We simulated the occurrence of both intraplate and interplate earthquakes in a simple spring-slider-dashpot model that includes both types of earthquake faults. We found that recurrence intervals of intraplate earthquakes are not controlled by the velocity of the relative plate motion but by the viscosity in the ductile fault zone, the coefficients of friction of both the interplate and intraplate earthquake faults, and the stress drop of intraplate earthquakes, when the stress drop of intraplate earthquakes is assumed to be only a small part of the total shear stress. These findings may open new avenues for the physics-based long-term forecasting of intraplate earthquakes.

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Important issues with regard to the generation processes of large inland earthquakes include how the stress concentrates and how nucleation starts in the deeper part of the seismogenic zone prior to the mainshock. We propose a model of earthquake generation processes that uses a constitutive law combining friction and flow processes. Using this law, we can represent fault behavior in which frictional slip coexists with flow processes at the frictional-viscous transition zone. We consider a limitted region where viscous deformation is high along the frictional-viscous transition zone, and investigate the role of this region in the nucleation process. During the interseismic period, slip velocity due to flow is much larger than frictional slip velocity in the region of low viscosity in the deeper part of the seismogenic zone. Large slip due to flow in this region is thought to cause stress to concentrate in the surrounding regions, and nucleation starts just above the low-viscosity region. Our numerical simulations indicate that the location of the nucleation process is determined by the nonuniform distribution of the depth of the frictional-viscous transition zone.

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The Alpine Fault accommodates around 60-70% of the 37 mm/yr oblique motion between the Australian and Pacific plates in the South Island of New Zealand. Uplift on the fault over the past 5Ma has led to the exhumation of the deep-seated mylonite zone alongside the present surface trace. Shear strain estimates in the mylonites reach 200-300 in the most highly strained rocks, and provide an integrated displacement across the zone of 60-120 km. This is consistent with the amount of displacement during the last 5 Ma, suggesting that displacement on the fault is localised within a 1-2 km wide ductile shear zone to depths of 25-30 km. Existing geodetic data, together with Late Quaternary slip rate and paleoseismic data, are consistent with the steady build-up and release of elastic strain in the upper crust driven by ductile creep within a narrow mylonite zone at depth. Faults of the Otago Fault System form a parallel array east of the Alpine Fault and accommodate c. 2 mm/yr contraction. Long periods of quiescence on individual structures suggest episodic, or “intermittently characteristic”, behaviour. This is more consistent with failure on faults within an elastico-frictional upper crust above a ductile lower crust. Localisation of crustal deformation may be initiated by inherited weaknesses in the upper crust, with downward propagation of slip causing strain weakening within the ductile zone immediately beneath. Inherited structures of great length focus a greater amount of displacement and hence more rapidly develop underlying zones of ductile shear.

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We show using numerical model experiments that upper crustal faults can impose ductile localization in the mid and lower crust over the seismic cycle, with strain-rates and integrated creep strain enhanced by a factor of 10, or a factor of 100 if lower crust is also thermally weakened. Imposed ductile localization is caused by the transfer in stress from the lower tip of the frictional fault to the mid-crust. Within the weak ductile mid-lower crust, this stress transfer also promotes significantly enhanced creep rates in a lobe that extends down-dip from the lower end of the fault. Comparison of model results with the Alpine Fault of New Zealand, shows how the interaction of faulting with other localization mechanisms can account for key aspects of the geodetic strain accumulating across the Alpine Fault. Localization of ductile strain in the lower crust imposed by faulting in the upper crust could explain the extension of major faults into the lower crust observed in seismic imaging.

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We investigate a novel fully coupled thermal-mechanical numerical model of the crust in order to trace the physics of interaction of its brittle and ductile layers. In a unified approach these layers develop in a natural transition as a function of the state variables pressure, deviatoric stress, temperature and strain-rate. We find that the main storage of elastic energy lies in the domain where brittle and ductile strain-rates overlap so that shear zones are attracted to this zone of maximum energy dissipation. This dissipation appears as a local heat source (shear heating). The brittle-ductile transition zone evolves through extreme weakening by thermo-mechanical feedback. The physics of the weakening process relies on repeated breaching of a critical energy flux threshold for feedback within this sub-horizontal brittle-ductile flow layer, thus developing unstable slipping events at postand pre-seismic strain-rates. The width- and the temperature domain of the feedback layer is controlled by the activation enthalpy Q of the material. For olivine rheology (Q-500 kJ/mol) the layer can be extremely thin <500 m and adheres to the 875 K isotherm. For quartz (Q-135 kJ/mol) the width of the feedback layer fans out into multiple interacting ductile faults covering a temperature domain of 450-600 K. The weakening by thermalmechanical feedback entirely controls the location and rejuvenation of upper crustal shear zones propagating from the detachment upwards in the form of listric faults. Within the detachment shear layer we identify an astonishing rich dynamics featuring distinct individual creep bursts. We argue that the rich ductile dynamics holds the key to earthquakes in the brittle field.

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The classical application of rate and state dependent frictional constitutive laws has involved the instabilities developed between two sliding surfaces. In such a situation, the behaviour and evolution of asperities is the controlling mechanism of velocity weakening. However, most faults have a substantial thickness and it would appear that it is the bulk behaviour of the fault gouge, at whatever scale, that is important. The purpose of this paper is to explore how bulk frictional sliding behaviour may be described. We explore here the consequences of applying the rate and state framework initially developed to describe the frictional behaviour at the interface between two interacting sliding blocks, to frictional behaviour within a layer of gouge that has bulk elasticplastic constitutive behaviour. The approach taken here is to replace the relative sliding velocity in the classical formulation with the maximum shear strain rate, D, and the characteristic length with a characteristic shear strain, γ_c. This means that the frictional behaviour of the bulk material now evolves with shear strain rate, D, over a characteristic shear strain, γ_c. This approach still does not address the problem of reproducing natural recurrence times between instabilities, but perhaps places the problem in a new framework.

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A good understanding of the rheology and strength of the whole crust is needed to obtain a physics-based earthquake prediction models. However, geodynamics-based and laboratory-based strength estimates disagree. Geodynamics tend to indicate that the actual strength of the plastic crust is less than deduced from laboratory experiments. Here, I evaluate lower crust strength from observations of transient postseismic deformation. Fault motion during an earthquake produces only a small stress perturbation, but that perturbation is sufficient to significantly affect the deformation rate of the aseismic levels of the crust, as observed by space geodesy. Even considering the non-linearity of plastic flow in geological materials, one cannot escape the conclusion that the pre-earthquake stress on the region where transient postseismic deformation occurs is not more than an order of magnitude larger than earthquake-induced stress perturbations. Using a simple shear zone model and assuming wet quartzite rheology, I show that such stress levels are not compatible with a km-scale shear zone, in spite of the geological evidence for localized deformation in the plastic crust. This implies that in plastic shear zones, rock strength is reduced. Possible explanations for the strength reduction include structural effects such as reduced grain size and/or a localized thermal anomaly associated with the shear zone.

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Slip events with an average duration of about 10 days and effective total slip displacements of severalc entimetres have been detected on the deeper (25 to 45 km) part of the northern Cascadia subduction zone interface by observing transient surface deformation on a network of continuously recording Global Positioning System (GPS) sites. The slip events occur down-dip from the currently locked, seismogenic portion of the subduction zone, and, for the geographic region around Victoria, British Columbia, repeat at 13 to 16 month intervals. These episodes of slip are accompanied by distinct, low-frequency tremors, similar to those reported in the forearc region of southern Japan. Although the processes which generate this phenomenon of episodic tremor and slip (ETS) are not well understood, it is possible that the ETS zone may constrain the landward extent of megathrust rupture, and conceivable that an ETS event could precede the next great thrust earthquake.

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An important observation associated with seismic activity on the Nagamachi-Rifu Fault is the existence of tabular, fluid rich zones at mid-crustal levels. These zones resemble the “bright spots” seen in many seismic images of the crust worldwide. The aim of this paper is to develop the mechanical foundations for the formation of such zones. To do so requires an understanding of the distribution of pore fluid pressure in a deforming crust. In a hydrostatically stressed porous material, the pore fluid pressure should equal the mean stress in order to keep the pores from collapsing. Past discussions of this subject imply very high pore fluid pressures, two to three times lithostatic. Considerations of plastic yielding together with continuity arguments, particularly at the plastic/viscous transition, suggest that pressures closer to lithostatic are more the norm. Particularly just below the plastic/viscous transition in compressive regimes, this leads to collapse of porosity with an associated collapse in permeability resulting in an over-pressured region comprising that part of the lower crust that is characterised by high mean stress. The base of the plastic region is at a strong discontinuity in stress difference where localised deformation occurs. Tabular, dilatant fluid filled regions develop at and above this zone in close association with dilatant tensional zones in the hanging-walls of faults and diffuse shear zone development in the upper to mid crust. Some of these dilatant zones ultimately develop into listric transitions between steeply dipping, upper crustal faults and shear zones associated with the plastic/viscous transition. These zones are also the sites of strong mineral alteration that may, particularly in ancient examples, also contribute to the delineation of “bright spots” in seismic images. For high geothermal gradients another class of fluid filled layers, in the form of “stagnant fluid zones”, develops below the region of high mean stress in the viscous lower crust. Mineral alteration associated with this second class of fluid rich layers is predicted to be asymmetric in distribution as opposed to the first class that would be homogeneous in the mode of alteration.

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Gold deposits in the Archaean Eastern Goldfields Province in Western Australia were deposited in greenstone supracrustal rocks by fluids migrating up crustal scale fault zones. Regional ENE-WSW D2 shortening of the supracrustal rocks was detached from lower crustal shortening at a regional sub-horizontal detachment surface which transects stratigraphy below the base of the greenstones. Major gold deposits lie close to D3 strike slip faults that extend through the detachment surface and into the middle to lower crust. The detachment originally formed at a depth near the plastic-viscous transition. In orogenic systems the plastic-viscous transition correlates with a low permeability pressure seal separating essentially lithostatic fluid pressures in the upper crust from supralithostatic fluid pressures in the lower crust. This situation arises from collapse in permeability below the plastic-viscous transition because fluid pressures cannot match the mean stress in the rock. If the low permeability pressure seal is subsequently broken by a through-going fault, fluids below the seal would flow into the upper crust. Large, deeply penetrating faults are therefore ideal for focussing fluid flow into the upper crust. Dilatant deformation associated with sliding on faults or the development of shear zones above the seal will lead to tensile failure and fluid-filled extension fractures. In compressional orogens, the extensional fractures would be subhorizontal, have poor vertical connectivity for fluid movement and could behave as fluids reservoirs. Seismic bright spots at 15-25 km depth in Tibet, Japan and the western United States have been described as examples of present day water or magma concentrations within orogens. The likely drop in rock strength associated with overpressured fluid-rich zones would make this region just above the plastic-viscous transition an ideal depth range to nucleate a regional detachment surface in a deforming crust.

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Seismicity in both compressional and extensional settings is a function of local and regional stresses, rheological contrasts, and the distribution of fluids. The influence of these factors can be illustrated through their effects on electrical geophysical structure, since this structure reflects fluid composition, porosity, interconnection and pathways. In the compressional, amagmatic New Zealand South Island, magnetotelluric (MT) data imply a concave-upward (“U”-shaped), middle to lower crustal conductive zone beneath the west-central portion of the island due to fluids generated from prograde metamorphism within a thickening crust. Change of the conductor to near-vertical orientation at middle-upper crustal depths is interpreted to occur as fluids cross the brittle-ductile transition during uplift, and approach the surface through induced hydrofractures. The central South Island is relatively weak in seismicity compared to its more subduction-related northern and southern ends, and the production of deep crustal fluids through metamorphism may promote slip before high stresses are built up. The deep crustal conductivity is highly anisotropic, with the greater conductivity along strike, consistent with fault zone models of long-range interconnection versus degree of deformation. The central Great Basin province of the western U.S. by contrast is extensional at present although it has experienced diverse tectonic events throughout the Paleozoic. MT profiling throughout the province reveals a quasi one-dimensional conductor spanning the lower half of the crust which is interpreted to reflect high temperature fluids and perhaps melting caused ultimately by exsolution from crystallizing underplated basalts. The brittle, upper half of the crust is generally resistive, but also characterized by numerous steep, narrow conductors extending from near-surface to the middle crust where they contact the deep crustal conductive layer. These are suggested to represent fluidised/altered fault zones, with at least some fluids contributed from the deeper magmatic exsolution. The best-known faults imaged geophysically before this have been the listric normal faults bounding graben sediments as imaged by reflection seismology. However, the major damaging earthquakes of the Great Basin appear to nucleate near mid-crustal depths on near-vertical fault planes, which we suggest are being imaged with the MT transect data, and where triggering fluids from the ductile lower crust are available. In both compressional and extensional examples, the fluidised fault zones are hypothesized to act to concentrate slip, with major earthquakes resulting in asperities along the fault surface.

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Magnetotelluric imaging of the San Andreas Fault has shown that seismically-active segments are characterized by a zone of low resistivity in the upper crust. Similar resistivity features are observed on other major strike-slip faults, and may have a common origin in a region of fractured rock, partially or fully saturated with groundwater. Other strike-slip faults show possible zones of reduced resistivity in the mid and lower crust that may be related to zones of ductile shear. Additional MT surveys are required to elucidate the role of fluids in controlling the seismic behaviour of major faults, both in and below the seismogenic zone. A set of synthetic inversions show that MT data is sensitive to the geoelectric structure of a shear zone at mid-crustal depths.

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Deep low-frequency (DLF) events have often been discussed in association with fluid (magma) activity around volcanoes, in this paper, however, we will show another example of DLF activity beneath active faults. On October 6, 2000, a Mw=6.7 crustal earthquake occurred in the western Tottori, southwest Japan. Beneath the focal region of the earthquake, DLF earthquakes had been observed prior to the mainshock and the activity increased after the mainshock. The events are distributed around 30 km depth and seem to be located around the downward extension of the seismogenic fault. Three types of DLF events have been observed and their seismological features indicate fluid activity around the focal region of the DLF events. If the downward extension of the seismogenic fault in the lower crust exists, fluid activity indicated by the DLF events possibly affect the aseismic slip process of the deep portion of the fault and likely control the occurrence of the earthquakes on the seismogenic portion of the fault.

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We published a CD-ROM “Geothermal Gradient and Heat Flow Data in and around Japan” (Tanaka et al., 2004a), which includes values of heat flow and geothermal gradient data. This compilation was intended to improve the understanding of variations in the thermal regime in and around Japan. Our current knowledge of the heat flow distribution can be increased by including information derived from geothermal gradient data contained in this compilation. In southern Kyushu, the pattern of heat flow is significantly modified by incorporation of estimates of heat flow from geothermal gradient data.

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The high-quality database of seismicity of Japan (JMA, Japan Meteorological Agency) and an extensive compilation of thermal measurements (Tanaka et al., 2004a) are used to quantify the concept of temperature as a fundamental parameter for determining the thickness of the seismogenic zone. Qualitative comparisons between each data of heat flow and geothermal gradient, and the lower limit of crustal earthquake hypocentral distributions beneath the Japanese Islands show that, as expected, the lower limit of seismicity is inversely related to heat flow and geothermal gradient. However, the density of the data is not uniform but highly clustered and spatially concentrated instead. To fill the data gaps, the available data were interpolated onto regular grids of points with a spacing of 0.25°× 0.25°. Gridded heat flow or geothermal gradient and D₉₀, the depth above which 90% of earthquakes occur, correlated well with each other. The evaluated temperatures for D₉₀ range between 250°C and 450°C except for higher heat flow data. The consistency of temperature for D₉₀ over a large depth interval almost all over the Japanese Islands support the concept that the temperature is the dominant factor governing the focal depth in the crust. A comparison of our results with other tectonic regions could provide evidence for variations in temperatures for D₉₀.

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K-Ar ages and fission-track ages of granitic rocks in the Hatagawa Fault Zone (HFZ), NE Japan were measured to examine the cooling history of the HFZ. The HFZ is an NNW-SSE trending fault zone in the Cretaceous granitic rocks, and consists of a conspicuous cataclasite and two types of mylonite with a sinistral sense of shear. The cataclasite zone is NNW-SSE trending and continuous over at least 40 km with a maximum thickness of 100 m. One type of mylonite is low-T mylonite, which is mainly developed for a length of 6 km along the HFZ. The other is high-T mylonite, which is widely distributed in the HFZ. Most of K-Ar ages of hornblende and biotite from granitic rocks are about 110 Ma and show no obvious differences along the strike of the HFZ or among different granite bodies. This implies that the granitic rocks in the HFZ have a similar cooling history and cooled rapidly from closure temperature of hornblende to that of biotite. Zircon fission-track analysis shows little possibility of reheating of the granitic rocks. This supports the formation of cataclasite and mylonite during the cooling of the granitic bodies. Fission-track ages of zircon and apatite from the samples in and near the areas where the low-T mylonite is developed are older than those for other areas. Infiltration of near-surface derived water into the low-T mylonite after plastic deformation may account for the accelerate cooling of granitic bodies.

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Many pseudotachylytes and their related fault rocks are found in the Hidaka metamorphic belt representing an ancient crustal section. On the basis of field observations of the pseudotachylytes and related fault rocks, nature of seismogenic faulting in the Hidaka crust is examined. The field observations suggest the following conclusions. (1) Two structural types of pseudotachylytes are distinguished: layer-parallel and layer-oblique. The latter are scattered in the metamorphic belt, but the former occur only in the southern part of the metamorphic belt. (2) An abundance of the layer-parallel pseudotachylytes suggests that earthquakes occurred repeatedly and frequently in the southern part, where complicated and duplicated crustal structures occur with many lowtemperature thin mylonite zones. The southern part with such crustal structures was an ancient seismogenic area containing asperities and having a radius of a few tens of kilometers in the Hidaka crust. In the seismogenic area, the layer-parallel pseudotachylytes resulted from seismic slip on the mylonitic foliation within the lowtemperature mylonite zones with strong preferred orientation of micas. (3) The layer-parallel pseudotachylytes and the subsequent layer-oblique pseudotachylytes post-date the latest and very-low-temperature mylonitization in the metamorphic belt. The former pseudotachylytes formed just above the upper side of the brittle-plastic transition zone.

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Geological studies on exhumed pseudotachylyte-bearing mylonite zones in S-type tonalite were carried out in the southern Hidaka metamorphic belt, Hokkaido, Japan. Mylonitization is characterized by (1) development of composite planar fabrics, (2) grain size reduction, and (3) change in modal composition an increase in mica content and a decrease in quartz content from protolith to mylonite. Mylonite zones are heterogeneously concentrated in the host rocks. At microscopic scales, shear deformation is concentrated heterogeneously in finegrained layers along C-surfaces. Most of the pseudotachylyte layers are subparallel to the C-surface, and tend to overprint thick mylonite zones. The heterogeneous development of mylonite zones, which may be activated as layers of co-seismic slip, should be incorporated into numerical modeling of seismogenic zones.

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Lithological heterogeneity of low P/T metamorphic rocks in southern area of Hidaka metamorphic belt (HMB) was formed through historical development of HMB while these rocks had been laid in ductile lower crust. Many strain-localized mylonite zones (<100 m in thickness) are preferentially developed within S-type tonalite and pelitic gneiss, which are characterized by a large modal amount of phyllosilicates (biotite+muscovite+chlorite) and quartz, compared to other lithofacies in HMB. Mylonitic foliations are more conspicuous with close to the center of the shear zone associated with increase in amounts of phyllosilicate minerals, indicating fluidenhanced weakening mechanisms were operated in plastic shear zones. Pseudotachylyte veins are observed exclusively in these mylonite zones, which were generated during exhumation stage of HMB. We conclude the seismic slip zones in southern HMB had been initiated in the ductile lower crust by concentration of localized plastic shear zones within the phyllosilicate- and quartz-rich lithofacies, which were heterogeneously formed by old metamorphic and magmatic events. Then these zones were further weakened by fluid-enhanced plastic deformation, and finally seismic slips occurred at the bottom of seismogenic upper crust, during exhumation of HMB.

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Following the Hyogoken Nanbu earthquake (January 17, 1995, Mw=7.2), three drillholes were sunk through the Nojima Fault (Awaji Island, Japan). Textural and petrographic studies of the Geological Survey of Japan (GSJ) drill cores allow recognition of two deformation episodes. The first one is older than the deposition of the Middle to Late Eocene Kobe Group, corresponds to a left-lateral movement on the Nojima fault and is expressed by pseudotachylytes, kinking of biotite crystals in the low-strain rocks and an intense laumontite hydrothermal alteration. The second one displaces the basal unconformity of the Kobe group, corresponds to a right-lateral reverse displacement and is expressed at least by carbonate-filled hydraulic fractures and thin gouge zones. Different important deformation mechanisms are recorded by the fault rocks, but questions relating to the attribution of deformation and alteration features to one or other deformation episodes remain unresolved.

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The mechanism of hydrogen generation during the mechanochemical treatment of biotite was examined by grinding experiments using a ball mil in H₂O or D₂O as a grinding media. From the linear relationship between the amount of generated hydrogen and the increase of the surface area of ground powders, the hydrogen productivity of biotite is estimated to be 0.036 μmol/m², which agrees with the previous results in spite of the difference in the grinding conditions. D₂ analyses by a mass spectrometry indicate that the produced amount of D₂ accounts for only 10% of the total hydrogen and that more than 90% of hydrogen takes a form of a mixture of HD and H₂. The observed isotope distribution clearly indicates that hydroxyls within the crystal structure canv e a major source for the generation of hydrogen. Hydrogen generation originated from hydroxyls may indicate the higher hydrogen productivity of phyllosilicates than those of quartz and alkali feldspar.

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We investigated the crustal deformation around the northern Itoigawa-Shizuoka Tectonic Line Fault Zone by continuous observation with a dense GPS array. The observation revealed concentrated deformation around the East Matsumoto-Basin Fault while the more active Gofukuji Fault has smaller strain rate. By examining the physical meanings of geodetic strain rate and the geological slip rate data, we estimate a larger seismic potential around the Gofukuji Fault. The deep slip model proposed for the strain concentration around the Omachi area was not validated. Inelastic deformation may play an important role as a cause of strain concentration.

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The northern Fossa Magna (NMF) is a Miocene rift basin formed in the final stages of the opening of the Sea of Japan. The northern part of Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the NMF and forms an active fault system that displays one of the largest slip rates in the Japanese islands. Reflection and refraction/wide-angle reflection profiling and earthquake observations by a dense array were undertaken across the northern part of ISTL in order to delineate structures in the crust, and deep geometry of the active fault systems. The ISTL active fault system at depth (ca. 2 km) shows east-dipping low-angle in Omachi and Matsumoto and is extended beneath the Central Uplift Zone and Komoro basin keeping the same dip-angle down to ca. 15 km. The upper part of the crust beneath the Central Uplift Zone is marked by the high Vp and high resistivity zone. Beneath the folded zone of the NMF, the middle to lower crust shows low Vp, low resistivity and more reflective features. The balanced geologic cross-section based on the reflection profiles suggests that the shortening deformation since the late Neogene was produced by the basin inversion of the Miocene low-angle normal fault.

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The Toyama Trough and Fossa Magna basin are major transverse tectonic depressions located between northeast Japan (NEJ) and southwest Japan (SWJ). Itoigawa-Shizuoka Tectonic Line (ISTL), the western margin of Fossa Magna is a boundary between two tectonic provinces along the inner side of NEJ and SWJ. Abrupt lateral variations in thickness of the Middle to Upper Miocene strata are quite significant among these provinces in the North Fossa Magna. The development of the thrust/fold belt is attributed not only to horizontal compression but also to vertical block movements as basement-involved tectonics. In response to the Pliocene and later compression regime, the old fault-block boundaries were reactivated and produced differential block movement such as the tilting of the Central Upheaval Zone and uplifting of the Nishikubiki Belt. One possible model for the deeper geologic structure is presented that high-angled block faults on the surface among tectonic provinces originate in vertical weak zones in the deep seismogenic zone under the sedimentary layer. The folded Neogene system comprises the present-day thrust-fault province of the North Fossa Magna and the stress regime of strikeslip faulting occupies the basement, as inferred from focal mechanism solutions for small events. In order to account for the apparent discrepancy, a duplex stress field is possible for the active tectonics in the region.

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We used teleseismic P waves recorded by the J-array, the Hi-net and a temporal local seismic network to investigate the three-dimensional topography of the Moho and the Philippine Sea plate beneath central Honshu Island, Japan. An image of the subsurface discontinuities beneath the region, derived from receiver function analysis, depicts the Philippine Sea plate dipping toward the north with complex local curvatures. The Moho is clearly detected in the northern part of the area studied, and its depth increases to the center of the island. Receiver functions from the stations adjacent to the Itoigawa-Shizuoka tectonic line indicate the step-like topography of the Moho directly beneath this tectonic line.

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The relationship between recent seismicity and the active fault system was investigated using the precise hypocentral distribution by taking into consideration the heterogeneity of the velocity structure around the northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL), the western boundary of the northern Fossa Magna basin. Since a velocity structure suitable for the ISTL is necessary for precise hypocentral determination, deep seismic reflection and refraction profiling were undertaken in 2002 across the northern part of the ISTL and a reliable velocity model was obtained. The hypocenters were located using the station corrections derived from the velocity model. The obtained hypocenters were found to be distributed in a straight line in an almost north-tosouth direction. The hypocenters, whose lower limit was approximately 15 km, were distributed around a vertical plane. The focal mechanism solutions were primarily strike-slip and the direction of their P-axis was NW-SE to WNW-ESE. The fault planes expected from these mechanisms are consistent with the distribution of the hypocenters. However, they are not consistent with the thrust fault planes of previous large earthquakes, which were inferred from the near-surface feature of the active fault system. This suggests that the recent seismicity cannot be attributed to aftershocks of the previous large earthquakes and is not induced by the movement of the active thrust fault.

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An active fault segment at the northern Itoigawa-Shizuoka tectonic line (ISTL), Central Japan, which will potentially cause M8-class intraplate earthquake, was imaged by wide-band magnetotellurics. Three parallel profiles across ISTL revealed along-strike variation of the resistivity structure. Three resistivity models commonly showed the thickening conductors in the upper crust to east of ISTL which imply the heavily folded Miocene sediments with maximum thickness of several kilometers. Thus the upper crustal structure seems two-dimensional throughout the segment. We found mid-crustal conductors, top of which correlate well with the cutoff depth of seismicity. The seismicity clusters mainly in the resistive crust that is underlain by the mid-crustal conductors. This implies the local distribution of fluids below the brittle-ductile boundary and suggests that the fluid migration into resistive zone is triggering earthquakes. However, the distribution of these mid-crustal conductors is not consistent with the strike of ISTL, but rather it is better correlated with the negative dilatation anomaly inferred from GPS. This suggests the weakening of the crust by the existence of fluids.

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The northern Fossa Magna (NFM) is a back-arc rift basin filled with thick Tertiary sediments, which show strong NW-SE shortening deformation. In the NFM, there exist two major active fault systems, the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL) and the Western Nagano Basin active fault system (WNB), both of which have great potentials to cause destructive earthquakes. By reanalyzing five sets of refraction/wideangle reflection data, we successfully obtained detailed and consistent models of the crustal structure in the NFM region. It was a very effective modeling procedure to incorporate vicinal seismic reflection data and geologic information. The geometries of the active faults in the NFM region were revealed. The ISTL is east dipping, and the WNB is northwest dipping. The Tertiary sedimentary layer (<4.0 km/sec) west and adjacent to the ISTL extends to a depth of 4-5 km. The basement rocks below the Central Uplift Belt (CUB) form a wedge structure, which suggests the westward movement of the CUB basement rocks.

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The northern Fossa Magna (NFM) basin is a Miocene rift system formed in the final stage of the opening of the Japan Sea. The northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna. In order to understand the active tectonics in these areas, it is essential to explain the seismic velocity structures, deep structures of active faults, and microseismicity near the active faults. In the autumn of 2002, we conducted a seismic array observation across the northern part of the ISTL and the NFM to obtain a structural image beneath the NFM. Arrival times of local earthquakes and explosive shots were used in a joint inversion for earthquake locations and 3-D Vp and Vp/Vs structures. P- and S-wave arrival time data were obtained from 73 events including 4 explosive shots, and 3809 P- and 2659 S-wave arrival times were used for the inversion analysis. We obtained a seismic velocity model revealing good correlations with the surface geology along the profile. In particular, we found thick low-velocity zones beneath the NFM and the Komoro basin and a high-velocity zone beneath the Central Uplift Zone. Beneath the NFM, a low-velocity zone with low-to-moderate Vp/Vs extends to a depth of approximately 10 km. The low-velocity suggests the existence of aqueous fluid-filled pores with high aspect ratios.

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A gravity survey was conducted between October 2002 and December 2003 in and around the eastern area of the Itoigawa-Shizuoka Tectonic Line (ISTL). The total number of measurement points amounted to about 436 and 326 stations were taken at the same points as the seismic survey points. The location and altitude were decided by differential GPS and the accuracy is thought to be within several ten-centimeters. The data was compiled from existing data (Geological Survey of Japan, 2000), so the total number of compiled gravity maps is about 3, 540 points. All measured gravity data were referred to the International Gravity Standardization Net 1971 (IGSN71) and the normal gravity values were estimated according to the gravity formula of 1980. Terrain corrections were conducted within a range of 60 km by approximating the real topography to an assemblage of annular prisms interpolated by mesh terrain data and random terrain data of the gravity points. The effect of the sinking of the topography due to the Earth's curvature was taken into consideration. Bouguer corrections within a range of 60 km in arc distances were made using a spherical cap crust formula. The density for both terrain correction and Bouguer corrections was chosen to be 2, 670 kg/m³, because the mean surface density of the whole area is estimated to be a slightly large density by the CVUR method. The features of Bouguer anomalies is characterized by a low anomaly in Omachi city, in the northern parts of Matsumoto basin, and a high anomaly in the central highlands, Central Uplift Zone, about 10 km west of Ueda city. The low anomaly shows that the thickness of the sedimentary layer is more than 1 km from 2D/3D automatic analysis.

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The Itoigawa-Shizuoka Tectonic Line (ISTL) in Central Japan is a fault zone with a very high slip rate during Pliocene-Quaternary time. Our seismic reflection and gravity surveys across the northern segment of the ISTL at Matsumoto have revealed its geometry to a depth of -5 km. The fault plane was found to be of fairly low angle, in spite of the surface geologic observations that late Quaternary movements on this fault zone are dominantly strike slip. Partitioning of slip is taking place between the East Boundary Fault (thrust) and the Gofukuji Fault (left-slip), which constitute the fault zone and are parallel to and a few km apart from each other. However, these two faults are found to merge down-dip at a depth as shallow as 1.5 km below the surface. The geometry of the ISTL is significantly discordant with the orientation of the maximum shear stress acting regionally on Central Japan, indicating that the fault plane is of very low strength.

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The slip-rates on the northern extent of Itoigawa-Shizuoka tectonic line (ISTL) are estimated based on seismic reflection profiles, drill core data and analysis of tectonic geomorphology. The ISTL is a major tectonic line that passes through the Honshu Island of Japan, and its northern and central segments form an active fault system characterized by high slip-rates. In the Kamishiro basin, near the northern end of the ISTL active fault system, the rate of net slip is estimated to be 4.4-5.4 m/kyr over the last 28 ka, with a vertical-separation-rate of 2.2-2.7 m/kyr. In the Omachi area, south of the Kamishiro basin, the Quaternary slip-rate is estimated to be at least 2.9 m/kyr based on the balanced cross-section derived from reflection profiles and surface geology. The dip angle of 30° determined from the Omachi seismic profile suggests a vertical-separation-rate of at least 1.5 m/kyr. Based on compiled evidence from the available geomorphological and paleo-seismological data, vertical-slip-rates of 1.0-2.9 m/kyr are inferred for the region between Hakuba and Toyoshina over the past 3 ka. The northern ISTL exhibits dip-slip-rate of at least 2.9 m/kyr, with a constant average slip-rate of 2.0-5.8 m/kyr since the Early Quaternary. A paleoseismological data and long-term slip-rate along the northern ISTL has potential for a large earthquake.

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The Northern Fossa Magna is a Miocene failed rift and due to subsequent shortening, its basin-fill forms a fold-belt associated with active faults. Seismic reflection data across the middle part of the northern Fossa Magna acquired in late 1990s were reprocessed to reveal the deep geometry of active faults. The reprocessed seismic sections portray the folded and faulted structure of the Neogene basin-fill. The deeper extension of the Western Nagano Basin active Fault (WNBF), which has been revealed for the first time, can be traced down to 4 km, as a reverse fault dipping 40° westward. In the western part, the Itoigawa-Shizuoka Tectonic Line (ISTL) active fault is presented as an emergent thrust dipping 30-35° eastward. Based on the seismic profiles, surface geology and well data, the balanced geologic cross section was constructed. Using simple-shear model of the basin formation, the total amount of Miocene extension is calculated to be ca. 27 km and the total amount of late Neogene to Quaternary shortening is ca. 11 km. The basin formation and shortening deformation are well explained by the tectonic inversion model and fault reactivation.

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A two-dimensional finite element model was constructed along a cross section almost perpendicular to the Nagamachi-Rifu Fault Zone, in order to clarify the stress accumulation process on an intraplate earthquake fault. We explain the surface deformations observed by the dense GPS network and leveling surveys using models with heterogeneities in the crust. These heterogeneities are identified from various geophysical surveys in the region. We found that the observed surface deformations cannot be explained by a model having a weak zone in the upper crust, but can be explained by models having a weak zone in the lower crust. Models having an aseismic fault or fault zone in the lower crust can reproduce the spatial pattern of the observed deformations, but amplitudes predicted by these models are smaller than those observed. The weak zone in the lower crust probably plays an important role in the stress accumulation process on the Nagamachi-Rifu fault zone.

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A nationwide GPS array with more than 1, 000 permanent stations operated by the Geographical Survey Institute of Japan, has provided many invaluable deformation data such as co- and post-seismic, volcanic, and ongoing secular deformations in the Japanese islands. Based on daily coordinate data of the GEONET stations together with results from a regional network operated by Tohoku University, we derived a map of the strain rate distribution in NE Japan showing that there exists a notable strain concentration zone of EW contraction along the Volcanic Front. The area demonstrates active seismicity including some disastrous earthquakes. Recent seismic tomography studies have revealed the existence of inclined seismic low-velocity zones (LVZ) at depths shallower than -150 km in the mantle wedge sub-parallel to the subducted slab. The inclined LVZ reaches the Moho right beneath the Volcanic Front, indicating that the formation of the strain concentration zone is closely related to the rheological structure of the island-arc system.

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Quantitative analysis of collocated seismic velocity tomography and electromagnetic experiments is developed to elucidate the structure of the deep extension of the Nagamachi-Rifu fault, northeastern Japan. P and S wave seismic velocities obtained from a dense seismic network are examined and a ratio of spatial variation in P and S wave velocities d ln V_s/d ln V_p is chosen as a proxy for the influence of pore geometry. The analysis shows that the deep extension of the Nagamachi-Rifu fault reveals the d ln V_s/d ln V_p values exceeded 1.1. Such large values of d ln V_s/d ln V_p cannot correspond to equilibrium pore geometry, at which the interfacial energy is at a minimum, and indicate regions with non-equilibrium state where non-isotropic stress prevents the equilibrium pore geometry to be achieved. To specify a fine distribution of porosity and connectivity of micropore in the region, we carry out the joint analysis of the seismic velocities with the electrical resistivity data obtained by the magnetotelluric survey crossing the Nagamachi-Rifu fault. It is shown that the region at 10-17 km depths at about 20-40 km to the northwest from the hypocenter of the M5.0 earthquake occurred in 1998 exposes the highest connectivity among the adjacent areas, suggesting a strong deformation process.

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The Northern Miyagi earthquake (Mj 6.4) on 26 July, 2003, was a shallow crustal earthquake produced by high-angle reverse faulting. To construct a realistic geologic model for this fault system from depth to the surface, seismic reflection profiling was carried out across the northern part of the source fault of this earthquake. The common mid-point seismic reflection data were acquired using a vibrator truck along a 12 km-long seismic line. The obtained seismic profile portrays a Miocene half-graben bounded by a west-dipping fault. Consistent with gravity anomaly data, the maximum thickness of the basin fill probably reaches 3 km. From the regional geology, this basin-bounding normal fault forms the eastern edge of the northern Honshu rift system and was produced by rapid extension during 17-15 Ma. The deeper extension of the fault revealed by seismic profiling coincides with the planar distribution of aftershocks. The hypocentral distribution of the aftershocks shows a concentration on a plane dipping 55 degrees to the west with listric geometry. Thus, the basin inversion has been performed using the same fault; the 2003 Northern Miyagi earthquake was generated by fault reactivation of a Miocene normal fault.

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“SEKKI” phenomena often appear in the Japanese historical literatures as distinct red emission in the nocturnal sky. The Japanese word “SEKKI” means the red atmosphere. We compile 16 events of SEKKI for 12-19th centuries in the literatures. In order to understand the SEKKI phenomena, we compared 2 events of SEKKI on February 21, 1204, and September 17, 1770, with the characteristics of low-latitude auroras studied recently by modern scientific methods. We conclude that these historical SEKKI phenomena are probably giant low-latitude auroras.

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Repeated precise leveling in the earthquake swarm area of Ontake, central Japan has revealed uplift of 3-6 mm in proximity to the epicentral region of the most active earthquake cluster in 2002-2004. Although the uplift is small, the vertical displacement is significant even considering leveling error. This uplift is associated with increases in ³He/⁴He ratios and CO₂ δ¹³C values at a mineral spring in the region, indicating an upper mantle contribution. A region of low resistivity at a depth of 2 km beneath the uplift area has also been inferred, suggesting that the observed uplift is related to changes in a shallow seismogenic layer due to increased hydrothermal input from the earthquake swarm area.

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