The accurate trace of the Ota fault, part of the 1896 Riku-u Earthquake fault, was settled, based on geomorphic interpretation of old aerial photographs taken by American Army and stereoscopic topographic maps and images digitally processed. Fault features appearing at several locations along the fault scarps show that the Riku-u Earthquake brought crustal shortening of about 3 meters and vertical slip of 1.5 meters, and that the Ota fault was activated before 6500 y. B. P.
Gravity measurements have been conducted at 68 stations to focus on west-northwest trending Naguri fault (NGF), which is supposed to be the northern extension of Tachikawa fault. It is found in the Bouguer anomaly map that iso-anomaly contour-lines are distorted due to the NGF left-lateral horizontal displacement, the amount of which is presumed to be a few kilometers. The vertically dipping fault plane cuts the Chichibu formation and presumably the underlying Mikabu-Sambagawa metamorphic rocks at a depth of about 1.2km.
The Kitatake fault is a right-lateral active fault running WNW to ESE, through the central part of Miura Peninsula. Although the fault located in the densely populated area, little is known about its paleoseismological records. Thus, trenching survey was performed to reveal the latest paleoseismic event at the eastern part of the Kitatake fault at Nobi, Yokosuka City. At the trench walls, the fault trending N52-42°W 35-67°S, displaced the humic mud dated at 1540±80 y. B. P. Its upper termination is covered by the channel fill dated at 1020±70 y. B. P. Thus the latest paleoseismic event associated with the Kitatake fault is estimated to be between 1540±80y. B. P. and 1020±70y. B. P. The evidence of paleo-liquifaction in the sediments suggests that this site has been suffered strong ground motions after 460±90y. B. P.
The Tanna fault in the northern part of Izu Peninsular, central Japan, is a principal strand of the Kita-Izu active fault system, a conjugate system of faults for about 35 km that consists of left-lateral faults of N-S or NNE-SSW in trend and right-lateral faults of E-W or WNW-ESE in trend. During the Kita-Izu earthquake of 1930 (Magnitude 7.3), the Tanna fault that extends N-S, moved in a leftlateral sense. In 1997, one trench as its underground observation room, was excavated across this surface rupture line wih the maximum horizontal offset of 2.6m, at Hata, Kannami, Shizuoka prefecture. In this trench, the sharp and fresh fault plane with thin fault clay and some tracks of leftlateral displacement in landslide and debris flow deposits, is exposed. It is estimated that the surface rupture in 1930, make an arrangement of a left echelon type near this site because the fault plane in the trench, is obviously in a direction of southwest to northeast.
Precise air-photo interpretation of tectonic landforms in the south of Matsumoto basin was made in order to clarify the overall nature of active faulting along the Itoigawa-Shizuoka Tectonic Line. The fault zone consists of both thrust and strike-slip faults. At the eastern boundary of the basin is an east-dipping thrust fault, which has become inactive progressively from the south to the north in late Quaternary time. Three to four kilometers west of the boundary fault is another west-dipping thrust fault, which, however, is still active in Holocene time. Thus, the thrust front is likely to have migrated basin-ward in late Quaternary time. A purely strike-slip fault (Gofukuji fault) exists on the upthrown side of, and parallel to, the boundary fault; the Gofukuji fault has a slip rate as high as 8.6±1.0 mm/yr and is still active. Our observations indicate that the overall movement on the Itoigawa-Shizuoka Tectonic Line at depth is transpressional and is transferred at shallow depths into both thrust and left-slip movements. We therefore interpret that significant slip partitioning is taking place on the up-dip extension of an east-dipping fault, which was originated from a normal fault bordering the west margin of the Northern Fossa Magna rift of early and middle Miocene age, and has rejuvenated as oblique slip fault since Pliocene time.
A new sampling technique using ten meter long geo-slicers in addition to drilling was applied across the Kamishiro fault, northern part of the Itoigawa-Shizuoka Tectonic Line. Slip rates of the Kamishiro fault in the Kamishiro Basin is estimated be ca.1.5mm/yr since about 18,000 years B. P., based on the interpretation of geological section in size of 20m×25m and 14C dating of woody and peaty materials sampled.
The Hanaori fault is a large fault that extends NNE-SSW for about 50km from Imazu to Kyoto, in central Japan. Generally, it is well known that the most of Hanaori fault is active in late Quaternary time with right-lateral slip, but its history of recent faulting events does not become clear yet. So we excavated this fault at Tochudani, west of Imazu. As a result, it became clear that the Hanaori fault had displaced surely in historic time. Its age is after A. D.875-1028 year, the early Heian period. It is estimated that this is the latest faulting event of the Hanaori fault and the rightlateral displacement of at least 3.5 m occurred dring the event. The 1185 Bunji earthquake of M 7.4or the 1662 Kanbun earthquake of M 71/4-7.6, both of which are known as big historical earthquakes that gave disaster of extreme severity to Kyoto, has possibility to be identified this Hanaori's faulting event.
We excavated one trench on the Omiyagawa alluvial fan at Hieitsuji, Otsu City, Shiga Prefecture, in order to clarify geological records on recent surface faulting events of the Katata fault. The Katata fault is a major fault consisted of the western marginal active fault system of Lake Biwa, that may have occurred the 1662 Kanbun-Omi earthquake (M=71/4- 7.6). Although any faults have not cropped out, most of alluvial fan d e posits exposed on walls of the trench, have made a gentle flexure facing east. It is certain that at least one faulting event of the Katata fault have occurred in the past 16000 years.
The Gosukebashi fault zone which is a major active fault in Kinki district, striking NE-SW extends about 15 km along the eastern Rokko Mountains. Detailed geological and geomorphological studies on the southwestern part of the Gosukebashi fault zone were carried out in this study. The following results were obtained. 1) Distinct fault t o pographies showing right-lateral displacement are recognized along the Gosukebashi fault zone. 2) Fault trace l i neaments and fault outcrops show that the fault is a right-lateral strike-slip fault with a reverse displacement component. 3) It was formed that t h e drainages along the fault were dextrally displaced. There is a linear relation of D: aL between the offset (D) and the length (L) of upperstream along the fault. 4) The mean rates of displacement are 1∼4 m/103years in horizontal, an d 0.26∼0.32m/103years in vertical, respectively. 5) Geological and geomorphol o gical studies, such as aerophotograph interpretation and field survey, show that the Gosukebashi fault zone is still active in the Holocene.
A series of small-scale fault topographies associated with the Hyogo-ken Nanbu Earthquake were observed along the Nojima Earthquake fault. The results are as follows: (1) Such fault microtopographies as fault scarplets, cracks and a earth rise appeared at the top of the terminal facet consisting of relatively fresh granite. (2) They are recognized on the slope where the Nojima Earthquake fault is not exposed or has become indistinct. (3) Slope collapse is not in evidence, where small-scale fault topographies were formed. (4) Opening cracks were widely recognized, except in the area along a earth rise. (5) Small-scale fault topographies were classified into two groups as follows: relatively large-scale oblique-slip faults, and small-scale normal faults and cracks. (6)Oblique-slip faults show eyebrow or angle in plan view, and the part of the bend or curve corresponds to the highest part. Normal faults are linear. (7) Strikes of oblique-slip faults deviate from 10-40 degrees east of the Nojima Earthquake fault, whereas normal faults strike parallel or conjugate to the oblique-slip fault. (8) The lengths and displacements of small-scale faults were significantly shorter and smaller, respectively, than those of the Nojima Earthquake fault.
The Senzan fault is an active reverse fault directing N-S to NNE-SSW in the central Awaji Island. During the 1995 Hyogoken-nanbu earthquake, a characteristic surface break appeared along the Nojima fault located at the western side of the Awaji Island, however, no surface break had occurred along the Senzan fault. We excavated an exploratory trench on the Senzan fault to detect the age of the faulting events, and we confirmed a reverse fault making a contact between slope deposits and a granitic rock. The youngest age of the hurried soil deformed by the fault is 1161±67 AD. This means that the last faulting event of the Senzan fault occurred after the 12th Century. This event may correspond to the historical Keicho Fushimi earthquake in 1596 AD.
Conjugate fault system of strike-slip type with some vertical component was associated with the 1927 Earthquake (Table 1; Fig.1) in Tango Peninsula, Central Japan. Detailed field observations have been described in many reports and papers which were published by lots of researchers and organizations. Those data are here compiled as a list (Table 3) with detailed location maps (Figs.8 ABCD, Fig.9). Based on the data compiled, distribution of major surface faults and a m ount of displacement on the fault traces are shown in Figs.7AB and Table 2. The followings are summarized by descriptions and reviews of surface faults and fault topographies. 1) The Go m ura fault zone is composed of en echelon arranged several faults, generally trending with the direction of N30°W. The total length having clear fault morphology is 14km on land, and the morphology become obscure to the southeastern part. 2) The seismic faults had ap p eared on the pre-existed Quaternary faults. The features of the morphology are not so obvious, as these faults belong to the active faults of lower class B or class C in term of the average slip rate. 3) Each fault within the Gomura fault zone has the maximum value at the central part on the distribution for the amount of displacement at the 1927 earthquake, which decrease to the both ends. 4) Each fault is arranged with en echelon, and these make the Gomura fault zone with altogether. The distribution of displacement, especially of left slip, shows the wavy pattern. 5) Nevertheless, the maximum value of the left slip in the G o mura fault zone is recognized around Go, Amino Town. 6) Based on the relationship between the southeastward length and the distribution of amount of displacement is known, the northwestward extension to the Japan sea bottom is estimated as about 4km or slightly longer. If the subordinary faults such as the Kuchi-Ono and Mie faults are included in this zone, the length become much longer. 7) The G o mura fault zone was a very clear seismic fault, and was associated with large amount of displacement throughout the whole zone. Especially, cumulative fault topography is distinctive along the Gomura (Takahashi) fault. The offset features of stream valley and ridge of about 100m in maximum are seen along this strand. These features are explained by the repetition of the displacement in 1927 event in the late Quaternary period (about half Million of years). 8) The recurrence interv a l of the Gomura fault zone is longer than several thousands of years, possibly about 10,000 years. 9) Althou g h the Yamada fault zone, being another conjugate fault, has much clearer geomorphic and geologic features of offset than the Gomura fault zone, the amount of displacement in 1927 event was small and the length of the surface rupture was short. 10) The 1927 earthquake trigger e d small dislocations localy along the main part of the pre-existed Yamada fault zone (cf. Fig.7B). Therefore, the Yamada fault zone may dislocate in the time of the other large earthquakes in figure.