Abstract
The swarm earthquake occurred off the east coast of the Izu peninsula for about three months from June 25, 1980. Coseismic changes of groundwater level were examined at the 80 m-deep-observation well in Atami, Shizuoka Prefecture, located within 15 to 30km. northwest from epicenters. No mechanical error of groundwater monitoring system wass found throughh those observations.
The observation well is situated on the active Baien fault (Hoshino et al., 1978). with a height of 96m above sea level (Fig. 1). Strainers are installed from 15, to 30m and 60 to 70m deepp judging from temperature logging (Fig. 2) and other data, but the well deeper than 60.73m is buried with. debris.
This well had been a source of water supply to Atami City only in summer, but the pumping was stopped after 1974 or 1975. The authors installed an automatic water gauge in October, , 1979.
Table 1 shows the changes of groundwater levels caused by the Swarm earth quake from June 26 to June 30, 1980. There were 259 earthquakes swarm in magnitude (by Japan Meteorological Agency) over 2.5 during this period. Among them, 68 times of groundwater level changes were observed. Especially, the groundwater levels were fluctuated at all times when the earthquakes in magnitude over 4.0 were occurred. However, downward change was much larger than upward one. In general, an amount of groundwater level change by earthquake increased with magnitude of earthquake.
Figure 3 shows fluctuations of groundwater levels at the observation well every 30 minutes, of hourly precipitation, and of barometric pressures every 6 hours observed at the AJ'iro weather station from 21:00 hours on June 25 to 24:00 hours on June 30, 1980. During this period, earthquakes in magnitude over 4, 0 occurred nine times as shown in Table 2. Earthquakes A, B, C, and D were relatively large ones, among which C was the main shock. It took 14 months that groundwater level got back to the former situation before the main shock. The decrease in groundwater level by earthquake C occurred not in a moment but exponentially for 40 minutes as shown in Fig. 4.
The net difference of groundwater level (ΔH in mm) between before and after each earthquake is related with its energy (E in joule) as shown in Fig. 5. The equation between two is as follows;
ΔH=5.75×10-9E0.75 (1)
Groundwater level is greatly affected by an amount of the precipitation. But no remarka ble effect of barometric pressure on it can be seen in Fig. 7. The relationship between precipitation. and net. increase of groundwater level by precipitation is not. identical, but it is nearly linear on a log-log scale when they total precipitation exceeds over 10mm (Fig. 8). This relationship it can be seen, whether an earthquake occurred or nct. A scattering of circles in Fig. 8 such as A7, A8, . A10, B3, and. B7: mainly depends on each antecedent condition.
The recession segment of groundwater hydrograph after the peak can be expressed by the following equation (2).
H=H0e-α(t-t0)+b (2)
Replacing e-α with K, equation (2) can be rewritten by equation (3).
H=H0 K(t-t0)+b (3)
where H: groundwater level at the time t on a recession segment of hydrograph, H0: that at the peak (t=t0), K: recession constant, t-t0: passed time after the peak, b: constant. Table 3 shows recession constant calculated from equation (3), after the peak of hydrogragh before and after earthquake C. The mean value before and after earthquake C is 0.97. A difference can be hardly recognized between them. On the other hand, the decreases of groundwater level after earthquake C are 0.81 for the first 5 minutes and 0.88 after that.
