Faults In Japan Research Articles

Preliminary results of spatial distribution of radon and thoron with associated parameters in soil around active faults in Japan.

The radon (Rn) and thoron (Tn) concentrations and other environmental parameters were measured in the vicinity of active faults in two regions with different geology structure of Japan. The range of measured values was from 1.2 to 74 kBq m$^{-3}$, 1.1 to 38.9 kBq m$^{-3}$, 0% to 4.35%, and 2$\times $10$^{-13}$ to 4$\times $10$^{-11}$ m$^{2}$, for Rn, Tn, CO$_{2}$, and soil permeability, respectively. Significant differences in the measured parameters were observed, suggesting that Rn concentrations near faults are strongly influenced by geological structure and fault type. Based on statistical analysis, good correlations were found between Rn, Tn, and CO$_{2}$ (carried gas ) concentrations. It is planned to continue the study with measurements at a number of additional sites.

Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone

Ultramylonites are among the most extreme fault rocks that commonly occur in the mid-crustal brittle–plastic transition and are mainly characterized by intensely sheared fine-grained microstructures and well-mixed mineral phases. Although the deformation mechanism of ultramylonites is key to understanding the rheological behavior of the mid-crustal shear zone, their microstructural development is still controversial owing to their intensely fine-grained textures. To investigate the possible crustal deformation mechanisms, we studied 13 mylonites obtained from the Kashio shear zone along the Median Tectonic Line that is the largest strike-slip fault in Japan. In particular, we investigated various mixed quartz–plagioclase layers developed within tonalitic mylonite, which are representative of the common mean grain size and crystal fabric of quartz among the studied samples. A high-quality phase-orientation map obtained by electron backscattered diffraction showed not only a wide range of quartz–plagioclase mixing (10%–80% in quartz modal composition) but also revealed a correlation between grain size reduction and crystal fabric weakening in quartz, indicating a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep in the mixed quartz-plagioclase layers. In contrast, plagioclase showed an almost consistent fine grain size and weak to random crystal fabrics regardless of modal composition, indicating that grain size-sensitive creep is dominant. Combined with laboratory-determined flow laws, our results show that the Kashio shear zone could have developed under deformation mechanisms in which the viscosities of quartz and plagioclase are nearly comparable, effectively within 1017–1019 Pa·s, thereby possibly enabling extensive shearing along the Median Tectonic Line.

Right-lateral offset associated with the most recent earthquake on the Ikeda fault of the Median Tectonic Line, southwest Japan, revealed by ground-penetrating radar profiling

The Median Tectonic Line (MTL) is an arc-parallel strike-slip fault that accommodates much of the arc-parallel component of the oblique convergence of the Philippine Sea and Eurasian plates at the Nankai Trough. The MTL in Shikoku is one of the fastest-slipping faults in Japan, with a late Quaternary right-lateral slip rate of 5–10 mm/yr. To estimate the right-lateral slip amounts of the past faulting events on the MTL, we acquired 2D and pseudo-3D ground-penetrating radar (GPR) sections across the ENE-trending Ikeda fault of the MTL in eastern Shikoku. We conducted the GPR surveys at the Higashi-Miyoshi site, where two terrace riser offsets mark the active fault trace. The 2D lines were about 28–64 m long, and the pseudo-3D data were sized 20 m × 30 m with a 0.5-m inline spacing. We used 50 MHz GPR antennas and conducted wide-angle measurements to estimate the electromagnetic wave velocity. We identified three paleochannels on the final depth-converted GPR sections, and two of them are deflected by the fault. A paleochannel at 0.6–1.4 m depth is observed on all inline sections of the pseudo-3D GPR data. We built a 3D model of this paleochannel and estimated the right-lateral and vertical displacements of ~ 3.5 m and ~ 0.5 m, respectively. This paleochannel offset is probably caused by the most recent surface-rupturing earthquake on the Ikeda fault, which may be the 1596 Keicho-Fushimi earthquake. This study demonstrates the usefulness of the GPR surveys to identify geological features displaced laterally and vertically by the most recent surface-rupturing earthquake.

Electrical Resistivity Structure Around the Atotsugawa Fault, Central Japan, Revealed by a New 2‐D Inversion Method Combining Wideband‐MT and Network‐MT Data Sets

AbstractThe Atotsugawa fault is one of the most active faults in Japan, and the strain accumulation at the fault is considered to be caused by an aseismic shear zone in the fluid‐rich lower crust. To identify the shear zone and investigate the origin of the aqueous fluid in the lower crust, we deployed a Network‐MT survey in addition to a conventional wideband‐MT survey around the fault and performed an inversion combining both the MT data sets. In the inversion, by modifying a conventional inversion algorism, we accurately represented kilometer‐scale dipoles of the Network‐MT measurement to provide constraints on the electrical resistivity structure. In the lower crust under the study area, there are localized conductive anomalies below the Atotsugawa fault, the Ushikubi fault, and the Takayama‐Oppara fault zone. Comparing our electrical resistivity structure with the seismic velocity structure, we interpreted that the lower‐crustal conductors are localized ductile shear zones with highly connected fluid. We considered that the localized ductile shear zones are responsible for the strain accumulation along the respective active faults. In addition, in the mantle wedge above the subducting Philippine Sea slab and its downward extension, a highly conductive portion is detected, which may be attributed to the fluid dehydrated from the Philippine Sea slab and/or the Pacific slab. The existence of the large conductive area supports the suggestion of previous seismic and geochemical studies that the fluid of the lower crust around the Atotsugawa fault originated from subducting slabs.

Okada, A. and Yagi, K.: Aerial Images of Major Active Faults in Japan

Okada, A. and Yagi, K.: Aerial Images of Major Active Faults in Japan

The fault in Japan’s stars: Shinzo Abe, North Korea, and the quest for a new Japanese constitution

In the wake of North Korea’s progressive missile testing that set even the usually stoic Japanese people into a panic mode, Japan has found itself at the mercy of its former enemies. In an ironic twist of fate, Tokyo’s security outlooks seem to have become hostage to the strategic calculations of its fiercest nemesis in the past. This paper asks whether Prime Minister Shinzo Abe’s desire for constitutional change is precipitated mainly by the resurgence of Japanese nationalist sentiments as what many of his critics claim, or if there are genuinely rational justifications for revising the country’s 72-year old Constitution. And if so, why has it been so elusive for many Japanese leaders? Using neoclassical realism theory, I analyze the structural contexts and domestic intervening variables that simultaneously drive and prevent the realization of constitutional change in Japan. I argue that state leaders like Abe and those who have come before him have always been prone to acquiring flawed and inaccurate perceptions of the systemic stimuli; susceptible to making irrational and unsound decisions; and ineffective at mobilizing the national resources demanded by their preferred policies and strategies. Thus, despite having rational justifications, the quest for constitutional change has remained elusive for many Japanese leaders. Success will require Abe to carefully harmonize domestic and international expectations; prudently balance Japan’s benign security intentions and hawkish military strategies; and shift away from his pragmatic–ambivalent style of domestic politics.

Interseismic crustal deformation in and around the Atotsugawa fault system, central Japan, detected by InSAR and GNSS

The Atotsugawa fault system is one of the best-known active faults in Japan. However, revealing the interseismic velocity field in and around the Atotsugawa fault system with high spatial resolution is challenging because of dense vegetation, steep topography, and heavy snowfall in winter. To overcome these difficulties, we combined ALOS/PALSAR data and GNSS data from our original stations in addition to the nationwide station network (GEONET). First, we removed the height-dependent phase change in each interferogram using a digital elevation model. Next, we removed the long-wavelength phase trend using the GNSS velocity field. Finally, we applied an InSAR time-series analysis, known as small baseline subset analysis (SBAS), to all the corrected interferograms. The resultant mean velocity field shows a remarkable phase gradient around the Atostugawa fault system. We found a sharp velocity gradient across the Ushikubi fault, a major strand of the Atotsugawa faults system, rather than the main trace of the Atotsugawa fault. Using InSAR, we found that the interseismic deformation inside the strain concentration zone is spatially heterogeneous and different from what we expect from the fault traces.

Japan unified hIgh-resolution relocated catalog for earthquakes (JUICE): Crustal seismicity beneath the Japanese Islands

We have generated a high-resolution catalog called the “Japan Unified hIgh-resolution relocated Catalog for Earthquakes” (JUICE), which can be used to evaluate the geometry and seismogenic depth of active faults in Japan. We relocated >1.1 million hypocenters from the NIED Hi-net catalog for events which occurred between January 2001 and December 2012, to a depth of 40km. We apply a relative hypocenter determination method to the data in each grid square, in which entire Japan is divided into 1257 grid squares to parallelize the relocation procedure. We used a double-difference method, incorporating cross-correlating differential times as well as catalog differential times. This allows us to resolve, in detail, a seismicity distribution for the entire Japanese Islands. We estimated location uncertainty by a statistical resampling method, using Jackknife samples, and show that the uncertainty can be within 0.37km in the horizontal and 0.85km in the vertical direction with a 90% confidence interval for areas with good station coverage. Our seismogenic depth estimate agrees with the lower limit of the hypocenter distribution for a recent earthquake on the Kamishiro fault (2014, Mj 6.7), which suggests that the new catalog should be useful for estimating the size of future earthquakes for inland active faults.

Recognition of shear heating on a long‐lived major fault using Raman carbonaceous material thermometry: implications for strength and displacement history of the MTL, SW Japan

AbstractThe extent to which movement on major faults causes long term shear heating is a contentious issue and an important aspect in the debate about the strength of major faults in the crust. Comparing the results of experimental work on the kinetics of crystallization of carbonaceous material with results of thermal modeling show that the Raman carbonaceous material (CM) geothermometer is well suited to studying shear heating on geological time scales in suitable lithologies exposed around exhumed major fault zones. The Median Tectonic Line (MTL), SW Japan, is the largest on‐land fault in Japan with a length of > 800 km. Application of Raman CM thermometry to pelitic schist adjacent to the fault reveals the presence of a rise in peak temperature of around 60 °C over a distance of around 150 m perpendicular to the MTL fault plane. The spatial association of this thermal anomaly with the fault implies it is due to shear heating. Thermal modeling shows the recorded thermal anomaly and steep temperature gradient is compatible with very high rates of displacement over time scales of a few thousand years. However, the implied displacement rates lie outside those generally observed. An alternative explanation is that an originally broader thermal anomaly that developed during strike slip faulting was shortened due to the effects of normal faulting. Constraints on displacement rate, width of the original anomaly, duration of heating and peak temperature imply a coefficient of friction, μ, greater than 0.4.

活断層研究の進展―『日本の活断層』出版前後

活断層研究の進展―『日本の活断層』出版前後

The applicability of frictional reactivation theory to active faults in Japan based on slip tendency analysis

To investigate the applicability of frictional reactivation theory to active faults, we evaluated the slip tendency of active faults in Japan. Slip tendency is defined as the ratio of shear stress to the frictional resistance acting on a fault plane. To estimate the stress field near active faults, we determined focal mechanisms for numerous shallow earthquakes in the intraplate region of Japan, using data from dense seismic observation networks. The stress fields were estimated by applying the stress inversion method to the focal mechanisms. We found that most active faults are well oriented with respect to the stress field, having slip tendencies of ≥0.6, which indicates that fault reactivation theory is applicable to active faults, and that the present day tectonic stress field has contributed substantially to the development of active faults. Conversely, several steeply dipping active faults in northeast Japan, as well as the source faults of the 1995 Hyogoken–Nanbu earthquake, are mis-oriented, with slip tendencies of <0.6, which may indicate that highly pressurized fluids have contributed to earthquake triggering on these mis-oriented faults. We also discuss the applicability of slip tendency analysis to earthquake hazard assessment; i.e. estimating the probability of a future rupture on active faults. Our results indicate that slip tendency does not correlate with elapsed time since the most recent earthquake event. This observation shows that slip tendency may not be an efficient parameter with which to assess the risk of an earthquake occurring in the near future on a specific fault.

Zircon multichronology: Fission‐track, U‐Pb, and combined fission‐track–U‐Pb studies

<scp>Z</scp>ircon multichronology: <scp>F</scp>ission‐track, <scp>U‐Pb</scp>, and combined fission‐track–<scp>U‐Pb</scp> studies

Release of mantle and crustal helium from a fault following an inland earthquake

Static stress changes caused by megathrust slip of the 2011 Mw 9.0 Tohoku-Oki earthquake considerably affected the seismicity patterns in inland areas, resulting in the occurrence of numerous earthquakes along several active faults in Japan. On June 30, 2011, the Mj 5.4 central Nagano earthquake occurred at a shallow depth of 5km, indicating the reactivation of the Gofukuji fault in Central Japan. This study was undertaken to elucidate spatial and temporal changes of 3He/4He ratios around a source region before and after an inland earthquake using both existing and new and helium isotope data from hot spring and drinking water wells. Gas samples near the Gofukuji fault and its surrounding active faults are characterized by an increase in postseismic 3He/4He ratios. In contrast, the postseismic ratios decreased by up to about 30% away from the mainshock epicenter. Episodic faulting could either release stored crustal (radiogenic) helium from host rocks, or enhance the transfer of mantle volatiles through permeable fault zones, such that subsequent fluid flow near to the source region could then explain the spatio-temporal variations in 3He/4He ratios.

Characteristic Earthquake Model and Seismicity around Late Quaternary Active Faults in Japan

A total of 172 late Quaternary active fault zones in Japan are examined to determine whether the Gutenberg–Richter relationship or the characteristic earthquake model more adequately describes the magnitude–frequency distribution during one seismic cycle. By combining seismicity data for more than 100 active fault zones at various stages in their seismic cycles, we reduced the short instrumental observation period compared to the average recurrence interval. In only 5% of the active fault zones were the number of observed events equal to or larger than the number of events expected by the Gutenberg–Richter relationship. The average and median frequency ratios of the number of observed events to the number of expected events from the Gutenberg–Richter relationship are only 0.33 and 0.06, respectively, suggesting that the characteristic earthquake model more appropriately describes the magnitude–frequency distribution along the late Quaternary active faults during one seismic cycle. Moreover, the larger an average slip rate is or the shorter an average recurrence interval, the larger the gap in magnitude tends to be between the characteristic earthquake and the largest among other events. A fault zone with a shorter average recurrence interval and/or a larger average slip rate has generally produced more earthquakes in the past or is likely to be at a more mature or developed stage. Thus, these tendencies may reflect a change in the magnitude–frequency distribution related to the maturity or development of fault zones.

Estimates of stress and strain rate in mylonites based on the boundary between the fields of grain‐size sensitive and insensitive creep

Microstructural analyses of mylonites next to the Median Tectonic Line (MTL), SW Japan, reveal a transition in the dominant deformation mechanism of quartz from grain‐size‐insensitive dislocation creep to grain‐size‐sensitive grain‐boundary sliding (GBS). The transition occurred under greenschist‐facies conditions (∼300–400°C) during grain‐size reduction by dynamic recrystallization. The stereologically corrected grain size for the transition is approximately 4.3 μm. At the boundary between the fields of dislocation creep and GBS, as calculated from creep constitutive relations, the differential stress and strain rate for this corrected grain size are estimated to be ∼280 MPa and 1.2 × 10−11 s−1 for 300°C, and ∼110 MPa and 1.0 × 10−10 s−1 for 400°C. The strain rates estimated for the mylonites next to the MTL are much higher than those estimated for the surrounding metamorphic rocks (∼10−14 s−1), and the displacement rates calculated based on the thickness of high‐strain mylonites and their strain rates are comparable with the average slip rates of the most active intraplate faults in Japan. These inferences suggest that the high‐strain mylonite zones next to the MTL are the exhumed downward extension of a seismogenic fault in the ductile region. The zones were highly localized (&lt;10 m) and experienced very high strain rates (10−11 to 10−10 s−1).

Reexamination of the present stress state of the Atera fault system, central Japan, based on the calibrated crustal stress data of hydraulic fracturing tests obtained by measuring the tensile strength of rocks

To investigate past faulting activity and the present physical state of intraplate faults in Japan, we reexamined the crustal stress measured by hydraulic fracturing tests (HFTs) incorporating the tensile strength of rocks. The tensile strength was measured by fracturing hollow cylindrical rock samples which were obtained close to the Atotsugawa, the Atera, and the Rokko‐Awaji fault systems in central Japan, where in situ stress measurements had been conducted with HFTs. The measured tensile strength data reveal that the reopening pressure was obviously biased in the conventional HFT with a large compliance system. We reestimated the reopening pressure using the measured tensile strength and recalculated the in situ stress around the Atera fault system. We found that the shear stress decreases toward the fault. Although the past long‐term slip direction of the Atera fault system has been considered to be left‐lateral from the geomorphological features, the reestimated stress suggests that the present slip direction is right‐lateral, a finding that is also supported by the present‐day horizontal crustal deformation observed by triangulation and GPS surveys. The amount of accumulated right‐lateral dislocation estimated from the stress data using the dislocation model of Okada (1992) was 2.1 m. Because the current slip rate from the GPS survey is 1.9 ± 1.4 mm/yr, the accumulation period of the right‐lateral dislocation becomes approximately 1100 ± 800 years if the slip rate is stable. This estimation suggests that during the latest earthquake the Atera fault system underwent right‐lateral dislocation.

Seismic Risk Assessment of Active Faults in Japan in Terms of Population Exposure to Seismic Intensity

This study evaluates and compares seismic risks associated with inland crustal earthquakes in Japan on the basis of published data available on the Japan Seismic Hazard Information Station (J-SHIS). First, taking account of prediction uncertainty of the attenuation law of seismic intensity, the evaluation method for population exposure (PEX) to seismic intensity is presented. The method is applied to 333 seismic events potentially caused by main active faults (154 cases) and other active faults (179 cases). The relationship between population exposure and the probability of occurrence of seismic events (“P-PEX relation”) and the resultant seismic risk curves are obtained. Generalized risk index which incorporates the effects of focusing on urgency (probability) or significance (PEX) is defined, producing various risk rankings of active faults.

活断層からの地震発生予測

In order to assess the probability of the occurrence of future large earthquakes based upon the past activities of active faults, we divided active faults into behavioral segments, and adopted the cascade earthquake model, that is, a model that considers that an earthquake is sometimes caused by a single segment and sometimes caused by multiple segments. Using this model, we can evaluate the rupture probability of active faults by a uniform standard without any inconsistencies with field data. The result was published as the Rupture Probability Map of Major Active Faults in Japan.

About this title - Thermochronological Methods: From Palaeotemperature Constraints to Landscape Evolution Models

Thermochronology - the use of temperature-sensitive radiometric dating methods to reconstruct the thermal histories of rocks - has proved to be an important means of constraining a wide variety of geological processes. Fission track and (U–Th)/He analyses of apatites, zircons and titanites are the best-established and most sensitive methods for reconstructing such histories in the uppermost kilometres of the crust, over time scales of millions to hundreds of millions of years. The papers published in this volume are divided into two sections. The first section on ‘New approaches in thermochronology’, presents the most recent advances of existing thermochronological methods and demonstrates the progress in the development of alternative thermochronometers and modelling techniques. The second section, ‘Applied thermochronology’, comprises original papers about denudation, long-term landscape evolution and detrital sources from the European Alps, northwestern Spain, the Ardennes, the Bohemian Massif, Fennoscandia and Corsica. It also includes case studies from the Siberian Altai, Mozambique, South Africa and Dronning Maud Land (East Antarctica) and reports an ancient thermal anomaly within a regional fault in Japan.

Long-term forecast of large earthquakes on active faults in Japan: estimation of earthquake frequency

Long-term forecast of large earthquakes on active faults in Japan: estimation of earthquake frequency