The statistical distribution of earthquake magnitude is governed by the well-known Gutenberg-Richter (G-R)law. The seismogenic fault-length is transformed through the Matsuda formula into earthquake magnitude. The statistical distribution of the magnitude such transformed can also be governed by the similar law. On the basis of the list of active faults in Japan, we derive two kinds of statistical distribution formulas of seismogenic fault-length, which are consistent respectively with the modified G-R formula and with the Makjania formula.
High-resolution digital elevation models (DEMs) have been widely adopted as data sets that can be used, together with image processing and geographic information system technology, for geomorphological analysis. For example, shaded relief maps and bird's-eye-view images derived from DEMs are effective ways to depict topographic relief. However, not all geomorphologic features appear in DEMs because they usually consist of digital data derived predominantly from pre-existing topographic maps. Therefore, before conducting geomorphology research with DEMs, a detailed review should be conducted, to determine whether DEM resolution is sufficient to reveal the specific geomorphological features required. The aims of this report are: (1) to discuss the appearance of tectonic landforms on DEMs as they vary with DEM resolution; and (2) to present a case study on a new tool, the digital photogrammetry workstation (DPW), which can be used to construct DEMs with better detail directly from aerial photograph pairs or satellite image pairs. Well-known tectonic landforms around Chino and Fujimi along the Itoigawa-Shizuoka tectonic line are used as examples. This illustrates that 50m-resolution DEM is insufficient to indicate whether a scarp is of tectonic origin, because references such as tectonic bulges or river offsets are too subtle to appear. DEMs with 10-m resolution or better, constructed by using a DPW, might prove to be more useful in research on active faults, with the caveat that a certain amount of manual revision is inevitable when constructing DEMs.
After the 1995 Hyogokenn-Nanbu earthquake, local governments started survey of active faults by the fund from Minister of Education, Culture, Sports, Science and Technology. By the end of 2000,77 faults have been surveyed by local governments. We collected 73 seismic reflection profiles obtained by these surveys and made a database. The database consists of figures of reflection profiles and a table of profile lines. The table contains activity and geological conditions of faults, information on a reflection survey, and a faults' appearance in a reflection profile. We divided faults' appearance into four types based on the pattern of reflection surfaces in profiles; (A) Step (a fault appears as steps of reflection surfaces), (B) scarp, (C) discontinuity ( reflection surfaces are recognized on one side of a fault, but disappear on the other side), and (D) step and scarp. Using this database, we considered the relation between faults' appearance and geological conditions, fault type and activity, and found the following features. (1) Reverse faults appear as type (A), (B) and (D), while strike slip faults as type(C). (2)Most of faults of type(B) are characterized by thicker sedimentary layers and lower activity than those of type (A)and (D). We have to collect more reflection profiles of normal and strike slip faults in order to investigate features of these type of faults.
The software“Coulomb”is designed to let one calculate static displacements, strains, and stresses caused by fault slip, point sources of inflation/deflation, and dike expansion/contraction under an isotropic elastic half-space. The principal feature of the program is ease of input, rapid interactive modification, and intuitive visualization of the results. The program largely implements GUI (Graphical User Interface) environment such as menus, submenus, check-items, and dialogue boxes to ease operation. The internal graphics are suitable for publication, and these can be easily imported into illustration and visualization programs for higher-quality figures. We hope this program extensively supports researches for active faulting.
The Itoigawa-Shizuoka tectonic line (ISTL) active fault system is one of the longest and the most complex active fault systems in Japan. The system comprises the northern, the middle, and the southern segments. In the vicinity of the boundary of the middle and the southern segments (Hakushu - Nirasaki), tectonic landform related to the ISTL, active fault system is investigated and mapped. As a result of investigation, the following things became clear. The Hakushu fault, with vertical slip rate of 0.2-0.4mm/yr, branches into two faults in a southernmost part. The eastern fault is reverse fault, and the western one is left lateral fault. Mukawa faults comprise many short faults with low activity (<0.1mm/yr). Mukawa faults are assumed to be a northernmost part of the Shimotsuburai fault. In the same location, broad flexure (Mukawa flexure), trending E-W, is formed. The Mukawa flexure uplifts southern side, with vertical displacement rate of 0.6mm/yr. It is estimated that the Mukawa faults accommodates a part of the displacement of the Mukawa flexure. Since branching end of the Hakushu fault and the Shimotsuburai fault are confronting each other, it is assumed that the segment boundary exists here. The Hououzan fault has been considered to be an active fault formerly, based on the displacement of the terrace surface. As a result of detailed investigation, however, it become clear that the terrace was not deformed by any fault. Therefore, it is thought that the southern part of the Hououzan fault is not active recently. Based on the slip rate of faults, the displacement of the middle Pleistocene strata, and the estimation of subsurface structure, the history of fault activity is inferred as follows: (a) In late Neogene to early Quaternary, the Kamanashiyama - Hakushu - Hououzan faults were active. (b) In early to middle Quaternary, active segment jumped from the Hakushu - Hououzan faults to the Kamanashigawa - Shimotsuburai faults. These faulting uplifted the Akaishi Range, the Koma Mountains and the Nakayama. (c) In late Quaternary, the Kamanashigawa fault became inactive, and the Hakushu fault reactivated. The Mukawa faults (Mukawa flexure) were newly formed, and the active region of the fault system became the Kamanashiyama - Hakushu - Mukawa - Shimotsuburai faults. These faulting uplifted the Akaishi Range and Koma Mountains.
The Miyashiro fault is a reverse fault extending in the direction of NW to SE for 7 km along the western margin of the Nobi Plain, central Japan. This fault appears to have played an important role in separating the Nangu Mountains from the Nobi Plain during the Late Quaternary. We studied tectonic geomorphic features along the Miyashiro fault where it crosses the Himori fan in Tarui Town. The latest faulting event of the Miyashiro fault oc c urred after 1,730 y. B. P., with larger than 1.5 m vertical offset. The vertical slip rate of this fault is as high as 0.5-0.6 mm/yr during the past 15,000-20,000 years. If the average offset during a single earthquake is 1.5 m or larger, the average recurrence interval of earthquake might be 2,500-3,000 years or longer.
The Hanaore fault is a right-lateral strike-slip active fault trending NNE-SSW in Shiga and Kyoto Prefectures. The northern part of this fault might move during the 1662 Kambun earthquake. However, paleoseismic activity in the southern part still has been unclear. We excavated a trench at Shugakuin area in Kyoto City to evaluate the seismic risk of the southern part of this fault. Some high-angle fault planes cutting fan deposits were observed on the trench walls. We divided the sediments on the trench walls into seven stratigraphic units named as unit A to F. As a result of our precise observation, the latest faulting event occurred after the deposition of the unit B2 and before the deposition of unit A. And the penultimate event occurred after the deposition of unit D and before the deposition of unit C. The radiocarbon ages from the trench walls and the results of previous trench studies indicate that the timing of the latest faulting event was about 2,500 to 1,500 years B. P., and the timing of the penultimate event was about 7,800 to 7,000 years B. P. The interval between the last two events is estimated to be about 4,500to 6,300 years.
A large-scale cross-stratified sediments of about 5.5 meter thickness which consist mainly of sands and gravels, crop out facing the Amano River, at Kisaichi on the northwestern margin of Ikoma Mountains. This is regarded as deposits formed in an artificial raising process of river bed. The upper part of silty layer beneath this sediments, was dated at 602±59yB(A. D.1290-1424). These facts indicate that the beginning of artificial raising of Amano River bed doesn't date back to the end of the 13th century. And this is an important data suggested too that the historical event of Ikoma active fault estimated by Togo(2000), is after the latter period of Kamakura era.
Surface rupture with maximum right-lateral displacement of 150cm and length of about 1 lkm was observed during the 1943 Tottori earthquake of Mw: 7.0, which struck the eastern Tottori prefecture, southwest Japan. We compiled all the previous reports and papers regarding the surface rupture associated with the earthquake as precise location maps (Appendix 1) and a data table (Appendix 2). We also showed the slip distribution along the surface rupture (Fig.3). The trace of the 1943 surface rupture exhibited the large step and quite sinuous geometry compared with other strike-slip surface breaks. This observation probably means that the causative fault for Tottori earthquake is at the infant stage on its way of evolution based on the Wesnousky's fault evolution model (Wesnousky,1988). Most of the surface rupture during the earthquake occurred along the geomorphologically detectable active fault (Shikano-Yoshioka active fault system). Thus, we can recognize the possible surface faulting event around the epicentral area based on recent understanding of tectonic geomorpholgy. However, the length of the subsurface seismogenic fault is underestimated by the presently proposed method (Odagiri and Shimazaki,2000) and this sh ould lead to crucial underestimation of the seismic moment and also strong ground motions. The method to estimate the lengt h Df the seismogenic fault must be reestablished.
The October 6,2000, M7.3 Tottori-ken-seibu earthquake involved a few surface fractures, which have been thought to be direct exposures of the source fault at the surface (surface fault trace) by several papers. However the fractures are unusually short (<1km) relative to the one estimated from the magnitude, and they are characterized discontinuity along the fault strike and split into two parallel-running breaks. Using freely slipping surface elements employing the boundary element method, I demonstrate that these fractures are more likely to be triggered slips to shed broadly distributed left-lateral shear stress produced by the subsurface fault. Even if the rupture of the source fault stopped at a depth of 1 - 4 km,0.5 km- to 1 km-length pre-existing weak surface fracture is calculated to have released 10 - 20 cm left-lateral slip, which amount is almost consistent with observed slip along the fractures. Thus, we cannot rule out the possibility that the fractures observed at the Tottori-ken-seibu earthquake were rather triggered slip.
In this paper, we discuss on the Kashima fault, an inland fault that extends E-W for about 18km in the peninsula of Shimane. This paper aims 1) to depict the detailed feature and distribution of fault morphology along the Kashima fault on large scale topographic maps,2) to apply the fault branching model and dip-slip distribution pattern for fault segmentation, and 3) to clarify subsurface structure of the Kashima fault by acoustic survey. Results of this paper are summarized as follows: 1) Evidences of right-lateral movement is continuously discernible along the Kashima fault.2) Active fault trace is remarkably linear suggesting that the angle of fault plane is high.3) Result of seismic profiling suggests that the Kashima fault ruptured after 25,000 years ago.4)Based on the pattern of dip-slip distribution and the feature of branching fault traces, the segments of Kashima fault are identified as one segment, which means the segments of the Kashima fault might rupture together.