Torao Tanaka

Seepage into a mountain tunnel and rain infiltration

Abstract To clarify the processes governing recharge of rainwater into an unconfined aquifer in a mountain basin, the seepage rate of water into a mountain tunnel was monitored continuously between May 1988 and December 1991 in Yura, Wakayama, Western Japan. The electrical conductivity and ionic composition of the seepage water was also measured regularly from June 1989. The mountain is composed of fractured sedimentary rocks such as sandstones and cherts. Seepage appeared only at specific locations on the roof of the tunnel. Seepage was measured through overlying sandstones of 10 m depth and cherts of 5 m depth. Time variations in the discharge suggested that seepage is formed both by rapid flow and basic flow components which correspond to the so-called fissure flow and matrix flow. This was confirmed by analysis of time variations in the concentration of chemical species in the seepage water. Fissure flow contributes the initial increment of seepage discharge, immediately after the occurrence of rainfall, and its velocity for sandstone can be represented by a function only of water content (θ), αθ n . α is constant and larger in winter than in summer, but n remains constant, at approximately two. Infiltration time in fissures through the base of the cherts is negligible, compared with that through the overlying soil layer. Whereas matrix flow through sandstones persists through the year, for cherts it disappears within a week after rainfall events and its decay is dependent on the seasonal magnitude of evapotranspiration. The behavior of the matrix flow for sandstones can be analyzed through a kinematic wave model, as can the fissure flow.

Using short- and long-term transients in seepage discharge and chemistry in a mountain tunnel to quantify fracture and matrix water fluxes.

Abstract Infiltration of rain-water into a fractured sedimentary rock mountain is explored through continuous observations of discharge rate, Q , and electrical conductivity, EC, of seepage water into a mountain tunnel. Also concentrations, C CO 2 , of carbon dioxide gas near the tunnel ceiling, and the chemistry of the seeping water are examined. Earthquake events occurred in the period of the seepage observation and influenced characteristics of the time trends in Q and EC. This provided a mechanism for the identification of rapid flow (fissure flow) and slow flow (matrix flow) in the infiltration components in the fractured rock base. Also, a cycling of discharge water from the matrix via the fissures and back into the matrix was expected to occur. C CO 2 increased due to rainfall events, and its response was with a phase-shift to increased Q . For a heavy rainfall event, the increase in Q was mainly caused by the occurrence of fissure flow, and as soon as Q began to decrease moderately after a rapid decrease from a peak value, C CO 2 showed a peak value. The C CO 2 peak seemed to coincide with increased matrix flow. Wetting in the rock matrix was assumed to behave as a shock wave. For a light rainfall event, where only matrix flow is likely to occur in the fractured rock base, Q increases were delayed in comparison to C CO 2 increases. The variations in C CO 2 due to rainfall events appeared to relate to the movement of the matrix wetting front, when high moisture contents were apparent. The wetting front was inferred to be pushing void-airs with high concentrations of CO 2 gas towards the tunnel. High CO 2 concentrations were assumed to be formed near the ground surface via dissolution of organic matter and respiration of plant roots. The chemistry of seepage water observed at two close locations is seen to differ distinctly. Time-variations in EC for one location (A1) are consistent with those for C CO 2 , while for the other location (A3) this was not the case. The variations are due to dominant anions in the seepage water; HCO 3 − for A1 and SO 4 2− for A3. These occur via dissolution of CaCO 3 and CaSO 4 into infiltrating water, and CO 2 gas plays an important role in the former process. The time trends and integrated interpretation of the seepage volumes, chemistry of seepage water, and the concentration of CO 2 gas are shown to be useful indicators for understanding rainwater-infiltration process in the fractured rock mountain, and for separation of the seepage into fissure flow and matrix flow components.

Precipitation, Groundwater and Ground Deformation

Strainmeters and tiltmeters installed on or at a shallow part under the ground surface record ground deformations caused by rainfall. Such deformations are detected to a depth of several ten meters with these instruments, and accordingly they are main noises to observations of crustal movements and/or earth tides with these instruments.

Geodynamical value of historical geodetic measurements: A theoretical analysis

Historical geodetic measurements have been used to infer on the displacement and strain states locally or regionally. They are also often used to invert for other geophysical parameters. However, historical geodetic measurements have been known to contain significant scaling and orientation errors, which may even be different in different parts of a network. These significant error sources may result in producing a wrong (or at least, a misleading) displacement or strain field. When such a displacement or strain field is further used to invert certain geophysical parameters, mis-interpretations may be expected. Thus, in this paper, we will perform a theoretical analysis to answer the following three questions: (i) are displacements obtainable from historical geodetic data? (ii) are strains obtainable from historical geodetic data? and (iii) what geodynamical value do historical geodetic data have?

Crustal movement research and GPS in Japan

Abstract Many GPS receivers are now used to monitor crustal movements in Japan as field surveying and stationary observation instruments. On continuous records of crustal movements with strainmeters and tiltmeters slow changes of secular trends are sometimes observed in a time range of several years. This kind of change cannot be detected and studied by conventional surveys. By using high precision GPS such fluctuations of crustal movements are successfully found and new facts relating geodynamics can be investigated.

Correlation Analyses of Horizontal Gradients of Atmospheric Wet Delay versus Wind Direction and Velocity

Total water vapor contents were monitored along lines of sight at such low elevation angles as 10 and 15 degrees with two water vapor radiometers, WVR1100™, WVR05 and 06. They were installed in two directions of N-S and E-W in Uji city, southwest Japan in the period from 1997 to 99. Results show that differences of wet delays between N and S directions, which correspond to the gradient of wet delays of microwaves, sometimes reach to 3 cm or more and continue to exist stably for a few days or longer. It is ascertained that the horizontal gradient of water vapor distribution in the N-S direction is caused by atmospheric conditions, especially by wind direction and velocity, and also probably by sunlight. Similar correlations are apparent between E-W gradients of wet delay and wind velocity. However, the data is not enough to draw definite conclusions on the E-W component.

Comparison of Crustal Deformations Observed with GPS and Strainmeters/Tiltmeters

Daily solutions from GPS observations can give uniform tectonic deformation and relative motions of stations more than 100km apart to each other. More precise wet delay correction is necessary in order to monitor local or the order of 10km deformations. On the other hand observation with strainmeters and tiltmeters show irregular changes which are probably associated with local stress accumulation or irregular concentration, but it is not easy to distinguish the irregular change caused by tectonic stress from that caused by meteorological or ground water disturbances. Combined observations with GPS and high sensitive strainmeters/tiltmeters are important to make clear the nature of observed irregular changes, and further to get information relating to the occurrence of earthquakes of M7 class and accordingly to approach the long-term earthquake prediction.

Earthquake Prediction by Geodetic Surveys and Continuous Crustal Movement Observations in Japan

Japanese Islands have been considered to be located at the eastern edge of Eurasian plate boundary to the Pacific and Phillipine Sea Plates. Recently, another plate boundary is proposed to divide northeastern from southwestern Japan(for example, Seno,1985), and by this the former is on the North American and the latter the Eurasian plate, respectively. Relative motions of these four plates control the tectonics of Japanese Islands, and consequently the occurrence of large earthquakes along these plate boundaries.