An earthquake of magnitude 6.1 occurred in Shizukuishi town, Iwate prefecture, northeastern Japan, on September 3,1998. The focal area of this event is located near Iwate volcano, which has been active since February,1998. The focal mechanism of the main shock is a reverse fault, whose pressure axis is nearly horizontal and almost east-west-a typical focal mechanism for shallow earthquakes beneath the northeastern Japan arc. Precise hypocenter locations of the main shock and its aftershocks delineate a fault plane that strikes northeast and dips northwest in the depth range 3-8km. The rupture probably began about 7km deep at the northern edge of the fault plane, then propagated southward along a deep part of the Nishine active fault.
The Iwate-ken Nairiku-hokubu earthquake of September 3,1998, caused surface rupture on the Shinozaki fault, at Shizukuishi, Iwate Prefecture, northeast Japan. The fault, dipping to the west and northwest, was traced about 800 m along strike. Reverse slip produced vertical displacement of 0.25-0.32 m. Landslides and building damage occurred in the northern and northwestern area of the fault. The southern part of the fault dovetails with the northern part of the previously mapped Nishine fault
The earthquake fault associated with the Iwate-ken Nairiku Hokubu earthquake were appeared along the existing active fault. It stretches in the direction of N-S approximately 800 meters. Some surface ruptures observed in paddy field and grass land. We made precise maps on a scale 1: 100 at three sites in order to obtain fault micro topography. The result are as follows; (1) The vertical displacement was ca.40 cm in central part of the earthquake fault, but it became 25-30 cm at the both ends part of it. (2) The surface of foot wall were flat. (3) A shallow depression appeared on the surface of hanging wall, and maximum depth of it were 10 cm. (4) A tectonic bulge appeared along the earthquake fault.
The western marginal active fault zone (Nishine faults group) of Shizukuishi basin, northeast Japan, were precisely mapped in detail. Their late Quarternary activity were reexamined, with implication to the surface fault of 1998 earthquake having occurred below this basin. Four faults named Fl, F2, F3 and F4 trending N-S have at most vertical slip rate of 0.5mm/yr. The F2 fault is the longest, ca. 13km, and have mainly contributed the differentiation of the basin from mountains. The Fl fault and 1998 coseismic surface fault are possibly a branch from the F2 fault. As fault movement at the 1998 earthquake was limited with a little of 40cm in vertical slip, the Nishine faults group still has some potential that large earthquake occur accompanied by their activation in future.
The geology at the northern and western margin of the Shizukuishi basin is composed of upper Miocene Yamatsuda Formation, Pliocene Masuzawa Formation, Plio-Pleistocene Tamagawa Welded Tuffs and units from Takakura and Iwate volcanoes, in ascending order. The geological structure of the margin is characterized by the Nishine fault system, which displaces these formations and units. The fault system, a set of west-dipping reverse faults, forms the topographic and geologic boundary between the Ou Backbone Range and the Shizukuishi basin, and it controls the vertical position of underlying units as well as Takakura and Iwate volcanic products. The vertical displacement of the Shibutami Welded Tuff, which is included in the Tamagawa Welded Tuffs and erupted between 0.7and 1.0 Ma, decreases to the north and south from an area where the tuff has been vertically offset some 300-400 m, for an average displacement of about 0.4 mm/yr. Slip occurred on the northern part of the fault system during the Iwate-ken Nairiku-hokubu earthquake of September 3,1998. The tuff is also displaced at least 250 m along the Matsuo fault for an average displacement of about 0.25mm/yr at the north of the Nishine fault system. This reverse fault strikes nearly parallel to the Nishine fault system and can be traced for 5 km.
To estimate the size of future earthquakes generated from active faults, it is important to define the extent of faulting from existing fault traces. This paper proposes new criteria for identification of segments for active strike-slip fault systems based on geometric surface rupture heterogeneity. Geometric and structural characteristics are often used for fault segmenta t i on, in cases that historical and/or paleoseismological data are not sufficient to reveal coseismic behavior of faults. We found an interdependent relation between the pattern of surface ruptures and the d i r ection of their propagation based on an investigation of recent earthquake fault ruptures. If a fault is associated with a simple baranching at one end in a plane view, it is rather simple to deduce the direction of future ruptures from its branching feature. If a fault brances at two ends, we may define a fault segment which raptures bilateralty and moves independanly at future earthuakes. If branchings of two faults face each other, we may expect a segment boundary between these branchings as a change in the direction of fault ruptures propagation take place between them. Regarding dip-slip distribution along a strike-slip faul t segment, the upthrown sides is, in general, located in the area on the fault block in the relative strike-slip motion on a faulted block. For example, along an E-W trending right-lateral strike-slip fault, upthrown side is located toward the east end on the northern block and toward the west end on the southern block. Thus, a fault segment may be identified based on the pattern of dip-slip distribution. We tested these criteria successfully on sever a rl recent surface fault ruptures and active faults, and applied them for segmentation of the active faults system of the Median Tectonic Line in Shikoku, Southwest Japan.
A seismic survey was conducted across the Tachikawa active fault in the western suburbs of Tokyo metropolitan area. The deep structure and movement of the fault are discussed based on the CMP stacked seismic section together with the geological data published previously. The results are as follows: (1) A flexure, about 150 meters in width, underlies the flexure scarp of the fault and continues vertically down to 1000 meters in depth. (2) The displacement of the fa u lt is about 100 meters upthrow of the northeast side between 300meters and 600 meters in depth, while the displacement at basement depth is about 100 meters downthrow of the northeast side. (3) The northeast side o f the fault had subsided relatively to the southwest side in the past. The fault movement was stopped for a while. Then the fault movement was reversed and the northeast side has been upheaved up to now. This is an inversion tectonics of the fault movement.
The eastern margin of Niigata plain is tectonically active region. However, east of Shibata City no active fault zone has been mapped. Interpretation of large-scale airphotos and geomorphological observation reveal that several traces of active faults run in the study area. These faults are ca.3.3km,3.3 km, and 4.3 km long. The topographic characteristics such as fault scarplet and flexure scarp suggest that there faults are reverse. The progressive deformation is also clear at several sites. As a result, Kajigawa fault zone has been active during the late Quaternary. Average vertical slip rate of Nagaminehara fault is 0.06-0.1 mm / yr., Shimotaya fault is 0.14 mm yr., Kamihazu fault is 0.12-0.15mm / yr.
Precise. aerial photograph interpretation of tectonic landforms in the eastern margin of the Kanazawa plain was made in order to clarify the overall nature of active faulting along the Morimoto-Togashi fault zone. This fault zone, about 25km long trending NNE, consists of east or southeast-dipping thrust faults, namely the Morimoto fault, the Nomachi fault, the Togashi fault, the Nagasaka fault, and the Nodayama fault. Vertical offset over 20m on a lower river terrace near Kanazawa city suggests that the overall rate of vertical slip on this fault zone is 1 m/kyr or more in the late Qauternary. The Morimoto, Nomachi, and Togashi faults, which constitute the main strands of this fault zone, have been active clearly in the Holocene; in paticular, the Morimoto fault has displaced an alluvial lowland surface formed after the Jomon. marine transgression of middle Holocene age. Such young offset features suggest that at least two faulting events have occurred since about 6000 years B. P.
The western extension of the Negoro fault, a segment of the Median Tectonic Line active fault system at the western part of Kink District, cuts the Middle Pleistocene deposits and is covered with Holocene alluvial deposits in the Wakayama plain, central Japan. We have carried out several drillings and a seismic reflection study using the S waves in this plain to clarify the subsurface structure around the MTL and to date recent faulting events; The buried reverse fault structure and stratigr a phic evidence with dates were clarified by seismic reflection method and a 80 meters deep drilling. As the fault shattered zone accampanies the Cretaceous Izumi sedimentary rocks, Sambagawa metamorphic rocks and ryolite intruded to the zone in the Miocene, this structure corresponds to the geologically defined MTL in the narrow sense and make a fault zone with the subordinary faults. The vertical displacement along th i s fault is estimated to be about 1 meter on the uncomformity between the Holocene sediments and the horizons of late Quaternary formations. The latest event of this fault may be younger than 8,000yBP, though the detailed age of the event and recurrence interval along the fault were not obtained by this study.
The Naruto-South fault is situated of about 1000m south of the Naruto fault, the Median Tectonic Line active fault system in the eastern part of Shikoku. We investigated fault topography and subsurface geology of this fault by interpretation of large scale aerial photographs, collecting borehole data and Geo-Slicer survey. The results obtained are as follows; 1) The Naruto-South fault runs on the Yoshino River deltaic plain at least 2.5 km long with fault scarplet. the Naruto-South fault is oblique by about 5-10°clockwise to the strike of the Naruto fault.2) Geologic boundary (MTL) suggested by borehole data, extends between Himeta and Ote-Beach about 8 km long, buried under alluvial deposits.. The strike and location is consistent with the Naruto-South fault and submarine fault. Therefore, the Naruto-South fault is presumed to have dislocated using the part of fault plane of the geologic boundary fault.3) We investigated to detect marks of the last faulting event by use of Geo-Slicer, and we confirmed a active fault with lateral slip component.
The Okamura fault, a segment of the Median Tectonic Line active fault system, is a master active fault along a northern range front of the Shikoku Range with well-defined geomorphic features related to its late Quaternary faulting. We have excavated five sites across the Okamura fault at Iioka, Saijo City, to date the times of recent faultings and surface-rupturing earthquakes. This paper summarizes geological characterisitics and late Holocene activity of the Okamura fault by comparing individual trenching studies, which have been reported by previous several papers. The Okamura fault trends N80°E and dips with high angle to almost vertical fault plane. This fault offsets all sediments except for artificially modified and cultivated soil immediately below the ground surface. Multiple V-shaped flower structures are recognized in the sediments near the top of the fault zone, and these are interpreted to have been formed by filling coseismic ground fissures associated with surface rupturing earthquakes. The most recent faulting on the Okamura fault is estimated to have occurred during the 4th to 7th century A. D. and several older events are also recognized in these outcrops by trench excavations. The recurrence interval is about 1000 years or slightly longer. A silty layer exposed on the north and south sides of the fault in trench ifi is offset rightlaterally 5.7meters. This lateral slip is associated with the latest event. The angular gravels of the Cretaceous Izumi Group which crops out in the upper reach of stream“a”southwest of the study area, are contained only at lower horizons on the north side of the fault. This observation suggests cumulative right-lateral displacement along this fault. The sedimentary environment of alluvial fan area has progressively changed with the lateral movement and largely different faces of sediments on the both sides of the fault were caused by the cumulative lateral offset
We discovered active fault traces on the extension of Kawakami and Okamura faults of the Median Tectonic Line active fault system in Shikoku based on new criterion that strike-slip faults are characterized by the pattern of dip-slip distribution; the upthrown side along strike-slip faults are, in general, located on the fault blocks in the direction of relative strike-slip motion. The northeast part of the Kawakami fault is recognized as a 13km -long NE-SW trending trace with low fault scarplets with the south side upthrown, and this does not agree with the pattern of dipslip distribution along a right-lateral strike-slip fault. Detailed interpretation of large-scale airphotographs taken by US army corps about 50 years ago, revealed ENE-WSW trending low fault scarplets across young alluvial plain, that is located on the northern extension of already mapped. The scarplets are less than 1 m high up to the north. The ENE-WSW trending Okamura fault h as been only recognized as an 18km-long distinctive fault scarplet known as“Nakahagi”cliff with the south side upthrown. We examined its eastern extension and found a low ENE-WSW trending fault scarplet with the north side upthrown in the south of Niihama city. The trace continues further east about 13km with obscure fault features. We trenched the newly found fault trace at two sites and obtained geological evidence of Holocene faulting. Thus, the geometric criterion we propose in this paper gives a quite different interpretation of fault distribution from those by the previous workers. We expect that this criterion may be tested together with the other geometric criteria on major active fault and compared with segmentation based on paleoseismological data.
The Minoh fault is an east-west trending fault zone on the northern foot of the Minoh mountains, in the northern Kyushu, extending from Kurume City to Ukiha Town. The fault zone consists of several normal faults in parallel or echelon. Recently we made a trench at Miyazono, Kurume City in the west-central part of the zone. The observations of the trench wall and drilling cores lead the following conclusion. 1. A normal fault with the northern side downthrown was expose d on the trench wall, by which the Pleistocene slope-deposits were brought into fault contact with the pre-Tertiary metamorphic rocks. Another normal fault was inferred to exist about 20m north of it from boring data, which indicate also the northern, lowland side is downthrown. 2. The fault is covered with undisturbe d strata of the later half of the 7th century to the 8th, indicating the latest movement of the fault was prior to the 8th century. This fault zone is known to generate the A. D.679 large earthquake in historical time in the west area. But in the present study it is not be abled to confirmed that the faults in this site moved or not at that time. 3. Long-term average vertical slip rates in the present area are estimated at about 0.1∼0.2m/1000years, based on the following observations: the weathered red gavel beds estimated at 300ky in age is displaced about 35m. And the AT tephra layer of about 25ky in age is displaced about 4.3m. 4. Total displacement since the Pleistocene bed began to deposit is about 50m a l o ng the above two fault in this area. The lowland-side fault ceased the movement earlier than the mountainside fault. This may indicate that the active segment in this fault zone migrated toward the mountainside.