Gamma distribution is used to fit Kumamoto's data of fault lengths by proper choice of two parameters a and β for“ Maxlen model” and a and b for“ Segment model”. The parameter α (or b) serves as a scale parameter but a change inα (or a) depends on the shape of the distribution curve. When a Maxlen model is segmented into k parts, approximate Maxlen-Segment relations α ∼ ka and β ∼ b hold good; this implies both the mean and variance are approximately proportional to k. Assuming that the fault movement is seismogenic, the earthquake magnitude can be converted from the fault length. Such converted magnitude is governed by a distribution law similar to the modified Gutenberg-Richter law proposed by Utsu.
It implemented gamma-ray survey as the geophysical exploration which detects a fault. Gamma-ray anomaly appears only near the fault plane. The occasion when doing measures for the different stratum, the older the period on the stratum is, the more the fracture width gets widely. The fault with the narrower fracture width is the more multitude. It is a small number as a fault with wide fracture width. The cumulation and power-rule are admitted as the regularity with fracture width. It deduced the number of the appearance and the occurring time on the active fault from the regularity with fracture width.
Since the magnitude 7.3 Hyogo-ken-nambu earthquake of 1995, several other damaging earthquakes have struck Japan, including the 2000 Tottori-ken-seibu and 2004 Niigata-ken-chuetsu earthquakes. The latter earthquakes did not have dominant surface ruptures comparable to their subsurface fault lengths, suggesting that seismic hazard assessments should include not only large earthquakes produced by mapped active faults but also moderately sized earthquakes with few surface clues. This short report aims to (1) calculate pre-seismic indexes for the following recent intraplate earthquakes in Japan: 1995 Hyogo-ken-nambu,1997 Kagoshim-ken-hokuseibu,1997 Yamaguchi-ken-hokubu,1998 Iwate-Shizukuishi,2000 Tottori-ken-seibu,2003 Miyagi-ken-hokubu,2004 Niigata-ken-chuetsu, and 2005 Fukuoka-ken-seiho-oki earthquakes; and (2) discuss these indexes based on a fault evolution model (Wesnousky,1999). Earthquake data for 1984-2005 were obtained from the Annual Earthquake Report of the Japan Meteorological Business Support Center and the High Sensitivity Seismograph Network Japan (Hi-net), which is operated by the National Research Institute for Earth Science and Disaster Prevention. Earthquakes of magnitude> 2.0 and depth< 20 km were selected using both 0.2 × 0.2 and 1.0× 1.0 degree rectangular grids with the hypocenter located at the center. The calculated statistical indexes include the cumulative number of earthquakes, magnitude-frequency relationship, Gutenberg-Richter parameters (b and a values), AS-function, and the LTA (long-term average) -function as a function of time. The preliminary results show (a) positive relationships for magnitude, the step width of abrupt seismicity rate changes, and the time until the main shock, although only four data sets were available for regression; (b) a large change in b-value amplitude as a function of time for earthquakes of magnitude 7.0 or larger, with the Hyogo-ken-nambu earthquake having the maximum amplitude and a dominant surface rupture; and (c) indication of seismic quiescences by the AS-function and LTA-function. Although these results were qualitatively interpretable from the fault evolution model, no qualitative relationship was shown from which to predict the time until the main shock and its magnitude. To improve the results, further objective quantitative measurements of continuous/discontinuous fault traces are necessary for incorporation into the fault evolution model.
The 2000 Tottori-ken Seibu Earthquake posed such questions as (1) why coseismic surface ruptures were not associated with earthquake the magnitude of which was 7.3 in JMA scale, or nearly the same as the 1995 Hyogo-ken Nambu Earthquake that accompanied surface ruptures along the Nojima fault and (2) whether the magnitude was predictable in advance or not using tectonic landform and empirical equation between fault length and magnitude in terms of the seismic hazard assessment. Here, we formulated the ellipsoidal rupture areas in order to resolve the difference between surface rupture length and subsurface rupture length in a fault plane. Rectangular fault plane models do not allow the difference. Using this ellipsoidal rupture area, we can improve the relationship between surface fault length, subsurface fault length and magnitude, especially for the fault with short surface length. We used the least-square method to estimate the best-fit coefficients of the ellipsoid fault model with observed parameters of 55 worldwide earthquakes listed in Stirlinget al. (2002). Using our empirical equation based the ellipsoid fault model, the 6km coseismic surface rupture (Fusejimaet al.,2001) of the 2000 Tottori-ken Seibu Earthquake would be as large as magnitude is 7.1. This estimate is far better than the empirical estimate of only 6.1 by Matsuda's (1975) relationship. We also discussed our new model in tectonic viewpoint by applying it to magnitude-frequency relation in certain seismotectonic provinces. Comparison between the Chubu and the Chugoku regions where the density of active faults are respectively high and low, shows the linear relationship between observed seismicity rate and fault length in both regions while the previous model showed a large gap.
Large earthquakes frequently generate co-seismic surface ruptures. These provide much information about surface displacement which produced in a single event. Geomorphologic and geologic studies, such as field investigation and fault trace mapping, are made after earthquake aiming to describe various features of surface fault. Seismologic studies are also carried out in order to estimate source rupture processes for the subjacent source faults. To make an initial model of the subjacent source faults, many fault model parameters are derived from the features of surface faults, for example, location of the surface rupture, style of the faulting and displacements along the fault. It indicates that surface faults contain voluminous information, and their information is important to make practical subjacent source fault models. We focus on the relationship between feature of surface faults and that of subjacent source faults, and make a research of examining the correlation among them. We start this study reviewing the feature of surface faults and fault model parameter and slip distribution of subjacent source faults as a basic data. Our targets are the 1968 Meckering, the 1979 Imperial Valley, the 1983 Borah Peak, the 1987 Superstition Hills, the 1990 Luzon, the 1992 Landers, the 1995 Hyogo-ken Nanbu, the 1995 Neftegorsk, the 1999 Kocaeli, the 1999 Chi-Chi, the 1999 Hector Mine and the 1999 Di. izce earthquakes. In these earthquakes, both surface fault and subjacent source fault are well studied. Compiling the information, we have made the catalog of surface faults and subjacent source faults. The items that contained in the catalog are location of the hypocenter; fault distribution; length; strike, dip and displacement of the surface faults; shape and slip distribution of the subjacent source faults, and so on. This catalog enables us to compare the features of these earthquakes on the unified point of view. Based on this catalog, we found following conclusions.1) Length of subjacent source faults are as long as length of surface faults, or longer than that of surface faults.2) Zones where surface displacements are large correspond to location of asperities on source faults in many cases.
The Kitakami Lowland, running parallel to the volcanic front of Northeast Japan, is typical tectonic depression delineated by active faults. It has been difficult to evaluate the long-term seismic risk on this fault zone, because timing and displacement of the most recent faulting has been poorly constrained. The active faults are located east of and parallel to the eastward thrusting of the Neogene system onto Quaternary sedimentary fill. The most east flexure scarps (FAF3) on alluvial fan are 1-2m high, based on the topographic profiles across scarps. In order to obtain data on paleoseismic activity of this fault zone, we have carried out Geoslicer survey and dug pits on the faulting alluvial fan along topographic profiles. The strata exposed on the Geoslicer sample and pit wall are alluvial fan deposits contained within some lenses of humus. Successive humus named Hu I dated at about 6000 years BP is parallel to the warping ground surface. The age from the most upper humus is about 2600 year BP. This age constrains the timing of most recent faulting event after 2600 year BP with 1-2m vertical displacement. It also suggests that no events occurred between 6000 and 2600 year BP along the FAF3.
An outcrop of the Senya fault, which moved associated with the Rikuu Earthquake in the year of 1896, was newly exposed on the west foot of the Senya hill at the Bodaizawa valley floor, Misato-town, Akita Prefecture (figs. l and 2). The fault dips 30° to the east and extends upward smoothly to the surface of horizontal buried top soil layer. Therefore, displacement of each layer (at least younger than layer II) on this thrust fault is about 1.2m in the vertical component, which is identical with the amount of height of the fault scarp (fig.4), this faulting event is recognized as a last event of the Senya faulting. This new fault exposure and the result of interpretation of the cadastral map (fig.3) and air photography revealed that the surface trace of 1896 faulting was convexly curved to the upstream ward in the valley floor. This is explained by assuming that the fault plane dipping toward the mountain is steeper in alluvial deposits than in the basement rock (Matsuda, et. al.,1980), or that many short faults with en echelon structure are distributed beneath the surface in the valley bottom (Imaizumi, et. al.,1989, in fig.5).
The recent activity of the Shioishi fault, northeast Japan, is investigated on the basis of the interpretations of aerial photographs, field surveys and trenching surveys. Some geomorphic surfaces show cumulative vertical offsets along the active fault. The vertical component is upthrown on the west side: 2 to 3 m on the Holocene L3 surface. The age of the most recent event could be constrained between 360 and 840 B. C. The timing of the penultimate one is bracketed to be between 9,550 yBP and 11,800 yBP. Thus, the estimated average recurrence interval is between 7 to 9.5 ky, and the ground surface has been vertically dislocated 2-3 m by single reverse slip.
The active fault system along the eastern margin of the Shonai plain is a reverse active fault system that extends NS for 40-km long along the western flank of the Dewa hills, northwestern Yamagata prefecture. The subsurface geometry of this fault system is poorly understood particularly in the southern part. To reveal the subsurface geometry of this fault system, shallow high-resolution seismic reflection profiling was performed across the Matsuyama fault. The common mid-point reflection data were acquired using a mini-vibrator truck and 180- channel digital telemetry recording system along a 6-km-long seismic line. The obtained seismic section demonstrates that the Matsuyama fault is a 35° dipping reverse fault, underlain by a subparallel blind thrust in its footwall. Eastward thinning of the Quaternary Shonai Group above the upper tipline of the blind thrust indicates that they were deposited concurrent with fold growth and fault activity. Based on the known rate of vertical displacement of the southern part of Matsuyama fault, the slip rate of the Matsuyama fault during Holocene is calculated as 1.1 mm/yr.
The western margin of the Nagano basin is bounded by north-to-northeast-trending west-dipping active reverse faults that extend for about 58 km. We conducted drilling surveys at Kusama site, Nakano City, in order to estimate timing of recent surface faulting on the central portion of the faults. At the site, a 6-m-high elongated tectonic bulge has been preserved in the basin on and parallel to the basal part of the 40-m-high southeast-facing warping scarp. We obtained 19 cores with length of 1 to 7 meters from the backlimb portion of the tectonic bulge and constrained its subsurface geologic structures with 12 radiocarbon ages measured by means of AMS, and identified active southeast-dipping reverse faults and related warping in the structures, indicating two surface faulting that had occurred after AD1420 (El) and just after AD690-890 (E2). El would be correlated to the 1847 M7.4 Zenkoji earthquake, during which the whole segments of the faults ruptured. In addition, timing of E2 is consistent with that of the penultimate faulting on the northern portion of the faults, which is estimated to have been between AD560 and AD1160 by compiling and re-interpreting the previous studies. Consequently, we suggest that the penultimate surface faulting on the northern to central portion occurred between AD690 and AD1160.
The Itoigawa-Shizuoka tectonic line (ISTL) is one of the major tectonic lines in Honshu Island, Japan, and its northern and central part forms an active fault system. As the slip-rates of active faults provide the basic information for understanding quantitative active tectonics, we estimated the slip-rates along the northern part of ISTL active fault system by tectonic geomorphological study and the aerial photogrammetric survey. We took large-scale air photographs along ISTL. When tectonic landforms were artificially modified, we measured terrace offsets using old air photographs. We described the geomorphological interpretation of deformed landforms in this paper and precisely mapped fault traces. As a result, vertical slip rates of the ISTL active fault system in this area except the northern edge have no significant changes. In the northern edge, the vertical slip rate of the east dipping fault is on the decrease, and that of the west dipping fault is on the increase. Consequently, we inferred that the west-dipping fault began activation instead of the east dipping fault.
We conducted a tectonic geomorphological survey along the northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL) with support from the Ministry of Education, Culture, Sports, Science and Technology of Japan as one of the intensive survey on ISTL fault system. This survey aims to clarify the detailed distribution of the slip rates of this fault system, which provides the essential data set to predict the coseismic behavior and to estimate the strong ground motion simulation. In order to achieve this purpose, the active fault traces are newly mapped along the northern part of the ISTL through interpretations of aerial photographs archived in the 1940s and 1960s at scales of 1: 10,000 and 1: 20,000, respectively. This aerial photo analysis was also supplemented and reinforced by field observations. One of the remarkable results by using this data set is a large number of, here 84, photogrammetrically measured landform transections to quantify the tectonic deformations. We could calculate vertical slip rates of the faults at 74 points, based on the estimated ages of terraces (H: 120 kyrs, M: 50-100 kyrs, Ll: 10-20 kyrs, L2: 4-7 kyrs, L3: 1-2 kyrs). The vertical slip rates distributed in the northern part of the study area show 0.2-5.5 mm/yr on the L terraces (less than 20 kyrs) and 0.05-0.9 mm/yr on the M and H terraces (more than 50 kyrs). The vertical slip rates of the faults located in the central and southern part of the study area are 0.2-3.1 mm/yr.
We intend to calculate the vertical slip rate of the Uozu fault zone for affording the basical data on earthquake disaster prevention at Central Japan. Vertical slip rate of the Uozu fault zone has been estimated to be over lm/ky by previous reports. But the slip rate has been calculated only by the offsets on fluvial terrace surfaces because the offsets due to the frontal fault on buried terrace surfaces have not yet been clarified. We obtained the displacement of the frontal fault on a buried terrace surface by arrayed boring and seismic reflection survey. Fluvial terrace surfaces are divided into higher (H1-H4), middle (M1) and lower (LH1, LL1, lowers) surfaces. Ages of the surfaces are estimated from optical and chemical analyses of tephra particles in loess deposits. Loess deposits on fluvial terrace surfaces include cryptotephras such as K-Tz (Kikai-Tozurahara: 90-95 ka), Aso-4 (85-90 ka), DKP (Daisen-Kurayoshi: 55-60 ka) and AT (Aira-Tn: 25-30 ka) LH1 terrace surface formed about 60 ka because the surface is covered by the 55-60 ka DKP tephra. Buried LH1 terrace surface is recognized by arrayed borings across the frontal fault. Buried LH1 terrace surface recognized in four boring cores is covered by buried loess deposits (including DKP and AT) and recent alluvium in ascending order. H2, H3 and M1 terrace surfaces are older than the 90-95 ka K-Tz tephra because the surfaces are covered by loess deposits (including K-Tz, DKP and AT). H2, H3 and M1 terrace surfaces are estimated to be 280-310 ka,240-260 ka and 155-165 ka respectively as long as the accumulation rate of loess deposits above each surface is constant. The frontal fault is detected in the seismic reflection profile as an east-dipping reverse fault. Vertical slip rate of the frontal fault is calculated as ca.0.1 m/ky from the offset of LH1 buried terrace surface. Vertical slip rates of other faults are calculated by using terrace surfaces as 0.2-0.4 m/ky. Vertical slip rate of the Uozu fault zone is summed to 0.3-0.5 m/ky and the value is half or one-third less than the rate previously reported.
The Morimoto-Togashi fault is a reverse fault extending for 25km along the eastern margin of the Kanazawa Plain. The Morimoto-Togashi fault is characterized by fault scarps several meters high on late Quaternary fluvial terraces and the formative age of fluvial terraces were estimated by Nakamura et al (2003). We measured the vertical displacement of fluvial terraces across the Morimoto-Togashi fault and estimated the distributions of vertical average slip rates. Vertical average slip rates for the Morimoto-Togasi fault are 0.13-0.35 mm/yr (northern part), are 0.05-0.70 mm/yr (central part), and are 0.24-0.40 mm/yr (southern part), respectively. This implies that the peak of vertical average slip rate exist the Kanazawa City area and the Morimoto-Togasi fault does not move with the Sekidosann fault.
The 28-km-long surface ruptures of reverse fault type were associated with the 1945 Mikawa earthquake (Mj 6.8) in the southeastern part of Aichi Prefecture, central Japan. Two traces of surface faulting had appeared along the pre-existed late Quaternary faults named as the Fukouzu (ca 18 km long) and Yokosuka (ca 10 km long) faults, showing a hook-like complex shape with N-S and E-W trending sections. All the existing data concerning the 1945 ruptures have been compiled by Sugito and Okada (2004), and then this paper describes geomorphic and geologic features, characteristics of the faults, and relationship between surface rupture and Quaternary fault. Based on trench excavation and boring exploration surveys recently carried out at several sites by the CRIEPI (Central Research Institute of Electric Power Industry), interpretation by detailed topographic maps and air photos, leveling of fault scarplets, and age estimation for the faulted terraces, characteristics of the seismic faults have been clarified. Average vertical slip rate for the Fukouzu fault are roughly measured to be 0.6 m to 0.9 m per one thousand of years. Recurrence time of the Mikawa earthquake type is estimated about 20,000 to 30,000 yearinterval. The Yokosuka fault is a subordinate fault with weaker topographic and geologic expression and has slightly lower slip rate than Fukouzu fault. Tectonic setting, geomorphological classification and underground structure etc. in this area are also discussed.