Faults In Japan Research Articles

Changes in paleoseismic studies of active faults in Japan since the 1995 Kobe earthquake

This paper discusses recent progress of onshore active faults studies in Japan, especially since the 1995 destructive Kobe earthquake, when the number of trenching studies, which are essential for the reconstruction of onshore paleoearthquakes, has rapidly increased. The timing and repeat interval of paleoearthquakes are here reviewed for the Miura Peninsula, south of Tokyo and the Awaji Island and Kobe-Osaka area, in central Japan, where trenching have been carried out very intensively in the last few years.

Resetting experiment of E′ centres by natural faulting —the case of the Nojima earthquake fault in Japan

The resetting state of E′ centres in quartz in fault clay gouge has been examined using the Nojima Earthquake Fault that moved during the Southern Hyogo Prefecture Earthquake in 1995. As a result, the E′ centres have never been reset by this faulting event. Furthermore, there is no regular tendency among the D e values obtained from the grain samples and the plateau region of the ESR ages does not exist in the case of the Nojima Fault. Therefore, it is not possible to judge the complete resetting of ESR signals by natural faulting. On the other hand, the increase of radiation sensitivity of the E’ centres may be ascribed to the formation of the precursors by α-rays emitted from the radioactive elements adhered around the grain samples. Since the adherence of the radioactive elements may begin with fracturing the quartz grains by faulting, the ESR ages obtained from the unsaturated E′ centres with high radiation sensitivity may give the age of beginning of the fault movements. The ESR ages obtained from these unsaturated E′ centres of the Nojima Fault, 0.88±0.3 Ma–1.51±0.51 Ma (or 1.07±0.36 Ma–1.75±0.59 Ma), are consistent with the geological age estimate of the beginning of the fault movements.

活断層のトレンチ調査結果を用いて推定した日本の内陸地震の長期危険度評価

活断層のトレンチ調査結果を用いて推定した日本の内陸地震の長期危険度評価

Monitoring degradation of the 1995 Nojima earthquake fault scarps at Awaji Island, southwestern Japan

The Nojima fault on the northwestern coast of Awaji Island, south of Kobe, was reactivated during the January 17, 1995 Hyogoken-nanbu earthquake. This fault rupture was dominated by right-lateral offset (max. 1.7 m) along a high-angle reverse fault which has a maximum vertical displacement of 1.3 m on the southeastern side. We repeatedly measured seven profiles across the fault scarp in two areas (Hirabayashi to the northeast and Ogura to the southwest) for approximately 1 year following the earthquake. The original profile of the fault scarp was an overhanging scarp at Hirabayashi and Ogura, corresponding to the 70–80 ° dip of the fault plane. The fault scarp at Hirabayashi displaces Plio-Pleistocene siltstones of the Osaka Group and is overlain by a thin bed of unconsolidated gravel. The Ogura area is entirely underlain by the Osaka Group. Scarp degradation at Hirabayashi occurred by collapse of the gravel bed and proceeded more quickly than at Ogura, where fault scarp degradation proceeded mainly by exfoliation of the Osaka Group siltstones. The degradation occurred at a very fast rate until March at Hirabayashi, and until June or July at Ogura. Since then, the degradation has been very slow. Our data strongly indicate that the scarp profile was initially controlled mainly by the dip of the fault plane, and scarp degradation has been primarily controlled by lithological factors. The degradation of the Nojima earthquake fault scarp proceeded much more quickly than that of normal fault scarps in the western U.S.A., where many observations of the initial stages of scarp degradation have been carried out. The extremely rapid degradation of the Nojima fault scarp in weak late Neogene siltstones might, in combination with rapid cultural modification of the landscape, explain the paucity of geomorphic scarps along the numerous active faults in Japan. This observation may also have implications for tectonic geomorphology and paleoseismicity studies in other countries characterised by weak bedrock and moderate to high rainfall regimes.

AMS 14C chronological study of holocene activities in active faults in Japan

A submarine active fault, the Toyooka-oki fault, located on the northwest end of Beppu Bay, northeast Kyushu, was investigated by observing the fault structure preserved in the submarine sediments using a seismic profiler, and by collecting 15–20 m long sediment cores using a piston corer, on both sides of the fault. A distinct accumulation of the vertical displacements induced by seismic events is recognized by correlating specific layers using the seismic profile data between the two cored sediments. Vertical displacements of offset horizons on both sides of the fault are larger for the deeper sediment layers as compared with the upper layers. AMS 14C dating was performed on a total of 32 calcareous samples collected from both cores and two seismic events are clarified using the age-depth profiles of sediment samples on both sides of the fault. We estimate vertical displacement and timing of the events of ca. 2.8 m around 1900 Cal yr BP and ca. 2.0 m around 4800 Cal yr BP, for the Toyooka-Oki fault.AMS 14C dating was applied to identify the previous activities of the Nojima fault, located on the northwest coast of Awajishima, Hyogo Prefecture, which was active during the 1995 Hyogoken Nanbu Earthquake. A trench survey conducted just after the earthquake confirmed that the Nojima fault has repeated displacements over in the past 2000 years. A definite one occurred about 2000 years ago and a possible one about 400 years ago, which may have been caused by the 1596 Keicho earthquake of estimated magnitude 7.5.

Stress Changes Caused by Strike-Slip Faulting and Secondary Faulting Induction in Conjugate Directions.

As regards strike-slip faultings generated inland in Japan, we occasionally observe conjugate fault activity in association with main faultings. In most cases, conjugate faults appear on the side of main faults where frictional force on conjugate faults decreases in response to the main faulting. We present two examples which typically show mutually conjugate activities. These are the pair of the 1978 Izu-Oshima-kinkai earthquake (M7.0) and the 1990 earthquake near Izu-Oshima Island (M6.5) and the pair of the 1984 Western Nagano prefecture earthquake (M6.8) and its largest aftershock (M6.2). It seems rare that conjugate fractures were propagated in regions where frictional forces on conjugate faults were expected to increase. These facts indicate that changes in frictional force plays an important role in the generation of secondary fault activity at least in inland strike-slip faults in Japan. In other words, the frictional coefficient in the Coulomb failure function may not be as small as 0.1∼0.3 but as large as 0.5∼0.7 in its value in inland strike-slip faultings in Japan.

Gently north-dipping Median Tectonic Line (MTL) revealed by recent seismic reflection studies, southwest Japan

The Median Tectonic Line (MTL), with a length of more 1000 km, is the most significant fault in Japan. It juxtaposes the high- P/T Sambagawa metamorphic rocks against the low- P/T metamorphic rocks of the Ryoke belt. The MTL was probably formed in the Cretaceous with many subsequent reactivations. The western segment of the MTL is still active with an almost pure right-lateral sense of motion. Although a great amount of geological information on the MTL has been accumulated, information about the subsurface, especially the deep-seated structure of the MTL, is still insufficient. It has been generally assumed that the MTL is vertical or steeply dipping at depth because of its straight surface trace and its recent lateral motion. Recently, new geophysical data have suggested that the MTL dips gently northward at depth. We have acquired a complementary set of geophysical profiles (seismic reflection and refraction, gravity and MT) across the MTL in east Shikoku. Our results confirm that the MTL dips northward at about 30 to 40 degrees from the surface to about 5 km depth, where it becomes listric. This fault geometry more reasonably explains the reactivation history of the MTL: the motion has occurred on a listric-type fault in so-called oblique or lateral ramp manner.

Direct ESR dating of fault gouge using clay minerals and the assessment of fault activity

The ESR dating method has been applied to the fault gouge of the Nojima Fault, the outcrop of which arose due to the southern Hyogo Prefecture Earthquake on January 17th, 1995, and to the fault gouge of the Median Tectonic Line (MTL) which is a major active fault in Japan, without considering the resetting of ESR signals. The ESR ages obtained for clay minerals in the fault gouges are considered to indicate the age of the beginning of the fault movements at the outcrop. Assuming no radon loss and the general values of water content and α-ray efficiency, the ESR ages of the fault gouges of the Nojima Fault and MTL are estimated as 0.55–1.16 and 6.76–9.28 Ma, respectively. The ESR age of the Nojima Fault is consistent with the age of the beginning of the Rokko Movements in the Kinki districts estimated geologically. On the other hand, the ESR age obtained from the MTL is younger than the KAr age (about 11 Ma), which is presumably overestimated due to the existence of the source minerals in the KAr dating sample. Since it is geologically known that the MTL at the outcrop had moved until the Late Pleistocene-Holocene, the fault movement of the MTL at this outcrop probably began at 6.76–9.28 Ma B.P. after the intrusion of the felsitic dyke (12–16 Ma) which is the source rock of the fault gouge into the fault boundary. The ESR dating of clay minerals in the fault gouge may be effective for assessing fault activity if no Quaternary deposit exists at the outcrop. The younger the ESR age of the fault gouge is, the higher the fault activity is. If not indurated and the freshest clay gouge can be collected at the outcrop, the ESR age of the most recent movement may be obtained from the clay gouge.

The 1995 Kobe earthquake and problems of evaluation of active faults in Japan

The Kobe earthquake ( M 7.2) of January 17, 1995, which was the most damaging earthquake in recent Japanese history, made manifest the need for reconsidering the method of evaluating active faults. An earthquake of this magnitude at this time was unexpected according to conventional evaluation, in which the potential magnitude of earthquakes at a certain site is estimated by considering the greatest earthquake in the past 400 years and the length of the active fault. The following characteristics of this earthquake made it appear unlikely by conventional understanding: (1) the Kobe earthquake involved several neighboring faults, which had been previously been identified as separate fault systems: (2) the surface rupture of about 10 km length was much shorter than the 50 km seismic faulting; (3) the interval of 400 years between the Kobe and penultimate Keicho earthquake of 1596 AD ( M 7.5), which has been revealed by historical documents and some excavations, is much shorter than the 2000 years estimated by calculating the average slip rate of displaced landforms. These shortcomings imply that active fault evaluation with the traditional characteristic earthquake model which deals with each fault separately, is not adequate for an area like Japan where active faults swarm. New concepts such as the block rotation model (Kanaori, 1990; Late Mesozoic-Cenozoic strike-slip and block rotation in the inner belt of Southwest Japan. Tectonophysics, 177: 381–399) considering the macroscopic tectonic framework for fault interactions are needed. Furthermore, fault dynamics cannot be ignored; physical and temporal parameters associated with faulting, such as moment release rate, must be considered for realistic and precise evaluation.

Recent Progress in Tectonic Geomorphology in Japan, with Special Reference to Active Fault and Related Tectonic Studies

Active fault and related tectonic studies in Japan are chronologically reviewed from the viewpoint of tectonic geomorphology. The future prospects are also described based on the trends of recent progress in active fault study and the surrounding social circumstances. Since the publication of Active Faults in Japan in 1980, the field in Japan has roughly been split into two domains: the reconstruction of detailed fault behavior in Holocene times; and the elucidation of the relationship between active faults and regional tectonics. The former describes various kinds of fault behavior through the application of the trench excavation method. Although the historical activity has been uncovered on many active faults, the surveys have become increasingly difficult due to the artificial modification of land. New proposed methods such as drilling, however, have not yet developed sufficiently to substitute for trenching. The latter domain discusses various topics related to the characteristics of long-term tectonic behavior from the viewpoint of landform evolution. Studies on the net slip-rate of thrust faults and the migration of thrust fronts have brought new knowledge and ideas to landform geomorphology and seismotectonic research. The morphotectonic and seismotectonic studies in offshore and onshore regions along the Nankai Trough have sufficiently explained the tectonic and landform evolution on the island arc scale in Southwest Japan. Based on present conditions, the author proposes the following directions which should promote progress in active fault study and to utilize the results for hazard mitigation in the future: (1) to distinguish the segmentation structure in the active fault system; (2) to establish seismotectonics models for various regions in Japan; and (3) reconsideration of the recurrence interval of large earthquakes.

Audio-Frequency Magnetotelluric Imaging of an Active Strike-Slip Fault.

A tensor audio-frequency (10, 000-1 Hz) magnetotelluric method was used to image the Kita-Izu fault system, a typical active strike-slip fault in Japan. Regional strike direction (N55°E) was first determined after decomposing tensor impedances; this direction is consistent with the strike of geomorphology and also with the strike of the fault. Local anisotropy and site gain were then corrected for each site. Finally, the impedances were inverted using two-dimensional modeling with a smoothness constraint. The model shows (1) that the fault corresponds to a discontinuity of the surface resistive volcanic cover and (2) that there is a significant conductive anomaly (∼ 1 Ωm) beneath the valley at 500 m depth from the surface, which may correspond to a fractured zone.

庄内平野東縁における完新世の断層活動と地震周期

Earthquake recurrence intervals and characterization of fault behavior provide important data for seismic risk evaluation. Historical records of destructive earthquakes are insufficient to characterize paleoseismicity of inland active faults in Japan, because most of the active faults have recurrence times of more than several thousand years. Geological and geomorphological investigation including excavation studies is indispensable for a long-term prediction of inland earthquakes. We present a study on paleoseismicity regarding the Kannonji fault, which is one of the eastern boundary faults of the Shonai plain in the northeast Japan, based on analysis of the borehole data close to the fault. Stratigraphic analysis and radiocarbon ages of the samples collected from the boreholes have revealed the three surface faulting events in the Holocene. The latest faulting event occurred after 2, 500yBP; the prior one occurred between 5, 500yBP and 4, 300yBP, probably between 4, 500yBP and 4, 300yBP. The third latest faulting event occurred around 6, 300-6, 000yBP. These data suggest that recurrence intervals of earthquakes are about 2, 000 years. According to a previous study by Suzuki and others, no surface faulting occurred at the borehole sites in association with the Shonai earthquake of 1894, although the earthquake was generated from this fault. This fact strongly indicates that other unknown large earthquakes on this fault plane possibly occurred in Holocene time causing no surface faulting. Therefore, a recurrence interval of large earthquakes from this fault is eatimated less than 2, 000 years.

京都市桃山断層の反射法地震探査

Seismic reflection survey was conducted in Fushimi-Momoyama, Kyoto in November 1990. A linear array was spread across the extension of the Momoyama fault, which strikes from north to south at the eastern edge of the Kyoto basin. The length of the survey line was 1200m. A main aim of this survey was to investigate subsurface structure up to the depth of about 1km which possibly reveals the activity of the growth history of the Momoyama hill as well as the Momoyama fault. Seismic reflection survey is capable of imaging sedimentary layers because of its relatively short wavelength. A hydraulic weight dropping machine named “Yuatsu impactor” was used as a source apparatus. Through the data-processing by the CDP method and reverse time migration, the following results mainly were obtained.(1) The basement and some sedimentary layers were identified. They are bent beneath the surveyline and their maximum dip is about 20 degrees to the west. The depth of the basement is about 300m beneath the Momoyama hill (east side) and about 600m in the Kyoto basin (west side).(2) From the change of the dip in each layer, we found that the movement forming the Momoyama hill began less than 0.9 million years ago. Additionally, from the depth difference of each layer, the relative velocity of faulting since then is estimated at about 0.2m per 1 thousand years. This indicates that the activity of this movement is B class as noted in “Active Faults in Japan” (1991).

断層の幾何学的バリヤと破壊プロセス

Geometric patterns of surface earthquake faults are closely related to rupture propagation processes especially in the case of strike-slip faults. In this paper, the author examined geometric features of four strike-slip earthquake faults in Japan, that is, the Kita-Izu earthquake fault system of 1930, Izu-Oshima Kinkai earthquake fault system of 1978, the Nobi earthquake fault system of 1891 and the Kita-Tango earthquake fault system of 1927. Bends or jogs along a fault can be called as geometric barriers, which are classified into two categories as extensional and compressional, from a view point of volumetric change (Fig. 1). Studies of earthquake surface faults of recent inland large earthquakes in Japan show that ruptures were initiated from compressional barriers and terminated at extensional barriers. A compressional barrier in the course of rupture propagation could be the position of next rupture nucleation. Compressional barriers can be commonly described as “fragmentation barriers” (King, 1987) because short faults of various orientation are very popular in and around compressional barriers. The location of severe damage area is also closely related to that of barrier.Study of geometric characteristics of active faults will become very important because they give us a key idea on rupture initiation and termination as in the case of surface earthquake faults. Recognition of barriers along an active fault system from geometrical investigation will provide us some locations of detailed observation for geodetical, geophysical and geochemical anomalies along precautious faults.

Increase of radiation sensitivity of ESR centres by faulting and criteria of fault dates

The ESR spectra and ages of the fractured rocks along the Itoigawa-Shizuoka Tectonic Line, a major fault in Japan, were measured. The ESR signal intensities tend to become larger with decreasing distance from the fault boundary, because the radiation sensitivities of the ESR centres have increased by faulting. According to fracturing and heating experiments, when the radiation sensitivities of ESR centres have visibly increased, the signals may have been almost completely reset by faulting. As a result of the increase of radiation sensitivity, the error range of the TD values for incompletely reset ESR signals is comparatively large, whereas it is smaller in the case of completely reset signals. Therefore, by assessing the error range of TD values we can determine whether ESR signals were completely reset at the time of faulting.

Applicability of ESR dating using multiple centres to fault movement — The case of the Itoigawa-Shizuoka tectonic line, a major fault in Japan

No direct method for dating the fault movement has been hitherto established, except ESR dating whose applicability is disputed however. The most serious question in ESR dating is whether accumulated ESR signals are completely reset at the time of fault movement. When the defect centres are partially preserved at the time of previous faulting, the date obtained is uncertain. We have shown (Fukuchi et al., 1985, 1986), however, that we can judge correctly whether ESR signals were completely reset or not when the ESR signal counting is done for multiple ESR centres of different stabilities for resetting. From the data for completely reset centres, actual date of fault movement can be calculated. This multiple centre method was tested with samples whose ESR signal intensities are judged to be about zero at the time of fault movement. The result, which confirms the applicability for the fault movement dated back to 100 ka BP, is presented in this paper.

Implications of mylonitic microstructures for the geotectonic evolution of the Median Tectonic Line, central Japan

The Median Tectonic Line (MTL), the most prominent onshore fault in Japan, demarcates the Cretaceous Hiji quartz dioritic gneiss of the Ryoke belt on the west from the high P/T type Sambagawa metamorphic rocks on the east in the Takato area. Toward the MTL, the Hiji gneiss grades into strongly mylonitized rocks characterized by grain-size reduction of quartz. In the mylonitic rocks, the development of fluxion banding (Sm) is remarkably influenced by the existence of K-feldspar, forming a myrmekitic intergrowth. Brittle microstructures indicative of truly cataclastic deformation are observed only in mylonitic rocks close to the MTL. Early deep-level ductile deformation apparently gave way to shallower, brittle deformation at a later stage. The attitude of stretching lineations ( Lm) and asymmetric microstructures observed in the mylonites suggest that sinistral strike-slip shearing with a subordinate component of vertical-slip took place during mylonitization in mid-Cretaceous time. The mylonitic rocks and their protolith, supposed to have constituted the eastern limb of the shear zone, were probably eroded out and lost by upheaval of the Sambagawa belt relative to the Ryoke belt.

Relation between hydrogen emission and seismic activities

Measurements of chemical composition of bubbles from a mineral spring at Yuya Spa situated close to the Median Tectonic Line, the longest active fault in Japan, showed that the periods of increased H2 emission coincided with occurrences of the Ohno earthquake swarm nearby. Four cases of the coincidence without exception were observed in the last three years. The fluctuation of H2 concentration ranges between <0.5 and 200 ppm, whereas other gases such as He, Ar, N2, and CH4 do not fluctuate much. The H2 concentration is correlated with the energy released by the seismic activity. This field evidence, together with the results of laboratory experiments conducted bySugisakiet al. (1983), leads to the conclusion that H2 observed at the mineral spring was produced by the reaction between groundwater and rock fractured in the seismic activities. The observation that H2 in the mineral spring tends to appear prior to an earthquake suggests that microcracks may occur in rocks prior to earthquakes. The precursory emission of H2 may be useful for earthquake prediction.

1980年坂出・一宇爆破地震動観測による四国東北部の地殻構造

Explosion seismic experiments were carried out in 1980 for the investigation of the crustal structure in the northeastern part of Shikoku. Sixty-three seismograms were obtained at 42 temporary stations aligned on the two N-S trending parallel profiles crossing the Median Tectonic Line which is the most prominent geotectonic line and the greatest active fault in Japan. A P-wave velocity structure of the surface and granitic layers was obtained from the first arrival time data. A structure of the deeper part of the crust was also roughly estimated from the later arrivals. The Median Tectonic Line was estimated to be a north-depressed fault with a vertical offset of up to 2km in the shallow velocity profile. A possibility of the extension of the offset further downward to the deeper crust is also suggested.

断層粘土中の石英粒子の表面構造による断層活動性の評価の試み Ｉ 表面構造の分類と形成過程

Surface textures of quartz grains from about 250 fault gouge samples with various types of fault in Japan are examined by means of the scanning electron microscope. It is disclosed that the surface morphology of the grains is variable, but quartz grains extracted from one fault gouge sample commonly show a type of texture different from those observed on the grains from the other samples. The surface textures observed under the microscope are tentatively classified into eight types; subconcoidal, orange peel-like, fish scale-like, moss-like, motheaten, stalactitic, pot-hole, and coral-like textures. These textures can be classified into four groups of I to IV. Arranged in the order of their apparent features, it is interpreted that the progressive corrosion of quartz grains by ground water has taken place after faulting. The change of this surface feature can assist in estimating the time elapsed since the last fault activity.