Three factors, the cumulative slip, the fault length and the width (thickness) of the crushed zone, are examined to compare the development of active strike-slip faults in Chugoku and Chubu districts. The cumulative slips since the latest Cretaceous in the both districts are about the same, but the slips of Quaternary are distinctly smaller in Chugoku than in Chubu, indicating the different growth history during Tertiary and Quaternary. Lengths of the major fault zones are similar in both districts, but the ratio of the frequency distribution of shorter faults to the longer ones is larger in Chugoku. In general, the widths of crushed zones are smaller in Chugoku than in Chubu. The length of fault zones and the width of crushed zones are both proportional to the Quaternary slip, implying the former two are mainly products of the Quaternary. Origin time of the late Quaternary faulting of these active faults is about the same (ca. l Ma ) in both districts, since the long-term slip rate in late Quaternary is much smaller in Chugoku than in Chubu. The Chugoku district is about the same as the Chubu in the development of the fault in respect to cumulative slip since the latest Cretaceous. However, it is less developed in Quaternary time than Chubu district as indicated by the smaller amount of the Quaternary slip and the width of crushed zones as well as the relative abundance of the shorter faults.
Statistical earthquake analysis, that is represented, for example, by the Gutenberg-Richter relation (GR; Gutenberg and Richter,1944) and by historical and/or instrumentally observed seismicity, plays an important role in seismic hazard assessment, especially to answer the fundamental question of whether large and small earthquakes are related. The aims of this report are: (1) to make compiled data sets of historical earthquake records, instrumentally observed seismicity, and seismogenically active faults from the viewpoint of seismotectonic provinces in Japan (Kakimiet al.,2003); (2) to show the GR relation for each seismotectonic province, in order to discuss the continuity of magnitude-frequency distributions for different sources; and (3) to make reference data sets of instrumentally observed seismicity, cross-referenced to distance from active faults. The magnitudefrequency distributions for the inland seismotectonic provinces form several groups, based on the relation between instrumentally observed seismicity, the historical earthquake record, and active fault data.
Matsuda's formula connecting the seismogenic fault-length L with the earthquake magnitude M is expressed in the form of log L: -α +β M with the two coefficients α= 2.9 and β =0.6. In the previous volume of this journal, we proposed a new method for statistically estimating these coefficients on the assumption that the statistical distribution of log L can be transformed to that of M through the above formula with unknown values of α and β. In this study, the proposed method is applied to Kumamoto's data of the maximum and the segmented seismogenic fault-length, and then we estimate the respective values α MAX, β MAX, α SEG andβ SEG. The actual seismogenic fault rupture is possibly realized within the ranges of αSEG< α<αMAX andβ SEG< β < β MAX. Kumamoto's data file includes the type of fault such as reverse or lateral. The coefficient β is related to the seismic energy contribution to increasing fault length, so that its value is presumed to be larger for lateral-type faults than for reverse-type ones. Using Kumamoto's data file, we examine its fault-type dependency, and prove as a result our presumption is right.
In Japan, the empirical formula proposed by Matsuda (1975) mainly based on the length of historical surface fault ruptures, is generally applied to estimate the size of future earthquakes from the extent of existing active faults for seismic hazard assessment. Therefore validity of the fault extent and defining individual segment boundaries where propagating ruptures terminate are essential and crucial to the reliability for the accurate assessments. It is, however, not likely for us to clearly identify behavioral earthquake segments by the observation of surface faulting during the historical period, because most of the active faults have longer recurrence intervals than 1000 years in Japan. Besides uncertainties of the datasets obtained from fault trenching studies are quite large for fault grouping/segmentation. This is why new methods or criteria should be applied for active fault grouping/segmentation, and one of the candidates may be geometric criterion of active faults. Matsuda (1990) used“ five kilometer” as a critical distance for grouping/separation of neighboring active faults. On the other hand, Nakata and Goto (1998) proposed the geometric criteria such as (1) branching features of active fault traces and (2) characteristic pattern of vertical-slip distribution along the strike-slip fault traces as tools to predict rupture length of future earthquakes. The branching during the fault rupture propagation is regarded as an effective energy dissipation process and could result in final rupture termination. With respect to the characteristic pattern of vertical-slip distribution, especially with strike-slip components, the up-thrown sides along the faults are, in general, located on the fault blocks in the direction of relative strike-slip. By applying these new geometric criteria to the high-resolution active fault distribution maps, the fault grouping/segmentation could be more practically conducted. We tested this model successfully on the active faults generated the 1943 Tottori earthquake, the Chojagahara-Yoshii fault zone in Chugoku district in Japan, as well as the active fault system in northern Luzon, the Philippines. Thus, we name this conceptual model as“ Morphometric Unit Model” and call the active faults grouped by the model as“ Packaged Active Faults” for individual seismogenic faults. Moreover, we come to know that active fault mapping with the model in mind enables us to find many new active fault traces (e. g., the Shigenobu fault along the MTL in Japan).
An outcrop across the tectonic bulge on the alluvial fan surface was observed in the Obanazawa basin, Yamagata Prefecture. Young fan deposits ( sand and gravel ) of this surface are down warping toward the east, concerned with the inferred thrust fault.
The Northeast Japan arc as an island arc is morophotectonically characterized by a north-south trending chain of inter-mountain basins relatively subsided by Quaternary reverse faulting. The Nagai Basin among them has been produced by the fault movement of western marginal fault zone, set between granitic mountains. Our precise airphoto reading improved geomorphic fault traces and their continuity, and our field investigation using Quaternary method and leveling survey clarified the late Quaternary activity of the fault zone, ca.22km long. Geomorphic fluvial terraces distributed in the basin are classified into 5 levels: H, Ml, M2, L1, and L2 terraces. These terraces have been displaced with the vertical accumulation of faulting that shows the mean rate of about 0.5 m/ka. This western marginal fault zone of Nagai Basin is probably originated from the Tanakura tectonic line truncating the late Cretaceous granitic rock. Its subsurface structure is estimated to be simple, dipping west with relatively high angle. Vertical displacement of the fault in Quaternary reaches more than 1400m. The last seismic event generated from this fault zone undoubtedly occurred since ca.2000 years ago, and a vertical slip at an event is around 2.5m. The earthquake time recurrence possibly is between 3,000 and 7,000 years.
This paper discusses the cumulative displacements and formation process of the Uonuma hills based on the geological cross sections and the dislocation of the fluvial terraces, central Japan. The Uonuma Hills has tilted to the west by the Muikamachi fault (NNW-SSE strike, length ca.30 km) at the eastern margin and the Suwatoge flexure (NNW-SSE strike, length ca.15 km) in the hills. Their active fault has the natures of the reverse fault of the west dip. The Uonuma Hills has uplifted with the uniform slip rate during the Quaternary by the Muikamachi fault. The slip rates are ca.1.0 m/ky at the north area and ca.2.0 m/ky at the south area of the Muikamachi fault. The longitudinal profile of the Uonuma Hills has the asymmetric form, which plunges from the south to the north, depending on the slip rate. Suwatoge flexure locates with the parallel to the Muikamachi fault in the northern area of the Uonuma Hills. The slip rate is ca.1.0 m/ky. Thus, the strains has been deposited and released with ca.2.0 m/ky slip rates, respectively, in the north and south areas of the Uonuma Hills.
The western marginal fault zone of Nagano Basin has been a tectonically active in late Quaternary, appearing as a reverse fault by strongly crustal shortening in the northern Fossa Magna area, central Japan. The Iiyama Basin is a northern part of the tectonically subsided basin by such faulting. However, two groups of evenly running faults have migrated forward since middle Pleistocene, accumulatively deforming the fluvial terraces, and the previous basin area around the Chikuma River changed the upthrown side. Average vertical slip rate is evaluated to be> 0.2 m/ka in the eastern group of migrated faults, and around lm/ka in the western group, based on the displacement and age of fluvial terraces. This indicates that both migrated fault groups have been active in late Quaternary, possibly partitioning the fault slip by horizontal crustal shortening.
We performed a geomorphological and geological survey to clarify the detail geometry and faulting of the active fault around the Gero fault of the Atera fault belt, which is one of the prominent faults in central Japan, and also considered the channel migration around the fault. This survey area is dominated by many tectonic landforms and fault outcrops. The northwestern part of the Gero fault is characterized in the fault of middle and low dips northeast, and the fault which branches toward north. We think that these structures were formed for the overlapping strike-slip faults, namely the Gero fault and the Yugamine fault. Judging from the topography and geology around the Gero fault, we think that the Norimasa River at one time passed the neighborhood of the northern part of Nojiri district through a stream bent to left-lateral offset. Similarly, the Ogawa-tani River also was a stream bent to left-lateral offset, and passed the Hachiya-toge Pass, probably. Amounts of the bend along the fault of the restored Norimasa River and Ogawa-tani River are about 2.8 km and about 1.0-2.5 km, respectively.
Pit excavation across a small uphill-facing fault scarp on a mountain slope could be a paleoseismological tool for active faults without suitable sites for traditional trenching. We tested this method on the Neodani fault, which is one of causative faults for the 1891 M 8.0 Nobi earthquake. We found an unnatural small uphill-facing scarp on a mountain slope just along the trace of the fault and decided that the scarp should have tectonic origin. However, pit excavation across it, radiocarbon ages from the pit, and following additional investigation on historical documentations reveal that it is probably one of ruins of a“ shishigaki” (man-made protection wall against damage to crops by wild animals), which was built during the late Edo period (1804-1810 A. D. ). With the knowledge of a shishigaki, we would have previously recognized that the scarp was not a tectonic one: key observations are presence of level land along the scarp that used to be a paddy field (now woods), and common lack of recognizable vertical offset of a mountain slope across the scarp. As ruins of a shishigaki are reported to be widely distributed in all Japan except for Hokkaido and Tohoku regions, great care should be exercised with them in recognizing small fault scarps on a mountain slope.
We have compiled all the existing data about geomorphic and geologic features of the surface rupture associated with the 1945 Mj 6.8 Mikawa earthquake, which struck the eastern part of Aichi Prefecture, central Japan. The 28-km-long surface trace of the rupture, including a 10-km-long submarine section in Mikawa Bay, showed a hook-like complex shape that consisted of three sections: two N-S-trending sections comprised of the northern 7-km-long section and the southern 14-km-long section, and the other section, extending for 7 km from east to west between the two N-S-trending sections. On the basis of the compiled data and reconstructed slip distribution along the surface rupture, we suggest that 1) nearly pure thrust faulting along the southern N-Strending section was predominant among the whole surface faulting during the earthquake,2) probably, this thrust faulting was a result of E-W to ENE-WSW-trending compressional stress in the upper crust, and 3) the E-Wtrending section behaved itself as a kind of tear fault, which was an oblique-slip reverse fault associated with the nearly pure reverse faulting along the two N-S-trending sections. The maximum amount of vertical offset was approximately 200 cm. The source fault model constructed by Kikuchi et al. (2003) includes two asperities, and the northwestern asperity covers a wider area and exhibits a larger amount of slip than the southeastern asperity does. This doesn't agree with the 1) above. Further investigations are needed to understand what causes the discordance.
Buried wood fragments -2.8m below the surface were measured as 4,500-4,600yBP in the alluvial plain at the southwestern part of Yanagase fault, which extends about 28km with NNW-SSE direction at the NE side corner of Lake Biwa, central Japan. As the former river course from the Lake Yogo lowland to the Nakanogo windgap is adapted as the distinguished fault reference across the Yanagase fault, culumative vertical and leftlateral amounts of displacement during late Quaternary are roughly estimated to be about 300m (west side down) and 0.4- 0.5km, respectively. The approximate age of basal gravels below the Lake Yogo lowland and gravels at the Nakanogo windgap is inferred to be around 0.4 million of years, presuming that alluvial sediments in the lowland have constantly been deposited to the basement. Using this age and amounts of displacement, average vertical and left-lateral slip rates along the Yanagase fault are obtained to be in the order of 0.75-1m and 1.0-1.5m, respectively per thousand of years. The westside down vertical displacement along the Yanagase fault sharply becomes larger to the southward, approaching to the Lake Biwa. This westside subsidence along this fault might has been triggered by the appearance and migration to the present situation of Lake Biwa since the late Quaternary(0.3-0.4my). This left-lateral late Quaternary faulting has simultaneously reactivated diversifiing former topograpy and using the pre-existed fault shattered zone. Although structural, geologic and geometric segmentations along the Yanagase fault zone is proposed by the characteristics of each fault, more detailed informations for these faults are required for the recognition to the earthquake and behavioral segmentaions. And concealed active faults under the alluvial plain are estimated by the photo-interpretation, geomorphological and geologic examinations on the southward extension of the Yanagase fault.
The Ujigawa fault with about 9km length has been discovered by a seismic reflection (P waves) survey in the central part of the Kyoto basin and has been identified as a quite large active concealed fault revealing a flexure within the thick sediments. To elucidate such characteristics as distribution, activity and faulting history of this fault, we have carried out several survey lines for seismic reflections and deep drilling surveys across this flexure zone, and have also performed volcanic ash analysis, carbon-14 dating, pollen and diatom analysis by using core samples obtained. In this paper, we report the results of these surveys, especially on the vertical displacement during the late Quaternary. Because of technical restriction in the survey methods, we can not discuss about the strike-slip movement. The Ujigawa fault extends with ENE-WSW direction from the confluence of the Katsura and the Uji rivers through the course along the Uji to the south rim of the Momoyama hills, where greatly changes its trace from the southeast edge to continue the reverse fault trending north to south. The southwestward extension of this fault, on the other hand, may trace along the south side of the Takatsuki-Tennouzan fault, part of the Arima-Takatsuki fault zone, which main zone is situated in the further west-southwest along the northern fringe of Osaka basin. From the identification of the wide-spread volcanic ash layers and the measurement of radiocarbon ages in the core samples obtained, the latest activity of the Ujigawa fault is presumed to have taken place after about 3,000 years ago. The amount of vertical displacement per single earthquake is estimated to be 1.2-2.1m and the average recurrence interval of earthquakes is roughly 16,500± 6,500 years. The average slip rate of vertical displacement for this fault was 0.09-0.12m per thousand years during, at least, the last 520,000 years. This value indicates that the activity for the Ujigawa fault belongs between the lowest rank of the class B and the highest rank of the class C in Japanese classification. The Ujigawa fault appears to have shared an important role in the separation of the Kyoto sedimentary basin, where the northern part has relatively thin layers of the Quaternary sediments with 200-300m thickness and the southern part accompanies relatively thick layers of 500-600m.
The Arima-Takatsuki fault zone consists of a series of ENE-trending right-lateral strike-slip faults along the northern margin of the Osaka plain. This fault zone is considered to have ruptured during the 1596 Keicho-Fushimi earthquake based on paleoseismic trench excavations across the fault zone conducted by the Geological Survey of Japan and numerous liquefaction features on archaeological sites in Kobe-Osaka-Kyoto areas. The surface offset during the earthquake was estimated to have been about 3 m based on offsets of rice-paddy dikes at two localities. In order to search for more data on the surficial slip during the 1596 earthquake as well as evidence for prehistoric earthquakes, we have interpreted large-scale aerial photographs and conducted geomorphic field investigations. We have identified several localities along the fault zone where geomorphic and/or artificial features are systematically offset about 3 m, confirming previous estimates of the surficial slip during the 1596 earthquake. Larger displacements of older geomorphic features also suggest repeated right-lateral slip on the fault zone in late Holocene time.
The Arima-Takatsuki fault zone (ATFZ) consists of active intraplate strike-slip faults that extend for about 42 kilometers in southwest Japan. The ATFZ is thought to have ruptured in a historic earthquake in A. D.1596. A trench opened for this study across the right-lateral Boshima fault reveals evidence for Holocene seismic events. We surveyed this trench wall excavated along trend with late Pleistocene to Holocene fault scarps and offsets of stream channel walls. A fault-perpendicular trench exposes mainly coarse-grained fluvial deposits, as well as liquefied silt, that define the main fault zone. Analysis of stratigraphic relations and radiocarbon datings in Holocene fluvial deposits indicate at least four events. Among them period of the most recent earthquake postdates the fifth to sixth century humic soil. Although the uppermost section of strata is artificially disturbed, our result indicates the faulting event along the ATFZ occurred in the historical time.
Chojagahara fault in southeast of Hiroshima prefecture and Yoshii fault in southwest of Okayama prefecture are both NE-SW extending right lateral strike slip faults being separated with 10km gap in Kannabe plain. Distinctive geomorphic evidence along the Chojagahara fault suggests that the fault has moved during the recent geological period. We identified young sharp lineaments forming left echelon stepping in the plain, cutting and disturbing the rectangular field pattern originated from“ Jon-lot system” on alluvial lowland, indicating their activity during the historical period. A trench was dug across one of the lineaments to confirm the latest event of the fault, and a fault outcrop that cuts up to the base of cultivated soil appeared on the trench walls. The fault strikes N530E, and dips 700N and the northern block has been pushed up. The radiocarbon ages agrees that the latest event took place after the“ Jorilot system”. The pattern of dip-slip distribution along these strike-slip faults, i. e. the northern side up in the east and southern side up in the west, also suggests that they continue to each other to form a long active fault system.
In the eastern part of Oita Plain, there are several concealed active faults with ENE-WSW direction, and they have been active in Holocene epoch. To presume the position and the activity of the faults which lay beneath the plain, the reflection method, the drilling survey and Geo-slicer investigation were done. The northern fault named Misa Fault passes the vicinity of an former coastline and the fault in the south named Shimura fault can be confirmed even in the vicinity of Otozu river. The Oita Group and equivalent layers are covered directly with Holocene deposit in the south of the Shimura fault. The deposit of Late Glacial-Late Interglacial Stage is thick covered with Holocene deposit on the north side. In addition, marine deposit of Late Interglacial Stage exists in the subordinate position and the Late Glacial deposit is considerably thicker than that of the south from the Misa fault. On the other hand, the fault confirmed in the Hioka district, thick deposit of Late Interglacial Stage distributes at the south of the fault, and is different from the Shimura fault. Judging from the geological features, the fault confirmed in the Hioka district has a similar character as the Funai fault. ' Thus, the Misa and the Shimura faults which are concealed by Late Quaternary deposit in this region, greatly restrict the distribution of the stratum since Late Pleistocene. The base of Holocene marine deposit(ca.10,000 years ago) is about 48m in amount of displacement and the K-Ah volcanic ash layer is 26.5-30m by the Misa fault, and the vertical slip rates are about 4.8m and 4.2-4.8m/1000 year each other. The activity of the Misa fault since Holocene is judged to be A class.
The Unzen Graben is located on the western end of the Beppu-Shimabara Graben and is bounded by normal faults on the north and south sides. The faults in the Unzen Graben have developed in association with the growth of the Unzen volcanoes and offset volcanic materials such as lavas and pyroclastic deposits. The detailed location and amount of offsets of these active faults have been reported by previous studies, but the timing of faulting was poorly constrained. The Ogura fault is located on the northern margin of the Unzen Graben and offsets the alluvial fan surfaces formed by the Chijiwa River. We excavated a trench across the Ogura fault at Chijiwa Town in order to reveal the timing of recent faulting events. The faults exposed on the trench walls strike N70° W-EW and dip 70° -90° S, and offset the terrace gravels of the Chijiwa River. These faults may have moved after the deposition of the AT volcanic ash (26-29ka), which was found in fine sands filling coseismic open cracks. We also found an outcrop across the Kusenbu fault inside the Unzen Graben. A layer containing K-Ah volcanic ash (7.3ka) is offset by the Kusenbu fault, suggesting that the fault has moved during the Holocene.
An intense earthquake occurred in western Iran, about 225 km west of Tehran at 7: 28 local time, June 22,2002. Though the moderate moment magnitude of 6.4(ERI, University of Tokyo) - 6.5 (USGS) calculated for this earthquake was not surprisingly large as contrasted with those historical major earthquakes in this country,261 people were reportedly killed and 1,300 injured. We investigated the activated fault trace and observed the outcrop of suface rupture near the epicenter area of main shock from 27 July to 30. We recognized surface rupture associated with this earthquake extended for about 700m east of Abdarreh village. Amount of displacement upthrown to the south is less than 10cm. It's expected that faulting occurred several times along the same bedding fault related to the flexural-slip by the growth of fold.