents obtained from the self-affinities of folds in the Northeast Honshu Arc. This indicates that there are two possible seismic modes for the smaller and larger ranges in the focal regions in the Northeast Honshu Arc.

Water distribution in dynamically recrystallized quartz grains: cathodoluminescence and micro-infrared spectroscopic mapping

Geological Society, London, Special Publications, 2005

The distribution of water in dynamically recrystallized quartz aggregates in granitic mylonites w... more The distribution of water in dynamically recrystallized quartz aggregates in granitic mylonites was investigated by cathodoluminescence (CL) observations and micro-infrared (IR) spectroscopic mapping. It is clear from CL observations and micro-IR spectroscopic mappings that the CL intensity and contrast depend on the density of defects (e.g. the content of molecular H2O and absorbed OH species). Dynamic recrystallization, as a recovery process of intracrystalline strain, decreases the densities of excess carriers originated from defects. The decrease in the densities of excess carriers decreases the CL intensity and contrast within each grain. To investigate the effect of water weakening in the rheological properties of the crust, CL spectroscopy and micro-IR spectroscopic mapping are essential in the determination of concentration and distribution of water in natural high-strain zones.

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