Matsuda's formula consists of the linear combination of log L and M, where L is the seismogenic fault-length and M the earthquake magnitude. The coefficients of this formula are determined in most cases by the least squares method applied to coseismically generated L and M data. In the present paper, however, we propose another method of calculating coefficients by using mutually independent L and M data, for example, L determined by geological surveys and M by historical documents of seismic damages. Such a method is available under the assumption that the statistical distribution of M of historical events is equal to that of M calculated from L through Matsuda's fomula. The method proposed here is applied to Kumamoto's L and M data, and then we determine the coefficients regarding the whole land, the northeast (east of 138°E longitude) and the southwest (west of 138°E longitude) of Japan. We find as a result a statistically significant difference in coefficients between the northeast and the southwest of Japan. The difference may possibly imply the geological contrast that reverse faults are predominant in the northeast but lateral faults in the southwest.
Geologists and seismologists in the United States have been compiling and publicizing databases on possible sources of earthquakes including active faults since early 1970s. The ideas about the database and risk assessment have evolved in close response to public demands raised by such unforeseen earthquake hazards in 1 989 Loma Prieta earthquake and 1994 Northridge earthquake. Though the forecasts of future earthquakes failed repeatedly, the scientific communities always analyzed the failure and improved the knowledge and technology. The faultrupture hazard zone mapping under the Alquist-Priolo act was supplemented by the seismic hazard mapping of liquefaction and landslides. Probabilistic earthquake hazard mapping of Southern California and entire United States clearly demonstrated the advantages of regional ground shaking assessment to the evaluation of fault activity.
“Digital Active Fault Map of Japan”provides detailed information about location of active fault traces that allows us to examine relation between school buildings and the active fault traces in detail by using the GIS software (ArcView). As a preliminary result of the examination,1005 schools out of 43360 are located the zone within 200m from the active faults, about 570 within 50m and approximately 200 on the fault traces. These figures suggest that one out of about 200 school buildings are located on the active faults, and roughly 100000 students are studying in potentially dangerous schools against surface fault ruptures in case of earthquakes. In order to avoid further risky development of public buildings such as schools and hospitals within fault zones, and to mitigate seismic hazards, it is necessary to introduce“Active Fault Law”in Japan.
We present reflection profiles of the Kannonji Fault (Yamagata Prefecture), the Senya Fault (Akita Prefecture), and the Torigoe Fault (Niigata Prefecture) obtained by using a simple observation system, which consists of a 4kg wood hammer source and a 12ch ( or 24ch ) recording device. Reflecting surfaces can be seen down to the two way travel time of 200ms for the Kannonji Fault and the Senya Fault. The reflection profile of the Kannonji Fault implies that the faulting has formed a wide flexure zone, and that of the Senya Fault reveals a fault surface between the Miroku and the Senya formations. These features found in this study are very similar to those obtained by large scale reflection surveys conducted across these faults by Yamagata Prefecture (1998) and Satoet al. (1997). We set two survey lines at the Torigoe Fault, one is across a flexure scarp on the eastern limb of the Torigoe anticline, and the other crosses a flexure on Holocene lowland. The detailed subsurface structure obtained by this survey seems to contribute for investigating the formation of the Torigoe Fault System. Thus we suggest that such a simple observation system is useful to grasp a structure of an active fault near the surface.
Flexural-slip faults are distributed in the western part of the Shinjo Basin, Yamagata Prefecture. The Nikke and Masugata faults are flexural-slip faults accompanying flexural-folding by the Sakekawa blind reverse fault. The Nikke fault ruptured after 20ka. The Masugata fault ruptured before 50ka, but there is no indication of fault event after 50ka. Flexural-folding that had caused the Masugata fault has been inactive since 50ka because faulting is migrated from the Sakekawa fault to the Motoaikai fault. Therefore, the two flexural-slip faults have not ruptured simultaneously since 50ka.
The Isehara fault extends north to south for a distance of about 13 km along the eastern foot of the Tanzawa Range, central Japan. We have conducted a drilling survey of the Isehara fault at Isehara City, Kanagawa Prefecture. Drilling from the hanging wall side of the Isehara fault reached the foot wall sediments. The Isehara fault thrusts late Pleistcene terrace deposits and Tertiary basement rock over unconsolidated late pleistcene terrace deposits. Based on correlating of strata on the hanging wall and footwall sides of the fault using 14C ages and tephras, rate of the slip on the fault surface was estimated at 0.56 -1.46 mm/yr.
Interpretations of aerial photos and large-scale topographic maps, field investigation and analysis of drilling data reveal the presence of previously unknown active faults inside and along the northwestern margin of the Udo Hill, Shizuoka Prefecture. Several parallel-oriented fault scarps trending NNE-SSW to NE-SW are developed along the northwestern margin of the hill, which form a∼1km wide active fault zone extending for 7 km. On the basis of the morphology and geometry of the fault traces, it is inferred that the fault zone consists of southeastdipping reverse faults. There are some active faults developed inside of the Udo Hill. The tectonic landform and drilling data show that the topographic surfaces formed from 70-90 ka to late Holocene are displaced by 1-8 m in vertical. The average vertical slip rate is estimated to be 0.1-0.2 mm/yr for the fault zone in the late Pleistocene.
Kiso-sanmyaku-seien fault zone, extending for 60 km along the western front of the Kiso Range, consists of three echelon faults which trend in the N-S to NE-SW direction. We trenched the southernmost Magome-toge fault at Kudaritani site. In the trench walls, we recognized three distinct faulting events: one in the Holocene and two in the Late Pleistocene. Another older events are also inferred from a prismatic gravelly clay bed deposited in front of the fault plain. Our precise sequential 14C dating of the humic soils revealed that the latest faulting event occurred 5,000-3,800 cal yr BP. The penultimate event occurred after the fall of AT tephra (29,000 cal yr BP) and before 11,000 cal yr BP, and the third recent event predated the fall of the AT tephra. Mean faulting interval is therefore estimated to be around 12,000 to 25,000 years. The average vertical displacement per event is roughly estimated to be 1.2-1.6 m.
The Ibigawa fault is a left-lateral strike-slip active fault trending NW-SE direction in northwestern Gifu Prefecture. This fault runs parallel to the surface ruptures associated with the 1891 Nobi earthquake, however, no surface faulting was observed along this fault during the 1891 earthquake. The Active Fault Research Center, GSJ, AIST carried out a trenching survey on the Ibigawa fault to assess the activity of this fault and to make clear the interaction and segmentation of active faults in this area. Three trenches were excavated at Tsuka area in abandoned Tokuyama Village (Fujihashi Village at present) to reveal the paleoseismic activity of this fault. There are three terrace surfaces formed in the Latest Pleistocene to Holocene around the trench site. We divided these terraces into the Lower I, Lower II and Lower III terraces. Trench A is located on the Lower III terrace surface, and trenches B and C are located on the Lower I terrace surface. High-angle faults cutting the terrace deposits and undeformed surface soil layers were observed on every trench walls. The calibrated radiocarbon date from the deformed gravel bed in trench A is 2,000-1,880 cal y BP, and the age of the surface soil covering the fault zone in trench C is 1,170-980 cal yBP. These dates show that the age of the last event of the Ibigawa fault is about 2,000 to 1,000 cal y BP. The penultimate event seems to have occurred in about 3,900 to 2,700 cal y BP. The left-lateral offset of the base of the Lower III terrace deposit in trench A is estimated to be about 6.5 m although it is still uncertain how many times of faulting produced this offset.
In the Kanazawa City area, northern part of central Japan, fluvial terraces are widely distributed along the Sai to the Asano rivers. The absolute age of these terraces has reported few, as primary visible volcanic ashes are not recognized within terrace deposits and overlying loamy soils, and outcrops are quite few in this area. In this study, we have carried out drilling survey on the terrace to obtain the whole sections of overlying loamy soil. We have extracted some concentrated zones of volcanic glass and mineral derived from well-known wide-spread volcanic ashes within the loamy soil and could estimate the approximate age of terraces. Late Quaternary fluvial terraces in this area are divided into seven levels: Noda I terrace to Kasamai II terrace in descending order. Noda I terrace is overlain by the Kikai-Tozurahara tephra (75-95ka) contained at the lower part of the loamy soil. The Kodatsuno terrace is overlain by the Daisen-Kurayoshi tephra (43-55ka) and Kasamai II terrace is overlain by the Aira-Tanzawa tephra (22-25ka). Especially, each tephra is contained the lowest part of loamy soil in each terrace. We estimate that the Noda I terrace, the Kodatsuno terrace and Kasamai II terrace were approximately formed at 85,000∼95,000 years ago,50,000∼60,000 years ago,25,000∼30,000 years ago, respectively. The Kasamai II terrace is vertically deformed about 15-20m by the Nomachi flexure along the Morimoto-Togashi fault zone. Considered from the approximate age of terraces and vertical displacement of active faults, average vertical slip rates for the Nomachi flexure is estimated to be 0.5∼0.8mm/yr.
Large inland earthquakes bigger than Mj 7.2 during the historical past on Japanese islands have mostly been generated from active faults (Matsuda,1998). The 2000 Tottoriken-seibu earthquake of Mj 7.3 (Mw 6.6), however, occurred in the area where distinctive active faults were not mapped before the earthquake, and the surface ruptures associated with the earthquake were small and sparse. Active faults are hardly recognized even by detailed interpretation of aerial photographs after the earthquake but sharp lineaments. In Chugoku district in southwest Japan is characterized by less densely-distributed active faults with lower activities than other areas in Japan, and the 1943 Tottori earthquake of M 7 occurred by reactivation of the Shikano fault with rather obscure fault traces. Taking this condition, in mind, we carried out detailed mapping of active faults and lineaments, and compared with their topographical, geological, seismological and tectonic settings, in order to develop a new technique to find potential seismogenic faults. The results obtained are as follows; 1) Active faults and lineaments were not evenly distributed, and the dense zone is recognized along the Japan Sea while the sparse zone in the central part of the district. The active faults known before are mainly located in the dense zone (Fig.1). 2) The lineaments mapped are mostly less than 10km long, and half of them strike to NE-SW or ENE-WSW and 30 per cent to NW-SE or WNW-ESE (Fig.2). NE-SW lineaments prevail in the western part of the district, and NW-SW lineaments are systematically distributed only in the western-most and eastern-most area of the district probably reflecting their tectonic setting under the present stress condition. 3) Lineaments with poor topographical manifest were not commonly recognized by individual geologist, and were generally short, scattered, isolated, random in strike, and independent from geological structures. These lineaments will not be considered as potential seismogenic faults. 4) Epicenters of the small earthquakes are characteristically distributed to the north of the backbone range probably coincided with the past volcanic front. On the contrary, the area to the south of the backbone range the seismicity is sparse, except for several swarms. These seismic condition well matches with the distribution of active faults and well-defined lineaments (Fig.3). 5) Most of the active faults and lineaments follow the pre-existed geological faults that had moved opposite direction to the active faulting, indicating their inversion movements under the present stress field. 6) Surface ruptures reported as earthquake faults associated with the 2000 Tottoriken-seibu earthquake are considered as results of subsidiary shallow-sheeted faulting spontaneously caused by stain release around the seismogenic faulting in depth, because many of them appeared spontaneously, and not always along rather well-defined lineaments. They are small in extent and displacement. Therefore, it is rather difficult for evaluate such minor surface fault ruptures, but such ruptures may not displace the surface in large extent.
The writers describe the location and active mode of a fault (Funai fault) lying concealed under the town area of Oita City, by analyzing lots of the drilling survey, the reflection method and the Geo-slicer investigation. The Funai fault is the eastern part of“Beppu bay south coast fault group”, which com p osed of the Horita-Asamigawaa fault, Beppu bay south coast fault and Funai fault, extended from west to east with extension about 20km. The very position in Oita plain is considered from Kasugaura to the south of Maizuru bridge located at the left bank of Oita river. It was c o nfirmed in the drilling survey, and the amount of vertical displacement of the K-Ah volcanic ash layer in a main fault is 16m at the Funai castle traverse line. According to existing bore hole data, the maximum amount of displacement of the Funai fault is 18m to the west of Funai castle. The average rate of vertical displacement after the K-Ah volcanic ash (6,300 years BP) is 2.2-2.5m/1000 year, and the Funai fault is estimated one of the A class active faults in Kyushu. The latest activity of the Funaifault is thought to be time between before 1,540yBP dated at the lower part of the uppermost mud layer which does not show the displacement and after 2,350yBP dated in the peat layer in upper sand and gravel bed which shows displacement. The problem in the future is the clarification of the fault continuation to the further east, and it is necessary to elucidate accurate distribution and the activity history of the concealed faults lying under whole the Oita plain.
Tarim Basin, Xinjiang Uyghr autonomous region, China, contacts with the Tian Shan and the Pamir at its northwestern corner. On the tectonic field, Tarim Basin is considered to be under thrusting into the Tian Shan from south and the Pamir is slipping left-lateral against the Tarim Basin. Recently, many earthquakes have occurred in this region, including the Artux earthquake of 1902 (Magnitude 8 1/4) and the Wugia earthquake of August in 1985 (Magnitude 7.5). Though these major earthquakes are caused by the active faults, the precise locations of these active faults, with some references and characteristics of their activities are obscure. The purpose of this paper is to show the distribution and characteristics of the active faults at the northwestern corner of Tarim Basin based on the interpretation of the CORONA Satellite Photographs. The following results were obtained: Active faults of this region, mainly based on the reference of the river terraces, can be classified into following two groups; a) the reverse faults in SW-NE direction of parallel with the Tian Shan, b) the reverse faults parallel with the King Ata Tagh, located along the northeastern piedmont of the Pamir. The latter group changes their direction from N-S to NW-SE into the North. Based on the directions of the above mentioned active faults and occurrences of the major earthquakes (Magnitude>7.5) amidst these active fault zones, these active faults are considered to be forming important boundaries both of the Tarim Basin to the Tian Shan and to the King Ata Tagh, namely to the Pamir. According to the seismic data of USGS National Earthquake Information Center, recent earthquakes in the past 30 years concentrate around the southern part of Wugia and the northern part of Wpaerh. In the other areas along these recognized active faults, obviously, few earthquakes have occurred. If there is no evidence of large earthquakes during the historical time in the other areas, these active faults have high potentials for the occurrences of the large earthquakes in near future. It is necessary for more detailed investigations, namely, on the last events, amount of dislocations and the activity intervals of these active faults, for not only understanding the kinematics of present deformations of the Tian Shan, the Tarim Basin and the Pamir, but also the prevention of earthquake disasters.