By excavation of the Ochikawa-Ichinomiya remain located on the flood plain along Tama River near the boundary between Tama City and Hino City, Tokyo metropolitan area, a fault was found in young alluvium. This fault is regarded as the continuation of Tachikawa fault, a major active fault existing in the left bank area of Tama River, because it is located on the southeastern extension of Tachikawa fault line, and its strike is almost parallel to the Tachikawa fault. Detailed investigation of the fault outcrop made it clear that the last faulting event on the Tachikawa fault had occurred after A. D.1020-1158, the mid-Heian period. At this place, the last faulting event was dominantly strike slip with horizontal shortening of about 0.6 m.
The Kamishiro fault, extending for at least 20 km from the north Hokujyo basin to the south of Lake Kizaki, is one of major active faults along the Itoigawa-Shizuoka Tectonic Line in central Japan. We investigated detailed features of fault morphology along this fault, by interpretation of large scale aerial photographs. It became clear that faulting characteristics at the both ends of the Kamishiro fault are essentially different. At its northern end, the fault has become inactive progressively southward with time. However, the southernmost portion of the fault is still active in Holocene time, and continues southeastward to the Eastern Boundary fault of Matsumoto basin. At the southernmost portion of the Kamishiro fault, we also found evidence that remarkable thrust-front migration have occurred recently. Young offset features along the main portion of the Kamishiro fault suggest that at least one or two faulting events have occurred in Holocene time, and the latest event occurred probably in historic time.
Two humic silty layers were found at the level of +8.0m and +8.5m above sea level in the sediments of the Nakanawa archaeological site near Kuwana, central Japan. Radiocarbon age of the lower layer is dated as 2040±100yrs BP, and the layer is considered to have been deposited in a marine or brackish environment near the coast judging from the diatom assemblage. These facts show that the site has uplifted at a rate of ca 4m/1000 yrs since 2000 yrs BP, and this implies that the Kuwana fault is A-class active fault. Many tracks of sandblow are also found in the sediments of the site, and the uplifting movements are considered to have been associated with the historical earthquakes such as the Kaho earthquake in1096 and Tensho earthquake in 1586.
The profiles obtained by multi-channel seismic-reflection method, those on the Karasuma and Katata survey lines, around the southern lake of Lake Biwa are discussed together with the interpretation of geological and geographical data published previously. The results are as follows: (1) Three key seismic reflection horizons were observed in the Karasuma and Katata survey lines. (2) These reflectors can be correlated with some stratigraphic horizons revealed by deep drillings. (3) The Katata fault in the Katata survey line is a reverse fault, and its total vertical displacement is about 500m at the top of the basement rocks, and its average rate of vertical displacement is 0.54mm per year (of B class on the order of 0.1mm per year).
The Kego fault runs through the central part of Fukuoka-city, extending southward to Kasuga, and Oonojo cities. In order to estimate the fault activity during the Quaternary time and to clarify the fault's buried Quaternary topography, the author gathered drilling data in the area from Aratsu to Takamiya in Fukuoka-city, and examined the distribution of depth down to the base of the Quaternary strata lying on the Paleogene bedrocks. The results obtained are as follows. 1. The basement on which the Quaternary strata lie changes in depth remarkably on the line connecting Aratsu and Hirao in Fukuoka-city. The basement on the southwest side of the line is shallower than 10 meters in depth, but on the northeast side, the basement is deeper than 60 meters in depth. 2. The largest difference in depth of the basement between two closely situated study points is as much as 53.0 meters at location b. Moreover, the largest gradient of the averaged slope between two study points is about 77°in the vicinity of location f. 3. It is difficult to assume the steep slope mentioned above has been formed by erosion. Thus, the slope is due to faulting along the Kego fault during the Quaternary time. 4. The Kego fault in Fukuoka-city consists of two echelon-arrange d strands a few meters apart: one is from location a to the west of location b, and the other is from location d to location f. These strands are situated 100 meters southwest to the fault line previously shown in“Fukuoka Jibanzu”.
To determine paleoseismic events from trench walls of active faults, precise observation of displaced horizon by fault are very important. However, fault traces in humic soil and peat are sometimes very difficult to detect. X-radiography was used to identify the fault plane in the samples from the Kamishiro trench across the Itoigawa-Shizuoka Tectonic Line. The fault which is hardly identified on the eyes was succesfully identified by X-radiography. We described the detailed sampling method and condition of X-radiography. X-radiography is useful tool for the precise determination of paleoseismic event form trench walls.
In the Japanese Islands, including continental slopes on the Japan Sea side, are numerous active intraplate faults, which would cause disastrous earthquakes in the future. Evaluation of seismic risk potential of each active fault is necessary and urgent, in order to choose a feasible number of target faults, against which disaster-prevention measures should be taken with a high priority. Seismic risk potential of a fault could be evaluated on the basis of the size and timing of earthquakes produced from the fault in recent geologic time. However, the seismic risk evaluation using these two criteria either (1) depends strongly on untested models (such as the characteristic earthquake model and a variety of fault segmentation models) and/or empirical laws derived from observations of historical surface faulting, or (2) requires data that are difficult to observe in the present state of technology. Moreover, possible stochastic nature of faulting processes would raise another issue. In order to avoid these difficulties for the time being, I propose another criterion. The rate of seismic moment release per unit length of a fault averaged over recent geologic time is a good indicator of overall seismic risk potential, and is determined only from the average rate of net slip on and the subsurface geometry of the fault. Seismic risk potential based on this criterion, however, is only a first-order approximation, and must be updated with paleoseismological data in the near future.
Trenching survey across a fault is the best way to obtain informations on the timings of past earthquakes. Profound implications for seismic-hazard evaluation and active-tectonic processes call for numerous trenching surveys. In this paper, some problems on event analysis in current trenching surveys are discussed. This is indispensable for making standards for trench investigation. The main conclusions are the following; 1. Structures indicative of activity of a fault should carefully be distinguished from those related to liquefaction. Heavy shakings caused by other faults close to the fault under investigation might have produced sand-blows, injected sand dikes, intense rumplings and so on. It is particularly required in the case of argument of fault segmentation. 2. Talus or colluvial apron derived from a fault scarp, and a drastical facies change in subsequent deposits do not always indicate a faulting event. 3. Short branch faults are not good indicator bracketing of the time of an earthquake. 4. When recognizing plural events, particularly based on the grade of deformed strata, one should pay much attentions on the accumulation of displacements. 5. Humus and peaty materials can be piled up on a slope. Therefore, it is necessary to observe carefully whether these strata are deformed or not.
Ground penetrating radar uses reflected electromagnetic waves to image the subsurface. Its investigation depth is 2-3m and its resolution is 20-30cm in soil. Because of its very shallow investigation depth, ground penetrating rbdar has been used for finding underground gas pipes, electric cables and buried remains. Ground penetrating ra d ar method has an advantage over the seismic reflection method in tenns of the spread density of sources and sensors. It is necessary in the seismic reflection method to spread sources and sensors in consideration of surface waves because the generation of surface waves makes it very difficult to distinguish reflected waves. However, in ground penetrating radar method, sources and sensors can be spread conveniently with high density for its high resolution because surface waves are not generated in the electromagnetic field. We have devised a new radar system to ap p l y the ground penetrating radar method to geological surveying. In the ordinary radar system, impulsive waves are used as transmission signals, but in the new radar system, sine waves are used with the frequency varied as a step function of the sweeping period of transmission signals. We can obtain the impulsive reception signals such as the ordinary radar system after the convolution integrals between sine-shaped transmission and reception signals. We call the new radar system the Step Continuous Wave Radar(SCWR) systen after the characteristics of its transmission signals. Its investigation depth is 10-15m in soil and 20-30m in rock with resolution of 50-60cm. The SCWR system will provide useful information because it can be carried out speedily and non-destructively on the ground around active faults, before trenching, down to about 10m in depth. In this paper, we will show the imaging of active faults with the SCWR system throu g h the results of the investigation around the Nojima faults which appeared in the Awaji Island with the 1995 Southern hyogo Prefecture Earthquake, and then we will pick up some current problems in the imaging, for example the effective arrangement of sources and sensors for the shorter period of field work and for the three-dimensional imaging with ground penetrating radar, and finally show a conception for the solutions of these problems.
The success of a trenching study is highly dependent on the selection of the trenching site. If there is a geophysical technique to image shallow subsurface structure without excavation, it can be a useful tool for site selection of the trenching point. Moreover, if the quality of the subsurface image obtained by the technique is high enough to estimate the characteristics of an active fault, it can be much more useful for the active fault study itself. From this point of view, we have starte d to apply the shallow seismic reflection method using a portable vibrator for imaging very shallow subsurface structure. The electro-dynamically driven portable vibrator is a high frequency seismic source which can generate seismic waves with more than 1 kHz. This paper describes the outline of the portable vibrator system and two feasibility studies using the system. The first feasibility study was conducted for the purpose of detection of pieces of styrene foam buried at known shallow depths. These targets were clearly imaged as the diffraction pattern at depths of 1.5m and 2.0m by using the system. The second one was conducted near the trenching site at Nashimoto on the Nojima earth quake fault for characterization of the very shallow part of the fault. The seismic reflection profile obtained at this site clearly showed displacements of the subsurface layers shallower than 10m, which might be caused by faulting. These two feasibility studies described here revealed that shallow seismic reflection method using the portable vibrator system was one of the potential methods for investigating an active fault characteristics.
Faulting and other surface deformations in recent geologic time are essentially relevant to understanding present-day tectonic processes, which in turn are a key to scientific, not empirical, earthquake prediction. Geologic records are indispensable because instrumentally observed records, such as geodetic measurements and microseismicity, are not sufficient in time to cover a whole cycle of strain buildup and release in a orogenic zone. Rheological structure of the Japan arc b ased on explosion seismology, heat-flow measurements, and laboratory experiments indicates that the western half of central and northern Honshu, including continental slopes on the Japan Sea side, is mechanically very weak; only the upper 15 kilometers of crustal rocks behaves elastic, and ductile lower crust is underlain directly by asthenospheric mantle. This zone of weakness was rifted and stretched during the early Miocene back-arc spreading event, and coincides broadly with the distribution of active faults. Since late Miocene time up to the present, the Japan arc has been subjected to east-west compression due principally to the westward convergence of the Pacific plate at Japan trench at a rate as high as ∼90 millimeters per year. If the megathrust at the Japan trench is locked, the plate convergence is to be ac c ommodated mainly in this zone of weakness. Actually, geodetic observations in the last 100 years have revealed that strain accumulation rates over the mechanically weak zone are on the order as high as 10-7 per year. However, geologically observed strain rates, based on slip rates on active faults and folding rates, are one order of magnitude lower than the geodetic rates. A possible explanation for this discrepancy between short-term (geodetic) and long-term (geologic) observations is that the strain accumulated in the last 100 years at abnormally high rates is likely to be released by slip on the megathrust at Japan trench, which would produce big earthquake(s) with magnitude 8 or greater. Only a fraction of plate convergence may be accommodated within the Japan arc as long-term deformation. Whether or not the above scenario is real, the process of strain buildup and release in the Japan arc-trench system is unique, and should be understood with more geologic, as well as geophysical, observations.
The seismic reflection profiling system was newly equipped in Earthquake Research Institute of the University of Tokyo in 1996 for the research of active faults. The system consists of the vibrator seismic source (Minivib T15000 of IVI), the digital telemetry recording system (G-DAPS 4 of JGI), and the data processing system(iXL of MIT). The source mounted on a 4 ton truck, can generate 10to 550 Hz sweep signal with maximum peak force of 2.7 ton. The recording system has wide applicability from very shallow to crustal scale profiling. This seismic profiling system is opened to the Japanese university researchers.