深部地下水の放出と地表現象

Stress changes associated with crustal deformations may induce migration of fluid within the crust. It is hypothetically expected that a volume of pore water, being suddenly pressured in response to an elevated stress level in some seismogenic zone, will tend to intrude up into a crack network, and incidentally emerge at the ground surface. Groundwater changes in temperature and concentrations of chemical constituents should be observed at the spot where the upwelling fluid comes out. This paper reports the following 4 transient events recently found in Japan as evidence of the near surface discharge of hot and pressured water of a deep origin: 1) Frequent rises in well water temperature were observed at Iwakuni,Yamaguchi Prefecture, southwest Japan. The largest rise occurred just before the 2001 Geiyo earthquake of M0.1, which indicated a possible precursor for the shock. A chemical analysis of ion concentrations of the hot water suggested that the temperature anomalies arise from contamination by intruding external deep water; 2) Gushing of groundwater at the sea bottom was considered to have occurred at the Akashi Strait 2 days before the 1335 Kobe earthquake of M1.3, based on an interpretation of the appearance of brownish-black seawater found by the captain of a passenger boat; 3) Upwelling of deep hot groundwater was occurred at Inagawa Town, Hyogo Prefecture, southwest Japan, which was associated with the 1335 Kobe earthquake. The well water temperature rose 3-4℃. at the time of the shock, and decayed with a time constant of 1-2 years; 4) Heating of ground rocks by upwelling hot water intruding into the fracture zone of an active fault, which is considered to be a precursor for the April 1, 1335 Niigata-ken Hokubu earthquake of M5.5, was confirmed by a LANDSAT infrared image in the northern Niigata area, central Japan, on a summer night in 133.. All of the above transient phenomena can be reasonably understood in the light of the hypothesis of pressured hot water upwelling from deep underground in response to crustal movements around seismically active regions

Sequences of Microearthquakes Near the Yamasaki Fault

Data obtained by a long-running observation made by the Tottori Microearthquake Observatory reaveal separately distributed fractured regions where microearthquakes occur in clusters. Near the central part of the Yamasaki fault a linear configuration of such fractured regions parallel to the fault trace over ten kilometers at a depth of about 11km exists, whereas the depth of fractured regions around the eastern end of the fault is about 13km. Shallow-seated as well as deep-seated fractured regions are also confirmed. In terms of the multiple time-series which consist of separate sequential events at the respective fractured regions, the space-time distribution of earthquake events near the Yamasaki fault can be described explicitly. Adjacent regions within about 30km have nearly common active periods and successive migration of activity occurs around the regions. The life time of activity for each fractured region ranges from less than several months, a year or so, to more than several years. Magnitude distributions for the respective fractured regions also show a regional difference between around the central part of the Yamasaki fault and around the eastern end of the fault

Microearthquake Waveforms Recorded at Tottori Microearthquake Observatory and Their Relation to Hypocentral Distributions and the Upper-Crustal Structure

The microearthquake waveforms recorded at the Tottori Microearthquake Observatory (TTT) during the period from Aug., 1971 to Dec., 1975 are investigated in relation to source regions, focal depths and the upper-crustal structure. Representative waveforms which originated in respective regions in and around Tottori city, western Honshu, Japan are presented in this paper. By using P and S-P times from TTT and its satellite stations we obtained locations of hypocenters, V_p/V_s values and P velocities of the crust. Some prominent phases which appear on the seismograms are interpreted as reflected waves such as SxS and SxP, which can be estimated to be reflected at the Conrad discontinuity. Waveforms of shallow events are considerably complicated. The phases which appear between P and S waves on the seismograms of shallow events are interpreted as SP waves and channel waves of SP type propagated through the superficial layer. Travel times of P waves and SxS waves reveal that the crustal structure beneath the Tottori area is slightly different from the model derived by the Kurayoshi and Hanabusa explosions

On the Use of Analog Filters in Evaluating Amplitude Spectra of Seismic Waves

To investigate the frequency characteristic of a particular part of a seismic-wave train, such as P, S and other prominent waves, it may be best to observe the filtered waves using filters of various frequency-bands. This paper presents a method to obtain a smoothed amplitude spectrum by means of analog-filters. The procedure is quite simple and it is easy to treat a number of data by this method. We take a series of band-pass filters, their transfer functions B_n(ω) n=1, 2, …and the peak frequencies ω_0, n n=1, 2, …being defined as follow : B_n(ω)=B_1(ω/n), ω_0, n=n・ω_0, 1 Then the spectral density at ω_0, n is approximately given by |F(ω_0, n)|≒a_n/n・a, where a_n and a are the maximum trace-amplitudes of the wave filtered by the n-th filter and of the impulse response of the 1-st filter, respectively. Comparison of the measured spectrum with the theoretical one for a rectangular wave shows good agreement. This method is applied to several examples of seismic waves from microearthquakes and found to be effective for distinguishing between various parts of the wave train in the frequency characteristic

Radon-gas Monitoring by Gamma-ray Measurements on the Ground for Detecting Crustal Activity Changes : Preliminary Study by Repeat Survey Method

Concentrated stresses due to deformation of the crust may generate highly compressed interstitial fluids at depths of more than several kilometers. Such fluids tend to migrate upwards through a crack system in the crust under high pressures. A mass of radon gas is considered to be discharged into the air from underground, because radon is generated from abundant radioactive uranium and radium in the crust. Its concentration in the air is increased by the compression of fluids within the crust. Therefore, the concentration of discharged radon gas in the air is one of the important indicators of crustal activity. As the emergence of radon may be random in space and time, we have to set up a dense network of observation stations. Before constructing such a net-work system, preliminary surveys were attempted. The presence of radon is confirmed by gamma rays released from 214Bi, an intermediate decay product of radon. Using a RE-100 scintillation counter, a product of Ohyo Koken Kogyo Co., Ltd., which enables data to be recorded continuously from two bands in the gamma-ray spectrum, we took measurements close to the ground surface to monitor underground radon emissions into the air. During the last 10 years, repeated continuous observations at a fixed station and on a moving automobile or train along fixed routes positively showed the validity of the method adopted in the study presented in this paper. Repeated ob-servations at a fixed point showed a long-term increasing trend of radon concentration in Inagawa Town, Hyogo Prefecture. Mobile measurements taken aboard a Shinkansen bullet train traveling from Kyoto to Tokyo disclosed burst-like emissions of radon at such sites as near Kyoto, as well as regional variations in the intensity of radon emissions