We examined several previously proposed models of the process by which large intraplate earthquakes are generated, and found the most plausible to be the ‘localized shear model, ’ in which seismogenic faults have downward extensions in the lower crust and the localized shear deformation on the downward extensions accumulate stress on the seismogenic faults. Further, localized shear deformation may accelerate before a large intraplate earthquake occurs. This model can explain various phenomena related to crustal deformation in the Japanese Islands: preslips, seismicity, distribution of active faults, and stress and strain state. It will play an important role in the forecasting and prediction of large intraplate earthquakes.

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High resolution images of crustal structure across the San Andreas Fault (SAF) were obtained by using the common conversion point stacking of teleseismic P-to-S converted waves recorded during the Los Angeles Region Seismic Experiments (LARSE-I and II). In the upper crust, several sedimentary basins were delineated in the images, including the San Fernando and the Santa Clarita Basins. The San Fernando Basin reaches a depth of 8 km under the northern edge of the San Fernando Valley. On the LARSE-I profile, the downward projection of the SAF truncates several lower crustal interfaces including the Moho on both sides. The Moho is vertically offset by as much as 8 km. Along the LARSE-II profile, the impedance contrast and slope of the Moho are seen to change across the fault. These results indicate that the fault penetrates into the lower crust and probably uppermost mantle as a narrow (<10 km) feature. The Moho beneath the San Gabriel Mountains is shallower (-26 km) than under the San Gabriel Valley to the south and the Mojave Desert to the north, suggesting that the mountain ranges were lifted en masse by horizontal compression. On the northeast side of the SAF, the Mojave Desert has a sharp and essentially flat Moho at a depth of -32 km. The lower crustal structure beneath the San Fernando and Santa Clarita Valleys along the LARSE-II profile south of the SAF is complicated as indicated by the large undulation and low impedance contrast of the Moho. These observations suggest that the deformation in the lower crust is localized and often concentrates near boundaries of crustal blocks or beneath those places which have experienced intensive faulting and deformation in the upper crust.

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Seismic images of deformation beneath South Island, New Zealand, are provided by a form of seismic exploration uniquely suited to the study of “continental islands”. double-sided, onshore-offshore seismic methods in conjunction with onshore refraction and teleseismic P-wave delay data. Four sets of independent observations and analysis are use to infer rock properties within this plate boundary zone: seismic and electrical indications of high-fluid pressures within the crust; P-wave delays from teleseismic anisotropy to show a high-speed zone in the mantle directly below the crustal root; Pn anisotropy of 11±3% distributed over a region > 100 km-wide; and an effective elastic thickness (Te) that is vanishingly small beneath the Southern Alps and surface outcrop of the Alpine Fault, but increases to values of Te > 20 km beyond the coastlines of the South Island. Together, these observations show that deformation in the crust and mantle becomes progressively wider with depth. A region of distributed deformation > 200 km wide is inferred for the upper mantle. We propose that the weakness and the wide zone of deformation are phenomena of plate boundaries where both strike-slip and convergence have persisted for several millions of years.

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Distinct phases reflected from mid-crustal reflectors (SxS and PxP phases) were observed in seismograms of aftershocks of 1998 M5.0 Sendai earthquake at nearby stations. We estimated the locations of the reflectors (bright spots) by using arrival time differences between these phases and direct waves. A clear reflector is located in the depth range of 15 to 21 km just beneath the fault plane of the M5.0 event. It dips toward the NNE direction with a dip angle of about 25. Other reflectors are also located beneath the fault plane of the event. Internal structure of the S-wave reflector was estimated from spectral amplitude ratios of reflected SxS-wave to direct S-wave. Observed spectral ratios show that S-wave velocity in the reflector body is -1.1 km/s, and its thickness is about 50 m. This suggests that a thin reflector body (bright spot) partially filled with fluids exists in the lower crust beneath the focal area of the 1998 event.

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An extensive seismic experiment was carried out in June 2001 around the Nagamachi-Rifu fault, northeastern Japan. We deployed seismic arrays on the hanging wall of the reverse fault, and observed seismic waves caused by chemical explosions and vibrators. Several later arrivals from reflectors, whose depths are about 9 to 21 km, are detected in the observed shot gathers. Some of them probably correspond to S-wave reflectors and scatterers previously estimated from the data of natural earthquakes. A clear reflector was found in and below the seismogenic layer at 4 to 4.5-s two-way traveltime (TWTT). It is located at the shallower and deeper extensions of the fault plane of M5.0 earthquake that occurred in 1998. Its extension to the surface seems to connect with the surface trace of the Nagamachi-Rifu fault. These suggest that the Nagamachi-Rifu fault has a low dip-angle in the mid-crust as a detachment fault, if the reflector shows the fault structures.

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A seismic reflection survey using both explosives and vibrators was conducted in June 2001 around the Nagamachi-Rifu fault, northeastern Japan. We carried out observations of four small aperture seismic arrays in the area to reveal detailed structures of the fault. Array analysis was applied to waveform data from 15 explosives to obtain P-wave scatterer distributions in the area. The obtained P-wave scatterer distribution correlates in space with microearthquake activities and heterogeneous structures such as S-wave reflectors, a structure of caldera, and Mohorovicic discontinuity. We could also image that a sub-horizontal layer with a length of about 10 km exists in the deep extension of the Nagamachi-Rifu fault beneath the seismogenic layer.

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The Nagamachi-Rifu fault is an active reverse fault which trends NE-SW across the central part of Sendai City for over 21 km distance. The fault does not emerge at the surface and, accompanied with the Dainenji-yama fault, shows wedge thrusting in the Tertiary sediments. The amount of net slip of the master part of the Nagamachi-Rifu fault is estimated to be one mm/year. Seismic reflection profiles across the fault plus a gravity anomaly reveal the thicker Neogene sediments on the hanging wall rather than on the footwall. The Neogene sedimentary basin was formed by normal faulting in early Miocene under an extensional stress regime associated with the formation of the northern Honshu rift system. Due to shortening deformation since the Pliocene, this Miocene normal fault reactivated as a reverse fault. Judging from the CMP deep seismic reflection profile and location of the 1998 M5.0 Sendai earthquake, the deep geometry of the Nagamachi-Rifu fault is listric.

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Detailed structures of fault low velocity zones (LVZ) have been studied by analyzing fault-zone trapped waves at various active faults. These studies demonstrate that widths of the fault LVZ are ranging from an order of 10 m to a few hundred meters. In order to evaluate the effect of fault LVZ on the earthquake rupture process, a model of the LVZ related to the plastic deformation around an edge of propagating earthquake rupture is proposed. In this study, earthquake rupture processes are regarded as Mode III crack propagation. The LVZ is identical to the fault damaged zone which is related to plastic deformation in the vicinity of the crack tip. The Mode III crack analysis gives a simple relationship between the width of the low velocity zone, the breakdown stress drop at the crack tip, and characteristic slip distance d₀ in friction laws. The parameters applicable to the Nojima fault producing the 1995 Hyogo-ken Nanbu earthquake show that the breakdown stress drop is 10 times larger than the static stress drop and d₀ is about 10 cm. These values are consistent with the value obtained by the other study for the Nojima fault so the present model is applicable for considering earthquake rupture within the damaged zone.

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A gravity survey was conducted in and around the Nagamachi-Rifu Fault. The density for both terrain and Bouguer corrections was chosen to be 2, 300 kg/m³, because volcanic rocks are dominant over the whole measurement area and the surface layer density is estimated to be low from geological considerations. The Bouguer anomalies are characterized by a low anomaly similar to those of a caldera and the basement structure inferred from two and three-dimensional analysis shows that the depth is often more than 1 km in the caldera region and that there is a circular structure. The gradient of basement is steepest at the southern margin, and it resembles to rim of funnel-shaped caldera.

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Fault zone waves have the potential to be a powerful tool to reveal the fine structure of a fault zone down to the seismogenic depth. Seismic fault zone waves include head waves, trapped waves and direct body waves propagating in the fault zone. 3-D numerical simulation is necessary to interpret the waveforms in the presence of low-velocity zones with relatively complex fault structure. We computed finite difference (FD) synthetic seismograms to fit the seismograms of explosions, which contain frequencies up to 25 Hz, recorded by a linear seismometer array across the Mozumi-Sukenobu fault, central Japan. We find fault zone head waves, direct P waves propagating within the low-velocity zone and wave trains following the direct P waves associated with the fault for both observed and synthetic waveforms. Thus, modelling of fault zone waves is expected to determine details of complex fault zone structure.

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We established a dense permanent GPS array around the northern and central Itoigawa-Shizuoka Tectonic Line (ISTL) in order to study loading processes of inland active faults. 1-4 years observation has revealed concentrated deformation (about 0.3 ppm/yr) around the East Matsumoto-Basin Fault, an active fault along the ISTL, consistent with historic triangulation data. The deformation pattern is explained by dislocation models incorporating horizontal detachment fault with steady slip in the upper crust. However, deformation data alone cannot determine the model and there exist different possibilities. The vicinity of the Gofukuji fault is being deformed less in spite of the large long-term slip rate. These observations indicate that the deformation pattern is laterally heterogeneous along the ISTL. The two faults are considered to be loaded by different mechanisms.

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The Median Tectonic Line (MTL) is the longest arc-parallel fault system in southwest Japan whose right-lateral strike-slip is related to oblique subduction of the Philippine Sea plate (PH). We constructed a dense Global Positioning System network along a 200 km-long traverse line across the MTL in 1998 to estimate deep fault structure and slip distribution. Horizontal velocities were determined at 65 sites through campaign measurements and show crustal shortening in the direction of the plate convergence. Using multi-rectangular segments and depthdependent coupling at the plate interface, we calculate and remove elastic deformation caused by the PH subduction. The residual velocity field shows right-lateral strike-slip block motion of about 5 mm/yr across the MTL, consistent with geological estimates. However, the block boundary does not coincide with the surface trace of the MTL, being displaced 20-30 km to the north. The residual velocity field is reproduced by a model with a 35-45° northwarddipping fault plane, full locking of the upper portion to a depth of 15 km, and steady slip of 5 mm/yr below. GPS results are supported by imaging of an inclined fault plane revealed by seismic profiling and currently low activity of shallow earthquakes.

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A dense GPS network was established in 1997 around Ou Backbone Range (OBR), northeastern Japan, by deploying 28 continuous GPS stations to complement the sparse portion of GEONET operated by the Geographical Survey Institute of Japan. The aim of the network is to investigate the present surface deformation and understand the relationship between earthquake occurrence and the deformation process of the island-arc crust. Our GPS data are analyzed using a precise point positioning strategy of GIPSY/OASIS-II. Results of GEONET in daily SINEX files have been supplied by the GSI. Producing grid data of horizontal velocities and taking spatial derivatives, we derived a map of strain rate distribution. The results show that the region between 38.8° and 39.8°N in the OBR experiences notable concentration of east-west contraction. The region coincides with the area of active seismicity, including the focal areas of large earthquakes occurring in 1896 (M7.2), 1900 (M7.0), 1962 (M6.5), 1970 (M6.2), and 1998 (M6.1) . Observed strain is larger than can be explained by the total moment release of earthquakes that occurred in the same period as this study. Possible sources of strain concentration may be viscoelastic deformation due to large earthquakes and/or aseismic slip along the deeper extension of the active faults.

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A numerical simulation of seismic cycles is performed using a two-dimensional model with a vertical strike-slip fault in an elastic layer overlying a Maxwellian viscoelastic half-space, where the frictional stress on the fault is assumed to obey a rate- and state-dependent friction law. Simulated seismic cycles in the viscoelastic Earth model are nearly the same as those in a uniform elastic half-space model. The simulated postseismic deformation on the Earth's surface due to viscoelastic relaxation is significant for time duration comparable to the viscoelastic relaxation time following the occurrence of an earthquake, and after that the deformation due to aseismic sliding of the fault dominates over that due to viscoelastic relaxation.

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The cut-off depth of seismicity in and around the Nojima fault broken by the 1995 Kobe earthquake occurring in the intraplate Japan was compared with a brittle-ductile transition depth of the widely-accepted strength profile model of the crust. It was found that the cut-off depth is much deeper than the transition depth under the assumption that wet granite is deformed at a strain rate of 10^-15/s. Such a small strain rate implies that the plastic flow is uniformly distributed below the seismogenic region. When the strain rate is assumed to be greater than 10^-13, the cut-off depth can be attributed to the transition depth. This suggests that deformation is localized in a narrow fault zone below the seismogenic region even in the intraplate region.

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In order to obtain information about the deformation and fluid-flow processes in seismogenic regions, we designed and constructed an original Japanese-type gas-medium deformation apparatus capable of achieving both high pressure and high temperature. The present apparatus can produce a basic environment in which the confining pressure reaches 200 MPa by argon gas, pore pressure reaches 200 MPa either by argon gas or water, and temperature reaches 800°C. We report a new gas-medium apparatus for rock deformation, presenting preliminary results as an example of test results.

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The occurrence of mylonite and cataclasite, mineral assemblages of cataclasite, and the K-Ar ages of surrounding granitic rocks and dikes were studied to examine the possibility that the Hatagawa Fault Zone (HFZ), NE Japan was experienced under the conditions of the brittle-plastic transition. The Hatagawa Fault Zone is divided into three structural settings: mylonite zones with a sinistral sense of shear and a maximum thickness of 1 km, a cataclasite zone with a maximum thickness of about 100 m, and locally and sporadically developed small-scale shear zones. Occurrence of epidote and chlorite, lack of montmorillonite in cataclasite, and the coexistence of cataclasite and limestone mylonite suggest that the cataclasite was deformed at temperatures higher than 220°C. Crush zones in the mylonite near the cataclasite zone were recognized in one outcrop; they have a structure concordant with the surrounding mylonite and some fragments in them are dragged plastically. Granodiorite porphyry dikes near the HFZ intruding into cataclasite and mylonite with a sinistral sense of shear exhibit no deformational features. K-Ar ages of hornblende from host granitic rocks and from one granodiorite porphyry dike are 126 ± 6 to 95.7 ± 4.8 and 98.1 ± 2.5 Ma, respectively. These indicate that the fault activity gradually changed from mylonitization to cataclasis within 28 m. y., and suggest that the HFZ underwent a brittle-plastic transition during its activity.

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A new approach for revealing the brittle origin of C-surfaces as localized high shear strain zones in S-C mylonites (mylonites with C-surfaces cutting through a mylonitic S-foliation) is presented. A compiled worldwide catalog of width (W) and displacement (D) data for shear zones indicates that ductile mylonites show a constant W/D ratio of 10^-0.3 and ratios of brittle ‘cataclasites’ vary in magnitude from 10^-1 to 10^-3, implying that the ratio is a diagnosis for discriminating ductile and brittle shear zones. A newly measured W-D data of shear displaced minerals along C-surfaces in granitic S-C mylonites from the Hatagawa shear zone in northeast Japan is added on the worldwide W-D catalog, being plotted on a brittle origin with the high W/D ratio of 10^-1.5. Using this result and a tribochemical wear theory which accounts for wear formation under hydrothermal conditions, C-surfaces in the S-C mylonite might have been formed by cataclastic deformation under the lithosphere stress level of ca. 300 MPa at temperature of 400°C with water for granite. This result suggests a high lithosphere stress level at the depth of the S-C mylonite formation where deformation is predominantly plastic.

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The thickness of seismogenic crust layer correlates with surface heat flow in most interplate seismic areas of the world (e. g., Sibson, 1982) . Although the inverse relationship between heat flow and the base of seismogenic zone is obvious, the quantitative relationships are less certain and there should be variability of the focal depths among different tectonic settings. Comparisons of the heat flow (Yamano et al., 1997), thermal gradient (Tanaka et al., 1999) and earthquake (Japan Meteorological Agency, JMA) databases for the northeastern Japan provide detailed geologic and geophysical information about the earthquake process of island arc. Temperatures in the crust were calculated using a steady-state, one-dimensional, heat conductive transport model with heat generation as a function of heat flow and thermal gradient. The evaluated temperatures for D₉₀, the depth above which 90% of earthquakes occur, range between 200°C and 500°C except for high heat flow and thermal gradient data. The consistency of temperature for D₉₀ over a large depth interval supports that the temperature is the dominant factor governing the focal depth in the crust.

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We investigated the deep crustal resistivity structure across Itoigawa-Shizuoka Tectonic Line (ISTL), one of the most dangerous active intraplate faults in Japan, by use of wide-band magnetotelluric (MT) method. The 28 MT stations were aligned perpendicular to the ISTL. A two-dimensional model was created in transverse magnetic (TM) mode where electric currents flow in N60°W-N120°E directions. The model showed good correlations with the surface geology. In particular, we found a thick (-6 km) surface conductor to the east of ISTL which corresponds to the heavily folded sedimentary layer. The Japan Alps to the west of the ISTL is characterized by the resistive upper crust, where the pre-Tertiary rocks crop out. The Japan Alps is underlain by a conductor below 15-20 km depth, which is consistent with the low seismic velocity anomaly. We also found a localized shallow conductor corresponding to the Mt. Tateyama volcano. The most important feature is the conductor in the mid-crust directly under the area of active folding to the east of the ISTL. This may imply a localized zone of fluids because of the enhanced porosity in a shear zone. The recent seismicity clusters in the resistive crust underlain by the conductor, and this suggests the fluid involvement in earthquake generation processes.

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The aseismic parts of shear systems at mid- to deep crustal levels can localise the supply of deeply-sourced, high pressure fluids into the shallower level parts of these systems in the seismogenic regime. Even during deformation at elevated temperatures in mid- to deep crustal shear zones, high pore fluid factors promote grainscale to macroscopic fracture growth and permeability enhancement. The evolution of permeability is governed by dynamic competition between crack growth and crack sealing/healing processes. Steady state creep below the seismic-aseismic transition leads to steady state permeability and continuous fluid flow. In contrast, within and near the base of the seismogenic regime, large cyclic changes in permeability can lead to episodic fluid flow and fluctuations in fluid pressure. At mid-crustal depths, temporal and spatial variations in pore fluid pressure and shear stress within shear networks influence rupture nucleation via cyclic changes in shear strength. Fluid pressure and shear stress cycling can also drive repeated transitions between interseismic creep and rapid, co-seismic slip. Reaction-weakening and reaction-strengthening, during hydrothermal alteration in fluid-active shear systems, can also drive transitions between seismic and aseismic behaviour on longer time-scales.

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Rock alteration and geochemistry of the fault rocks are examined to infer the characteristics of the fluid phase related to the ancient fault activity. The Hatagawa Fault Zone, northeast Japan, is an exhumed seismogenic zone which is characterized by close association of brittlely and plastically deformed fault rocks mostly derived from Cretaceous granitoids. Epidote and chlorite are dominant alteration minerals in both rocks. However, calcite is characteristically developed in the cataclastic part only. Decrease in oxygen isotope ratio and existence of epidote and chlorite, even in weakly deformed granodiorite, is evidence of water-rock interaction. The water/rock ratio is interpreted to be relatively small and fluid chemistry is buffered by host rock chemistry in the mylonite. The occurrence of calcite in brittle structures is explained by changes in water chemistry during shear zone evolution. CO₂-rich fluid was probably introduced during cataclastic deformation and increased CO₂ concentration resulted in precipitation of calcite.

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P-T conditions of cataclastic deformation associated with underplating in subduction zone are estimated from fluid inclusions within the post-mélange veins in the Cretaceous Shimanto Belt, Kii Peninsula, SW Japan. Both cataclasite and post-mélange veins cut the mélange fabric and are distributed along the thrusts in duplex structure. The homogenization temperatures range from 200 (±10) °C to 270 (±42) °C corresponding to entrapment pressures over a wide range from 145 (±5) MPa to 304 (-20) MPa. Our results have important implications in evaluating the relationship between underplating processes by duplexing and coseismic deformation in accretionary complexes.

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Recent studies of active Californian faults allow us to investigate the mechanisms of fluid flow and crack sealing along faults and to model fluid pressure evolution during earthquake cycles. The model is first based on the observation that fluids flow from depth along active faults (stables isotopes and traces elements analyses). The model is also based on the study of mechanisms of deformation of rocks near and within active faults at depth. Sampling was performed in faults that have been recently uplifted. Studies on thin sections of rocks produce evidence of the mechanisms of crack sealing and compaction of fractured zones as transient processes after each earthquake. We found that pressure solution creep and crack sealing are likely to control deformation and permeability change during inter-seismic period. Crack sealing and compaction processes are modeled to give the kinetics of these processes. Then numerical modeling of fluid pressure and transfer around active faults have been performed by integrating slow changes in permeability by crack sealing, gouge compaction and fluid flow from depth. This modeling shows that location and evolution of fluid overpressure varies during the inter-seismic period depending on the heterogeneity of the faults (nature of minerals and fluids, spacing between micro-fractures, thermodynamic and kinetics parameters).

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Two recent experimental studies on the frictional behavior of synthetic gouge-bearing faults under the operation of pressure solution are compared. One is triaxial shear experiments on quartz gouge at high pressure-temperature hydrothermal conditions (Kanagawa et al., 2000), and the other is rotary shear experiments on halite gouge at atmospheric pressure and room temperature in the presence of methanol-water mixtures (Bos et al., 2000). In spite of quite different experimental settings and conditions, the results of these two series of experiments are strikingly similar; both cataclasis and pressure solution being active during the experiments, gouge strength rate-controlled by cataclasis, two different frictional behaviors of slip hardening and softening, slip hardening associated with gouge compaction, distributed deformation and wall-rock failure, slip softening associated with localized slip along the gouge-wall-rock interface, and the transition from slip-hardening to slip-softening behavior according to decreasing rate of pressure solution. Although there is a difference in velocity dependence of strength between quartz and halite gouges, these similarities clearly demonstrate the important effects of pressure solution on the frictional behavior of gouge-bearing faults.

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Laboratory-determined permeability and compressibility data for natural fault gouge samples from the Median Tectonic Line (MTL) are presented and used to estimate hydraulic diffusivities in fault gouge zones. Bulk compressibility varies with effective pressure in a log-linear manner. Hydraulic diffusivity decreases significantly during the first isotropic loading partly due to a plastic compaction component, but does not significantly change during elastic unloading. Hydraulic diffusivity decreases with decreasing gouge grain size and is lowest in the very fine-grained centre of the fault zone, identified as the most recent principal displacement zone of the MTL. Previous models of fluid-controlled dynamic strength evolution during seismic slip are assessed using the data. The data suggest that the most recent principal displacement zone has a characteristic hydraulic diffusion length lower than the half width of the low-permeability zone. Hence pressurized fluid is unlikely to escape into the surrounding high-permeability fault rocks over the lifetime of an earthquake slip event, suggesting that thermal pressurization is likely to occur if the rupture plane is confined to the low-permeability gouge principal displacement zone.

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The strength of seismogenic faults is fundamental to earthquake mechanics and plate tectonics, affecting many subsidiary processes in the solid earth. The key to understanding fault strength is determining the pore pressures and hydraulic properties within the faults and surrounding crust. The debate has lasted over decades, with evidence provided for both strong fault and weak fault scenarios. A recently proposed hypothesis for quantifying the strength at which faults fail uses earthquake scaling arguments to show that earthquakes fail over a range of strengths in the upper 15-20 km, and at near-lithostatic pore pressure below this depth. This observation, if correct, has important implications for crustal hydraulics, plate tectonics, and earthquake hazard assessment. This paper summarizes the arguments for high pore pressure faulting, and explores its implications for earthquake stress drops, strength of the lithosphere, and earthquake scaling. The hope is to establish a general framework for quantifying the role of fluids in the earthquake process, and to demonstrate that high fluid pressure may dominate failure of the brittle crust.

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The Nojima fault in Hyogo prefecture, Japan, ruptured during the 1995 Hyogo-ken Nanbu earthquake (M_JMA = 7.3). The stress measurements at sites close to this fault have revealed that the direction of the largest horizontal stress is almost perpendicular to the strike of this sub-vertical fault and that, in the zone within about 100 m from the fault core axis, the ratio of the largest shear stress to the normal stress is significantly small compared with that of the outside. It is thus the logical consequence that the principal stress outside the zone tends to direct perpendicularly to the fault plane. A model called the fracture process model is introduced for the relationship between fracture strength and elastic property of rocks. Making use of this model on the assumption that the observed shear stress equilibrates to the shear strength of damaged zone, it is found that the elastic wave velocities estimated from the stress well explain the observed velocities of damaged zone. This model suggests further that the friction coefficient of fault can be smaller than 0.15 due to the characteristic deformation of damaged zone and that the pressurized fluid is not essential for the formation of weak faults.

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This paper addresses concepts presented in Session 7 of the conference on “Slip and Flow Processes Near the Base of the Seismogenic Zone” held at Sendai, Japan in November, 2001. The questions raised in this session were associated with the downward extensions of seismic faults into the lower crust. The important issue is whether asiesmic slip accelerates on downward extensions prior to large earthquakes on the upper, seismic part. If this is the case, then such movement may be measurable as a precursor to large seismic events as accelerated tilt and/or distortion at the surface. Associated issues involve the geometry of downward extensions of faults, the mechanisms of localisation in the lower crust, and the mechanisms for earthquake generation near the base of the upper crust. The outcomes from this session are that aseismic slip in the lower crust could be generated by several mechanisms of localisation including yield of an elastic-viscous-plastic material, softening (including, in particular, thermal softening) of an elastic-viscous material and ductile fracture. Fine scale modelling of localisation in the lower crust is still required to resolve the issue whether accelerated motion precedes seismic slip in the upper crust. Such modelling also demands a better understanding of crustal rheology than we have at present.

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A new model for growth of plastic shear zone is proposed based on the basis of a theory of fluid dynamics coupled with a rheological constitutive function, and is applied to a natural shear zone. Mylonite, ultramylonite and other ductile fault rocks are well known to deform in a plastic flow regime. The rheological behavior of these kinds of rocks has been well documented as a non-linear viscous body, which is empirically described asγ = Aτⁿ exp (-Q/RT), where γ : strain rate, τ : shear stress, Q: activation energy, R: universal gas constant, T : absolute temperature, and A and n are constants. Strain rate- and temperature-dependent viscosity is obtained by differentiating the equation, and simplified by substituting n = 1. Then, substitution of the equation into a diffusion equation, δ = 4√νt, derives an equation δ = 4 [ t/ρ · A exp (-Q/RT) ] 1/2, where δ: thickness of active layer of viscous deformation, ν: kinematic viscosity, and ρ: density. The duration of creep deformation along the ancient plastic shear zone (thickness: 0.076 m) is estimated to be around 760 s, in a temperature range from 300 to 500°C. This estimation is rather good agreement with intermittent creep during inter-seismic period, than steady state creep or co-seismic slip.

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The fault zone in the earth's crust is thought to consist of several regions from top to bottom: the upper frictional region, the brittle-ductile transition zone and the ductile region. The upper frictional region consists of the unstable frictional zone, the unstable-stable transition zone, and the stable frictional zone. Recent geological observations of fault rock suggest that at the deeper part of the seismogenic zone, co-seismic frictional slip coexists with interseismic flow processes. We propose a possible model for slip processes at the deeper part of the seismogenic zone in which the frictional slip and flow processes are connected in series. In this model, in the ductile region, power law creep is dominant. Around the unstable-stable transition zone, we assume that co-seismic frictional slip coexists with aseismic flow processes. We investigate simple 1-D and 2-D models where rate- and state-dependent friction coexists with power law creep that has a threshold stress. The results of numerical simulations show that the amount of slip during the interseismic period is greater in the case where friction coexists with power law creep than it is when only friction is at work. It is also found that, for the case where friction coexists with power law creep, frictional slip is largely inhibited in the ductile region.

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