It is empirically proved that the statistical distribution of fracture size, such as fault length, is governed by the power rule. Nevertheless, fault length data is in most cases plotted in semi-logarithmic coordinates. This implies that the data fits an exponential distribution better than a power-rule one. We introduce here three power-rule distribution models: ie. a composite model of two power-rule distributions, and a model transformed from the truncated Gutenberg-Richter magnitude-frequency formula (TGR) and one from the modified Gutenberg-Richter formula (MGR). As a result of AIC tests we conclude that TGR and MGR models are the most suitable to fault length data.
When calculating the probability of future earthquake occurrence, it is common to apply stochastic models to paleoearthquake data collected from fault trenching surveys. The Headquarters of Earthquake Research Promotion (HERP,2001) concluded that in terms of fiscal meaning, stability, and ease of comprehension, the best of the stochastic models is the Brownian Passage Time (BPT) model. This model requires two parameters: average recurrence interval and aperiodicity of the mean period between events (coefficient of variation). HERP (2001) derived a value of 0.24 for the common aperiodicity from four paleoearthquake data sets, which is roughly half the value of 0.50 derived by Ellsworthet al. (1999) from 37 worldwide earthquake records. The purposes of this study are (1) to construct a paleoearthquake database of active intraplate faults in Japan based on trenching results conducted after 1995 and (2) to evaluate aperiodicity using the BPT model and this database. Common aperiodicity ( ac), calculated from 23 paleoearthquake data sets in this study, is 0.49, approximately the same as the result of Ellsworth et al. (1999). This study shows no distinct relations between aperiodicity and active fault type, fault activity, recurrence interval derived from trench results, or distance between trench site and the center of the seismogenic fault system. There is a small positive correlation between the aperiodicities in this study and the number of neighboring faults within 30 km of trench sites. This might be consistent with the BPT model, in that aperiodicity of a given fault can be affected by seismic activity of nearby earthquakes, which alters the recurrence interval and future earthquake probability of the fault under consideration.
Active Fault Database in Japan was released to the Internet at March 2005 (http://www.aist.go.jp/RIODB/activefault/). The database keeps be constructed from 2002 by the Active Fault Research Center, GSJ/AIST. It is a part of Research Information Database (RIO-DB) that is managed by National Institute of Advanced Industrial Science and Technology (AIST). By its relational database management system, many users on various web browsers can search many data flexibly. The database has been composed of three parts; the primary part is a bibliography on literature that described the results of the investigation about active fault in Japan; the secondary part is a collection of the data collected from the literature; and the third part is a compilation of averages for parameters of behavioral segments that is corresponded with the Rupture probability map of major active fault in Japan compiled by the Active Fault Research Center, GSJ/AIST. The database system will be revised one after another, the addition of the data renewal function, data analytical function, GIS function and more. Coincidentally, the new data will be collected into the database stepwise.
Results of numerical analyses and field studies in the epicenter area of the 2000 Tottoriken-seibu earthquake suggest that several research techniques are effective for identifying subsurface active faults unaccompanied with remarkable surface earthquake faults. Field observation of the lineaments found in the area by detailed air-photo interpretation revealed that most of them are accompanied by faults and/or dikes, and some of them show minor slips at the time of the earthquake. Crustal deformation caused by the activity of the subsurface seismogenic faults is deduced by numerical analyses based on seismic and geodetic data. The vertical deformation assumed from the height changes of the terraces along the river across the epicenter area suggests that cumulative uplift and subsidence associated with left-lateral strike-slip has been continuing. Offsets rate of the streams and uplift rate of the river terraces surfaces show good agreement with the estimate from the parameter of the earthquake faults model. Identification of active faults without clear surface evidence before occurrence of earthquakes is a difficult issue, and the techniques adopted in this study will probably provide relevant information effective to locate subsurface active faults.
This paper clarifies the accurate distribution of the Omagari-Toyotomi fault, one of the prominent geological faults in northern Hokkaido, using the high accuracy analyses and the specific characteristics of the geological environments in the Horonobe Town. In the Kamihoronobe area, the central Horonobe Town, the slight linear landform runs along the border of drainage patterns which reflected the lithology. In addition, the outcrop of fault, the boundary between the Wakkanai and Koetoi formations, situates along the linear landform. Judging from the above description, we conclude that the linear landform shows the Omagari-Toyotomi fault. In the Hokushin area, the northern Horonobe Town, where the fault doesn't form the lithology boundary and the fault isn't reflected topography, it was found that the result of investigations using the drainage pattern, slight linear landform, the subsurface geological structure, and the number of diatom valves clarify the detailed distribution of the fault. We will have to clarify the accurate distribution of the fault such as the branch fault and the secondary fault, and in a wide area, with same methods as this study.
A seismic survey was carried out in 1999 around Ota fault on the eastern margin of Yokote-basin, Akita Prefecture, Japan. There are two kinds of seismic spread: one is straight line (A line) and the other is L-shaped array (B line). Clear reflections are obtained in many shot records. Then, we applied the mirror image method to travel time data of these reflection. Optimum image point is defined at minimize RMS residual of travel time between the observed and the estimated by grid search. Grid size is 10m. N-S and E-W profiles were obtained by this method for the line B. In N-S profile, there are some reflectors dipping from the surface rupture of Ota earthquake fault. In N-S profile, additionary, this result shows the south dipping reflectors. Our results correlated with the CMP profile of the line A.
The South part of Eastern boundary fault System of the Shonai plain is divisible into two principal parts. The north segment called Matsuyama fault trending mainly N-S at the north side of the Mogami river has been studied by previous several works on recent activity, but no precise investigation performed at the south side of the river. This paper focuses on the rate of slip and Holocene fault activity on the South part of the fault System of Shonai plain by detail drilling survey. Topography of surface deformation is also evident from study of large-scale aerial photograph interpretation. Twenty-nine arrayed drilling holes were carried out about 400m long across the flexure scarp zone on the alluvial fan terrace. The drilling-hole data were collected as 20m intervals in the most intensive examined place with 5 to 10m in deep. The upper layer mainly composed of sandy gravel and silt, and clay facies about 4m thick was recognized from the top to downward. Just below this layer, the authors found a respectable peat layer, which have probably deposited at about 5,500 to 6,000yrs. B. P. The deformation of the peat layer indicates the vertical offset of this layer is estimated approximately 3.7 m. According to vertical offset and possible determined age suggest that vertical slip rate of the fault is about 0.62-0.67mm/yr.
The Itoigawa-Shizuoka Tectonic Line (ISTL) lies along western margin of the northern Fossa Magna and forms an active fault system (Matsumoto Basin Eastern Margin Fault) that displays large vertical slip rates (ca.2- 3 mrn/yr). In this paper, we estimated surface deformation pattern in the Saigawa Hills, which locates to the east of the ISTL, using height of fluvial terraces along the R. Sai-gawa, R. Omi-gawa, and R. Aida-gawa. Next, based on obtained surface deformation pattern and estimated subsurface fault geometry of the active fault system using seismic reflection survey and geologic structures, we discussed a relationship between subsurface activity of faults and surface deformation. As a result, we concluded that deformation pattern of ground surface in the Saigawa Hills is regulated by subsurface fault geometry of the active fault system (flat-ramp structure), and that relative uplift has occurred above a subsurface ramp of the active fault system. By this research, we show the possibility that the activity of an active fault system with complex subsurface geometry can be evaluated based on deformation of ground surface in geomorphologic time scale.
The Itoigawa-Shizuoka Tectonic Line active fault system in central Japan extends for ca.150 km and it is one of the most active faults on land. It consists of east-dipping reverse faults, north-west trending left-lateral strike-slip faults and west-dipping reverse faults. The Shimotsutaki fault consists of the southeastern portion of the central fault segment. We performed trenching and Geoslicer survey at Shimotsutaki site. The site is located in depression where fine sediments have continuously deposited since several thousands years. The trenches and sections of Geoslicers exposed several high-angle faults cutting debris-flow deposit near the trench bottoms and thick humic layer covered with cultivated soil at the surface. We identified three faulting events and estimated the timing of faulting based on radiocarbon ages. The most recent event postdates 1500 yBP and the timing is consistent with previous estimation. The penultimate and ante-penultimate events may have occurred between 2000 and 3300 yBP, and 3300 and 5500 yBP, respectively. Judging from these data, the Shimotsutaki fault has activated at the interval of less than 3500 year and possibly at the average of ca.1800 year. This recurrence interval is shorter than previous estimation. Additionally, we could constrain the left-lateral slip-rate of 5.5 mm/yr as the maximum value, based on the age of debris-flow deposit exposed on the trench walls and evolution of fluvial terraces related to offset river. Such refinement of paleoseismological data would give us to understand fault segmentation and contribute more precise evaluation of seismic hazard along the Itoigawa-Shizuoka Tectonic Line active fault system.
The North Matsuda fault in the Kozu-Matsuda fault zone is one of the active fault system, showing the largest vertical component of slip rate onshore Japanese islands. The Kozu-Matsuda fault system is considered as a part of the northern margin of the Philippine Sea plate. It is necessary to obtain the horizontal shortening component of slip rate of the North Matsuda fault for a better understanding of active tectonics in the northern margin of the Philippine Sea plate. To reveal the subsurface structure of the North Matsuda fault, high-resolution shallow seismic reflection survey with about 3.2 km line length was carried out across the fault, A kind of accelerated weight drop system named as Yuatsu Impactor (JGI Inc. ) was used as seismic source which has wide band frequency spectrum. Both the standard shot intervals and group intervals of geophones were 10 m. The Sakawa River near the center of the line obstructed both shooting and geophone setting, and the half of the line in the Matsudayama Mountains was severely crooked. Although such a bad survey conditions, the seismic section after careful data processing shows a drastic structural change and a north dipping thrust fault with 18 degrees near the Sakawa River, which is suggested as the North Matsuda fault. The fault trace revealed by a result of the seismic reflection survey is exactly located along the Sakawa River.
The Kameoka basin is located to the west of the Kyoto basin. On the northeast side of the basin, two faults trending the northwest to southeast direction exist along the foot and the former edge of a mountain, respectively. They compose of the Kameoka fault zone with the length of about 13km (Okada&Togo ed.,2000). To elucidate such characteristics as distribution, subsurface structure and activity of those faults, we have carried out seismic reflections (P-waves) and deep drilling surveys across the faults. Volcanic ash and pollen analysis were also performed using core samples obtained by drillings. In this paper, we report the results of these surveys, especially about the characteristics of the concealed faults related to basin formation. By these surveys, three faults were detected along the three sections by the seismic reflection crossing the eastern half of the Kameoka basin, named as Fl, F2 and F3 faults from west to east. All faults incline to the northeast to form the reverse fault type uplifting to the northeast side. The Fl fault is concealed under the alluvial plain of the Katsura River and is an active fault having remarkable displacement of vertical direction to a few hundreds of meters. An accumulation of the displacement in the vertical direction is plainly recognized on the topographical and geological sections. The F2 fault appears in the wide deformation zone on the hanging. wall of Fl fault and is thought to be a subordinate fault of the F1 fault. From the distribution, the F2 fault is corresponded to be an active fault described by Okada&Togo ed. (2000) and identified at former edge of a mountain in the Kameoka basin. In this paper, we will call the Fl fault and the F2 fault as“ the Kameoka fault within the basin”. It is surely distributed about 4.6 km from the Umaji to the Hozu settlements in the southeast direction. Of the Kawarabayashi reflection profile, one reflection layer C has vertical displacement of 65m resulted from the activity of“ the Kameoka fault within the basin”. A pure seam from core samples of the layer is confirmed as so-called Oda Volcanic ash at 420-450ka. Therefore, the average slip rate of the vertical displacement is estimated at 0.15m per thousand of years or less, during the last about 430,000 years. We also found a fault scarplet (relative height 1.5-2.5m) on a low terrace. It seems to be formed by the F2 faulting since about 20,000 years ago. Hence the faulting of“ the Kameoka fault within the basin” since the late Pleistocene is certain, and also there is a possibility of the activity in the Holocene from the existence of the reverse-inclined terrace II at Umaji. Judged from distribution, the F3 fault is corresponding to "the Kameoka fault in the foot of a mountain" described by Okada&Togo ed. (2000). There is no evidence of the F3 faulting during the late Quaternary.
The West-Tottori Earthquake (Tottoriken-seibu Earthquake) of October 6,2000, JMA Magnitude (Mj) 7.3, accompanied no distinct surface fault and occurred in the active fault-free area. This was a peculiar example in the Japanese on-land earthquakes, considering that all the recent on-land earthquakes greater than Mj 7.2 produced distinct surface faults and occurred along the detectable active fault in the epicentral region. The moment magnitude for the West-Tottori Earthquake is estimated at Mw 6.6 or Mw 6.7. In the Japanese on-land earthquakes of about Mw 6.6 or 6.7, three in five cases were associated with no distinct surface faults and did not occur along the distinct active fault. This indicates that the West-Tottori Earthquake was not a peculiar_ one in relation to the earthquake size against surface faulting. As for the absence of the known active fault in the epicentral region of the Earthquake it is explained by very low slip-rate of the seismogenic fault which must be about 0.01mm/yr or smaller. The abnormally high seismicity of the 1900's in the San-in region is indicated by the succesive occurrence of four large earthquakes greater than Mj 6.8, and by comparison of Quaternary moment release rate of the region and possibly by historical records. This implies that there exists rise and fall in seismicity with the return period of 100 years or longer and the existence of triggering among areas several or several tens kilometers apart in the region in 1900's.
The West off Fukuoka Earthquake which occurred on March 20th 2005, caused great concern for the Kego fault, because the Earthquake fault appeared on the northwestern extension of the Kego fault system. This paper reports on the Kego fault as seen by trenching studies atYakuin, which took advantage of construction of the subway Nanakuma-line in 1998. To clarify the exact position and shape of the Kego fault, borehole data analysis was carried out along the Kego fault in Fukuoka city. The analysis clarified the existence of steep trough, which was formed by the Kego fault activity, which reached a depth of 85m to the bottom of the Quaternary deposits. Several borehole cores which drilled on parallel with Yakuin -trench revealed that the fault plain was almost vertical but reverse tilted near the subsurface. The Kego fault was found 14 m below the ground surface and about 5 m west of the Yakuin-trench site. The fault cut the upper Pleistocene deposits almost vertically and slipped left with a N 30-40° W strike. By the detail observation it is concluded that there is no clear evidence of the fault was found in the Holocene deposits, Sumiyoshi formation. but more than 2 events of the fault were estimated from the disturbed upper Pleistocene horizons. Shimoyama et al. (1999) reported that 2 fault events were found during the last 32,000 years at the Osanotrench, the southern part of the Kego fault, but they could not determine the age of the latest events. Yakuin-trench study concluded that the latest event occurred between 16,000 y. B. P. and 10,000 y. B. P. The mean return period of the Kego fault event was determined to be 15,500 years by the Osano-trench study (Table 2). From these data, it is calculated that the probability of the next fault event occurring during next 30 years is 0.4%. Fukuoka prefecture (1996) determined that the length of the Kego fault was 18.5 km, but this study revises the Kego fault length to 22.0 km, because of the northwestern extension was found in Hakata bay.
Trench investigation at Shindo relics excavated to clarify geological activity records of Senbonsugi fault which is one of the faults constituting westernmost part of Minoh fault system. Senbonsugi fault had actived at AD679, and wide cracks generated in the ground. These cracks had described in“Nihonshoki (Chronicles of Japan)”as width was about 7 meters.
The Unzen Graben, which is bounded by normal faults to the north and south, is located at the western end of the Beppu-Shimabara Graben. The faults in the Unzen Graben have developed in association with the growth of the Unzen volcanoes and deformed volcanic materials such as lavas and pyroclastic deposits. The detailed location and vertical offsets of the active faults have been reported by previous studies, but the timing of faulting was poorly constrained. In this study, we calculate vertical slip rates of the faults based on recently obtained radiometric ages of volcanic rocks and sediments. The active faults in the Unzen Graben can be divided into five groups, based on the slip rates and the timing of faulting. The faults located on the northwestern and southeastern margins of the Unzen Graben have vertical slip rates as high as 0.12 to 1.0 mm/yr. However, the faults located on the northeastern margin of the graben are less active and have slip rates of 0.28 to 0.38 mm/yr. The faults inside the graben have slip rates of 0.085 to 0.46 mm/yr, except for the Akamatsudani fault which has a slip rate of 0.91 to 1.7 mm/yr. These faults show cumulative vertical offsets and their slip rates tend to be higher closer to the Fugen Volcano. On the other hand, the faults outside the Unzen Graben have considerably lower slip rates of 0.026 to 0.068 mm/yr.
The Ulsan fault extends for 50 km along the NNW-SSE direction in the southeastern part of the Korean Peninsula; this is one of the most important active faults in Korea. Its paleoseismicity has recently attracted considerable attention. With the support of KOSEF (Korean Science and Engineering Foundation), excavation studies of this fault were conducted in 1999 as a part of the Korea-Japan cooperative research at Kalgok-ri in Kyongju city. The results obtained are summarized as follows. (1) The Ulsan fault plane has an eastward dip of approximately 30 degrees and exhibits typical reverse faulting. (2) It was reactivated three times in the past 30,000 years, in particular, twice after the age of AT tephra (approximately 25,000 years BP). (3) A vertical displacement of approximately 0.8 m occurred during the fault event, and the amount of net slip along the fault plane is calculated to be 1.6 m.
Gerald Lensen (1921-2004) was one of the leading scientists on active fault research in New Zealand and also his work is wel-known internationally. He has established the Earth Deformation Section within New Zealand Geological Survey (presently Institute of Geologic and Neuclear Sciences), and promoted this field of study. His main interest was the mapping of active faults and their progressive deformation through late Quaternary, as well as fault geometry. He contributed not only for the progress of the active fault study, but also he considered how this study can contribute to reduce sesmic hazard, especially in urban area like Wellington. His idea was realized for the relocation of Temarua dam site to move away from the Wellington fault. Town of Totara Park was desigend to avoid the Wellington fault, making a wide road on the location of the fault. He has come to Japan nearly 30 years ago and inspired Japanese colleagues by his energetic work and talk. He also welcomed Japanese scientific team to study late Quaternary tectonics. This short note summarizes his life and his contribution, with translation of obituary written by Pat Suggate and Peter Wood, which appeared in News letter of New Zealand Geological Society No.138,42-26,2005.