Seismic activities and groundwater

Hydrologic changes associated with earth-quakes be can be expressed as an induced change in both quality and quantity of ground-water. The interaction between volcanism, tec-tonic activities, and uplifting also resulted in aquifer compartmentalization, discontinuous groundwater flow, lower groundwater storage and complex groundwater flow pattern. High salinity, high fluoride, above average content of trace elements in groundwater observed in the East African rift is the result of volcanism and climate.

Fig. 1. Compressional and shear waves impacted on aquifer’s materials

(Roeloffs, 1998)

1.3.1 Changes in groundwater level

Four types of post-seismic responses may be distinguished (Wang, 2002). In type 1, the groundwater level declined exponentially with time following a co seismic rise. This was the most common response in the study area (Chelungpu earthquake, Simoes et al., 2007, Chelungpu, Taiwan, 1999) and occurred in unconsolidated sediments on the Choshui River fan. In type 2, the groundwater level rose exponentially with time following a co seismic fall as exemplified by Landers earthquake (Fig.

8). In type 3, the groundwater level continued to decline with time following a co seismic fall.

This also occurred in the deformed and frac-tured sedimentary rocks near the rupfrac-tured fault as exemplified by Northridge earthquake.

Finally, in type 4, the groundwater level, fol-lowing a co seismic rise, stayed at the same level or even rose with time before it eventually declined as exemplified by Hector Mine earth-quake (Earth and Planetary Sciences 333 (2001) (Fig. 2).

The Cooperative Agreement with the U.S.

Department of Energy, the Harry Reid Centre for Environmental Studies at the University of Nevada – Las Vegas (HRC), conducts ground water level measurements at selected bore-holes near Yucca Mountain, and collects quar-terly data from a network of 24 wells currently comprising the monitoring network. Most of these wells lie on the eastern flanks and along

crest of Yucca Mountain within Areas 25 and 29 of the Nevada Test Site. The plot of continu-ously monitored groundwater level changes due to the earthquake of Yucca Mountain is shown as Fig. 3, while the quarterly ground water level changes is shown as Fig. 4.

Groundwater Research Group, Osaka City, Island of Awajishima, which is situated very close to the epicentre of Kobe earthquake, Japan which occurred in January 17 1995, has reported repeatedly dramatic decrease of groundwater level in the Kobe Earthquake. It has been reported by Sato et al., 1995; Ground-water Research Group, that groundGround-water level started to drop soon after the Earthquake (Fig. 5). The water table in the central part of the Island before the Earthquake was close to the topographic surface and dropped to more than 70 m within 90 days after the Earthquake (GRG, 1996; Kumai, personal communication).

1.3.2 Rapid increase of discharge along active faults

The second earthquake -induced hydrological change has included change of relative dis-charge as a function of time exemplified by Fig. 5, data after Sato and Takahashi (1996).

Sato and Takahashi reported the volumetric change of discharge at six locations between May 1995 and June 1996 (Fig. 5). The overall discharge in June 1996 was 43% of that in May 1995 (Fig. 5). Although they did not measure the entire volume of discharge over the island, their locations are all situated at the major dis-charge points and also cover the studied area;

thus, it can be assumed that the ratio of their measurements represents the ratio of the whole volume of discharge. Oshima et al.

(1996) measured stream flow of four locations, located about 250 m from the coast, whose catchment areas extend about 500–600 m along the coast. They measured an overall discharge of 1.75 m³/min 290 days after the earthquake.

This value is about 1.3–1.6 times larger than the steady state value (integrated over a 500–600 m distance) obtained from the model which is described later, in the case where one-third of the annual rainfall is assumed to be the recharge rate. The increase of discharge also is significant compared with the annual fluctua-tions. Sato and Takahashi (1995) reported that Fig. 2. Earthquakes

induced-groundwater level changes (Earth and Planetary Sciences 333, 2001)

the water level of one of the ponds, which is located where Oshima et al. (1996) measured discharge, increased very rapidly just after the earthquake even though no water existed there

before the earthquake. Considering that the dam was artificially broken within a day after the Earthquake to prevent it from overflowing, the increase of discharge must have been sig-nificantly larger than the annual fluctuations.

1.3.3 Change of chemistry of discharged water Earthquakes have imposed drastic changes in the chemistry of groundwater as a post seismic change in groundwater quality. In Awajishima Island, the change in chemical composition was observed after the earthquake of August 1994 (Hake and Nishimura, 1994), the earth-quake of January-February 1995 (Sato et al., 1995), earthquake of March 1995 (Takamura, Kono, 1996) and the earthquake of October 1996 in a well of 1,205 m depth. The chemical composition of discharged water resulted in an increase in bicarbonate after the earthquake (Fig. 6). The change of chemical composition can be explained either by the mixing of dis-charged water at steady state and the deeper Fig. 5. Change of relative discharge

(normal-ized by that in May 1995) as a function of time. Small dots and dotted lines indicate each measurement and large dots and a line

indicate the overall change between May 1995 and June 1996. Data after Sato and

Takahashi (1996).

Fig. 3. Plot of continuously monitored groundwater level changes showing effects of earthquake Yucca Mountain (Sato et al., 1995)

Fig. 4. Plot of quarterly groundwater level changes (Sato et al., 1995)

water represented by the 1,205 m well; or by the discharge of water that had been side-tracked into dead-end pores and/or slow path-ways through relatively less permeable rocks due to the possible earthquake-induced micro-fracturing, or some other reasons. A remark-able change in the concentration of radon in the groundwater was observed after the 1995 Kobe earthquake (Fig. 7).