The Yanagase fault is a remarkable active fault in central Honshu, Japan, extending north-nothwestwards from the northeast of Lake Biwa to the northeast of Tsuruga City.
Both geomorphological and geological detailed studies have been carried out along the fault zone to clarify the history of its movement especially in Quaternary time.
A linear fault line valley consisting of four different streams in a straight arrangment is developed along the fault zone over a distance of about 25km, which is recognizable as a lineament in a broad sense. Geomorphic features caused by left-lateral faulting such as shutter ridges, offset streams are detected along the southern part of the lineament. And the west-side block of the fault shows subsiding topographic features such as rather wide alluvial plain around Lake Yago, but the east-side uplifting such as elevated river terraces of two to five steps, composite alluvial fans, high triangular terminal facets and so on.
The paleozoic system consisting of slate, sandstone, chert, greenstones is sheared along the fault in a zone of a few hundred meters wide, in which rocks are strongly crushed to be transformed into fault gouge in a maximum width 50 meters or so. Porphyrite dykes within the fault zone are also crushed at some localities.
Fault displacement in late Quaternary fan deposits has been observed at several places in the southern, part of the fault zone. However no evidence of Quaternary fault movement has been found out in the northern part, although the fault zone is overlain by Quaternary fan deposits here and there. Each fault shows different sense of displacement, that is lateral-slip, east-side uplift dip slip and west-side uplift dip slip. The youngest deposit cut by the faults has been dated back to about 4, 000 years B. P. in Carbon-14 age.
Judging from the west-dip imbrication of gravels observed in the highest terrace deposits along the wind gap east of Nakanogo, the Yogo River had been running once eastward from Nokanogo through the wind gap to Shimo-niyu. The fault movement with west-side subsidence and east-side uplift occured at 80, 000 to 100, 000 years ago along the southern part, seems to have changed its direction to the south as in the present.
It is concluded the Yanagase fault has moved repeatedly since late Paleozoic time, and has been active in its southern half in late Quaternary time, on the contrary not or scarcely active in the northern part. Its dominant sense of movement in late Quaternary is supposed to be left-lateral, west-side subsidence and east-side uplift.

View full abstract

The Fujikawa fault that was discovered three years ago is important for the prediction of the apprehended outbreak of the Tokai earthquake. The latest activity of the fault was recorded in 1854at the time of occurrence of the Ansei Tokai earhquake (M=8.4). The Fujikawa fault runs northward from themouth of the Fuji river across the western slope of Mt. Fuji. The southern extension of the Fujikawafault is the Suruga Bay fault that is assumed to run along the Suruga trough. The Suruga Bay-Fujikawa faultsystem is a great left-lateral strike-slip active fault: the Suruga Bay fault shows a left-lateral offset of submarine topography of about 20 kilometers, whereas the Fujikawa fault shows a rapid average velocity of leftlateral displacement of 3.3cm/y. The fault system dislocates the Nankai trough that is regarded as a boundary between the Philippine Sea plate and the Eurasian plate. The surface expression of the Fujikawa fault is very slight because faulting has proceeded simultaneously with the growth of Volcano Fuji and the accumulation of the Fuji River fan-dellta deposits that has been promoted by postglacial rise of sea level. The surface trace of the fault is discernible by analysing abundant storage of water well dataand historical documents in addition to rarely discovered geological and geomorphological evidence.

View full abstract

The methods to evaluate fault activity must be applicable to many kind of faults in the basement terrain, in the case of important engineering problem such as the aseismic design of nuclear power plant.
In this paper, the authors have reviewed some problems which have been discussed about the activity of faults, from both scientific and engineering viewpoints home and abroad. Some characteristic features of faults have been shown through many data of dams and nuclear sites, such as fault length, distribution density, fracture thickness, texture and materials in fracture zone. Some typical methods, applicable to various kind of actual faults both inland and seabottom, are presented through the recent studies on fault activity evaluation.
In conclusion, the authors have proposed a proper evaluating system of fault activity for nuclear power plant, considering the scientific and technical development and the rule required to aseismic design in the present.

View full abstract

Fault density and “fault dimension” were measured to each dominant fault direction in some fields of Neogene sedimentary rocks and granitic ones.“Fault dimension” is expressed here as with the displacement of the fault U, fracture width W and fault length L. The linear relations were recognized among these values on the log-log co-ordinates.
In relation to the fault density along the scanline, Poisson's distribution was confirmed by the fault survey. And the frequency-fault dimension curve was in general agreement with the theoretical log-normal distribution.
Total amount of displacements was sometimes found to be large and relatively large faults were often found, in the part of high fault density. These geological features of the fault distribution are explained by the numerical deformation analysis for the continuous rock body with the conbination of elastic dislocation theory and FEM.
Monte Carlo simulation utilizing the poisson's distribution leads the distribution of the position of faults along the scanlines in the objective area and above mentioned numerical deformation analysis can put the fault size on each synthesized fault position.
These ideas were extended to some fields and the distributions of the discontinuities in the rock mass were well estimated in the restricted area described on this paper.

View full abstract

Groundwater discharge into under ground gallery and rain fall have been automatically recorded during about 14 years at Inuyama Crustal-Movement Observatory of Nagoya University. The whole discharge can be devided into two parts from the view point of the seepage pass, and one is the discharge through fracture zone developing near cut-face of the gallery and another is through weathered zone below the ground surface. These discharges have been recorded individually.
The response of former discharge to rain fall as an aid to understanding the flow behavior in fracture zone was also examined.
At the same time, we simulated numerically the steady condition of the groundwater flow around the gallery assuming constant infiltration rate into the groundwater table. The transient behavior of the flow caused by depletion of infiltration rate after stopping of rain fall was also simulated. The depletion curve of infiltration rate which was used in the simulation was assumed from the record of decreasing discharge through weathered zone after rain fall.
Results obtained are as follows;
1) Discharge through fracture zone has not so good response to rain fall, and this nature is mainly caused by the nearly constant water flow in the saturated zone below the groundwater table.
2) The transient discharge change calculated from the simulation of transient groundwater flow shows good accordance with observed records.
3) The proper modeling of hydro-geological structure of the area is very important to simulate the groundwater flow in rock basement.

View full abstract

Hydrogeochemical investigations were carried out for the water appeared in some pilot tunnels which were under construction in granite regions. The samples obtained are, in general cases, falling water near the cutting face of pilot tunnel and, in another cases, flowing water from horizontally drilled holes for prospecting fissure water. Such water is considered to be conducted through networks of open discontinuities and fault-sheared zones developed in granitic rock masses above and around the pilot tunnels. Cations, anions and other dissolved substances are analysed throughout the investigations. Data of hydrogeochemical analysis are compared with each other and with stream and spring water distributed above and around the tunnelling sites. As results, the authors pointed out that mineral water, defined as higher contents of Cl^-, HCO₃^-, SO₄^2- ions, is distinguished among the water obtained. Mineral water and common water are concealed irregularly and alternatively within the fault-sheared zones. Results of TR (tritium ratio) measurements indicate that large portions of above-mentioned falling water is supplied from the rainfalls before 1955 A. D., but some parts are recharged from the rainfalls after 1955 A.D. So, “intrusion” of rain water is rapid successively to the construction even though the case of pilot tunnel. In one case, 10^-1 order's TR-value is measured from the cutting face of the central part of the Enasan tunnel.

View full abstract

The construction of an undersea tunnel below the Tsugaru Strait between Honshu and HokkaidoIslands in Japan through incipiently metamorphosed Miocene andesitic rocks have provided a remarkableopportunity to study the geochemical method to predict the gushing out of seawater into undersea tunnel, through fissures in zeolite facies metamorphosed rock mass from boundless overlying sea.
Close relathioship between discharge rate and K (and Ca) content of the seepage waters was found.K and Ca contents of the seepage waters generally increase and decrease respectively with the increase ofdischarging rate. The K content of seawaters, after extensively interacted with rocks, became extremelylow when the discharge rate is lower than hundred liter per minite. The Cl content of the seepage water canbe used to indicate the mixing ratio of seawater/meteoric water. Therefore, K×Cl value is proposed to bethe most important chemical parameter which clearly reflects physical properties of waters such as migrationvelocity of waters within the Miocene rock mass, water/rock ratio, water pressure and discharge rate.The variation of K and Cl contents of seepage waters from pilot bore holes and faces of tunnels providedcritical informations on how rapidly fluids are penetrating the rocks between the undersea tunnel and seafloor, permitting necessary precautionary measures to be taken to avoid disastrous entry of seawater.

View full abstract

In pure geology, displacement and direction of faults are important concerns. On the other hand, forconstruction and maintenance of tunnels, the conditions of fractured rock and fault clay caused by faultsare taken seriously. However these faults often bring trouble to tunnel construction, since they have extensivescale, secondary faults, and form wide fractured zones in many cases. In such cases, if possible theyare kept away from the tunnel route and if the route crosses them unavoidably it is often difficult toexcavate through them.
In this paper, we will first describe the outline of the problems in survey, design, construction, andmaintenance of railway tunnels in fractured zones. Next, we will report the examples of the geological surveysfor the Rokko Tunnel and the Shin-Kanmon Tunnel. The former crosses many faults in the Rokkomountains and the latter crosses the fractured zone under the straits of Kanmon. We also will report theexample of the seismic prospecting for the Shin-Sasago Tunnel in order to foresee where the tunnel crossesthe fractured zone.
In addition, we will describe the construction methods to excavate through the fractured zone for theRokko Tunnel and Shin-Kanmon Tunnel. The Mukaiyama Tunnel was constructed by NATM, and thegreater convergence of the tunnel section was measured in the fractured zone even though rock bolts wereadded.
At last, we will report the case in which the Inatori Tunnel was damaged by a seismic fault.

View full abstract

Strong and watertight conditions are prime factors as to dam foundations. Fault and crushed zone arelarge deteriorated factors. One of the most important aims of geological survey is to find faults and cracks, and to evaluate fault conditions. Fault and crack effects on dam constructions are great and dependon dam types, scales and relative positions of faults and a dam.
It is a role of geologist to determine if the proposed dam sites should be abandoned or condemned, conducted appropriate treatment, or should be changed on dam type or scale, even after considerable geologicalinvestigations on the damsite.
The author has reviewed these themes on faults and cracks, faults and dam types, foundation treatmentsand active faults in view of dam constructions with several references, construction reports and hisexperiences.

View full abstract

Weak layer, such as fault or fractured zone has great effects on stability of slope. These effects are divided into two types. One is the weakness of rock masses which consist natural slope or cutting slope, caused by the fault movement, and the another is decrease of slope stability due to forming the slip surface in fault or fractured zone.
As for the former types, one example is explained, which relates to the cutting slope along road, route 41.
As for the latter type, three examples are explained. For shigeto-landslide in Kochi prefecture, ground water ran into the sliding mass along the faults. Slightly weak zone compared to surrounding parts was found after slope failure occurred, and at that time, many field tests were carried out.
Concerning the slope failure which occurred in the fault zone located in the upper part of a cutting slope, stability analysis using finite element method is performed. In this case, distribution of point safety factor in slope, calculated by using computer, is considered in connection with four stages of slope, namely, natural slope, cutting slope for railway, cutting slope first designed for road, cutting slope designed for road with amendment. The change of safety factor by condition of faults by which rock mass forms a sliding wadge is discussed by using vector analysis.

View full abstract