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Hydrogeochemical Anomalies in the Springs of the Chiayi Area in West-central Taiwan as Possible Precursors to Earthquakes

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Hydrogeochemical Anomalies in the Springs of the Chiayi Area in

West-central Taiwan as Possible Precursors to Earthquakes

S. R. SONG,1W. Y. KU,1Y. L. CHEN,1,2C. M. LIU,1H. F. CHEN,1P. S. CHAN,1

Y. G. CHEN,1T. F. YANG,1C. H. CHEN,1T. K. LIU,1and M. LEE3

Abstract—Water samples from both hot and artesian springs in Kuantzeling in west-central Taiwan have been collected on a regular basis from July 15, 1999 to the end of August 2001 to measure cation and anion concentrations as a tool to detect major earthquake precursors. The data identify chloride and sulfate ion anomalies few days prior to major quakes and lasting a few days afterward. These anomalies are characterized by increases in Cl)concentrations from 34.9% to 41.2% and 71.5% to 138.1% as well as increases in SO4

2)concentrations from 232.7% to 276.8% and 100.0% to 155.1% above the means in both

hot and artesian springs. The occurrence of these anomalies is probably explained first as stress/strain-induced pressure changes in the subsurface water systems which then generate precursory limited geochemical discharges at the levels of subsurface reservoirs. Therefore, finally leading to the mixing of previously separated subsurface water bodies occurs. This suggests that the hot and artesian springs in the Kuantzeling area are possible ideal sites for recording strain changes serving well as earthquake precursors. Key words: Chloride ion, sulfate ion, hot and artesian springs, anomaly, earthquake precursor, Taiwan.

1. Introduction

On account of highly active seismicity and a major destructive earthquake of magnitude ML = 7.3 which occurred in a densely-populated area in Taiwan on 21

September, 1999, a large-scale research program to monitor active faults and identify earthquake precursors was jointly initiated by the Central Geological Survey, MOEA-ROC and the Institute of Geosciences, National Taiwan Univer-sity. In one subprogram, weekly measurements of cation and anion concentrations are made in both hot and artesian springs in Taiwan to establish background concentrations and to identify geochemical earthquake-related anomalies. The

1

Institute of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei 106, Taiwan. E-mail: srsong@ntu.edu.tw

2Institute of Applied Geosciences, National Taiwan Ocean University. 3Central Geological Survey, MOEA.

Pure appl. geophys. 163 (2006) 675–691 0033–4553/06/040675–17

DOI 10.1007/s00024-006-0046-x

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purpose of this subprogram is to evaluate potential sites at which regular monitoring systems should be set up in the future.

The destructive Chi-Chi earthquake with magnitude ML= 7.3 occurred in

west-central Taiwan, causing a total of about 80–90 km in length of surface ruptures along the Chelungpu fault, with the largest measured vertical offsets reaching as far as 5–8 m (CHEN et al., 2001). The epicenter of the earthquake was located about 15 km east of the surface trace of the thrust fault at 120.82°E and 23.85°N and had a hypocenter depth of about 12 km (CHUNGand SHIN, 1999; MA et al., 1999; KAOand CHEN, 2000), near the town of Chi-Chi in Nantou County in west-central Taiwan (Fig. 1). This earthquake became one of the largest inland events in the past century, causing the death of about 2,400, injuring another 10,000 and destroying more than 100,000 buildings. Numerous aftershocks, including one event of ML = 6.8, were distributed around the main shock in a

large area of central Taiwan (KAO and CHEN, 2000). Following the Chi-Chi earthquake, another large quake with magnitude ML= 6.4 struck the Chiayi area

on October 22, 1999 in west-central Taiwan. The epicenter, 2.5 km northwest of Chiayi City was located at 120.40°E and 23.51°N and had a hypocenter depth of about 12.1 km (Fig. 1) (CWB, 1999).

One important goal of geoscientists has long been the detection of valuable short-term precursors of earthquakes, and, indeed, many types of precursors, including chemical and hydrological changes in subsurface fluids prior to large earthquakes. Among these, gases involved in hydrothermal processes (Rn, He, CO2, CH4, H2, Ar and N2) and water chemistry (Cl), F), NO3)and SO42)) are the

most unambiguous precursors (HAUKSSON, 1981; KING, 1986; SUGISAKI et al., 1996; TSUNOGAI and WAKITA, 1995, 1996; TOUTAIN et al., 1997; SONG et al., 2005). These geochemical and hydrologic anomalies are generally related to changes in groundwater circulating systems because of earthquake generation (THOMAS, 1988; SUGISAKI et al., 1996; KING et al., 1999). Thus, geochemical anomalies observed in groundwater have provided useful information for earthquake prediction in seismic countries (e.g., KOIZUMI et al., 1985; BARSUKOV et al., 1984/1985; GUIRU et al., 1984/1985). Meanwhile, preceding the major 1995 Kobe earthquake (ML = 7.2) and the 1996 Pyrenean earthquake (ML= 5.2)

(TSUNOGAI and WAKITA, 1995, 1996; TOUTAIN et al., 1997), anomalies of ions in commercialized bottled groundwater and spring water, respectively, have been recently detected.

This paper contributes to this field by presenting the results of a two-year study investigating hydrochemical changes in hot springs by collecting water samples from both hot and artesian springs in response to earthquakes in the Chiayi area of west-central Taiwan. Furthermore, the possible mechanisms inducing the chemical changes in the respective subsurface water systems are discussed.

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Figure 1

Regional structural sketch map of west-central Taiwan with the locations of the September 21 and October 22, 1999 earthquake epicenters and focal mechanisms. The legend in the lower right-hand corner indicates: 1: Pre-Tertiary basement; 2: Early Pleistocene tectonic belt; 3: Late Pleistocene tectonic belt; and 4: Escape blocks. The legend in the middle on the left-hand side indicates: A: Normal fault; B: Thrust fault; and C: Strike-slip fault. Shown on the map are: CCF: Chaochou-Chuchih fault; CF: Chukou fault; CHF: Chelungpu fault; LHF: Linnei-Hsinchu fault; MF: Meishan fault; and STF: Shihtan-Tuntzechiao fault (modified from BIQ, 1990; YANGet al., 1994). Inset map shows the tectonics in the vicinity of Taiwan

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2. Sites and Geological Background

Taiwan is located within the complexity of the oblique collision zone of the Eurasian continental plate and the Philippine Sea plate (Fig. 1). Presently, the Philippine Sea plate is moving WNW at about 70 mm per year (SENO and MARUYAMA, 1984), and it is believed the mountain-building process is still in progress (TSAI et al., 1981; YU and CHEN, 1994; YU et al., 1999). A dominant collision zone frequently inducing folding and fault thrusting, i.e., the Chelungpu thrust fault, may exist in west-central Taiwan. At the latitude of southern Taiwan, the Philippine Sea plate is riding up over the continental shelf of the South China Sea. Such active movements over the last 5 million years have been creating the island of Taiwan (HO, 1986; TENG, 1987, 1990), and more recently, rapid crustal movements and widely distributed active structures have induced at least tens of large earthquakes with magnitudes over 7.0 in the last few hundred years (YUet al., 1997, 1999; CHANGet al., 1998; CHENGet al., 1999).

The Chiayi area is located in west-central Taiwan, and its pre-Tertiary basement high, called the Peikang High, is below. This Peikang High is an indentation block controlling the structures and seismic activities around the Chiayi area during the orogeny of Taiwan (Fig. 1) (LU, 1994; LUand MALAVIEILLE, 1994). The curvilinear active Chukou fault thrusts westward onto the Peikang High, with its northward extension connecting the Chelungpu thrust fault (Fig. 1) (BIQ, 1990; LU, 1994). South of the Peikang High there is a large transtension zone (BIQ, 1990; YANGet al., 1994), while in the middle, an active strike-slip fault, the Meishan fault, cuts through on the southern edge of the Peikang High (BIQ, 1990; LIN et al., 2000). Thus, earthquake epicenters in this area, one of the most highly active seismic areas of western Taiwan, distribute in a semicircular formation around the High (SHINand CHANG, 1992). The epicenters of the September 21, 1999 Chi-Chi earthquake and the October 22, 1999 Chiayi earthquake are located about 35 km to the north and about 2.5 km to the northwest, respectively (Fig. 1).

The village of Kuantzeling is located in the southeastern part of the Chiayi area and is well-known for its hot spring spas. It is in the western foothills of Taiwan, where a passive margin shallow marine clastic sequence of the late Tertiary age crops out. Fossilferrous, fine-grained and little metamorphosed, the strata have been deformed by folding and faulting. The outpouring of hot spring waters is located on the axis of the Kuantzeling anticline, and a thrust fault, the Liuchungchi fault, cuts through it (HSU and WEY, 1983). The local geological structure, heat flow, silica geothermometry and Tritium data indicate that the hot springs may have come from a deep old water reservoir, over 2 km in depth rising along the fault fracture zone (CHENet al., 2001). The temperature and pH value of the Kuantzeling hot springs are 79°C and 8.1, respectively. According to historical records, the perturbations of the hot spring system, i.e., the bursts of steam and increased outpouring, occurred a few days prior to, during and a few days after the 1964 Paiho earthquake, one of the

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most devastating earthquakes on Taiwan in the last hundred years, with magnitude 6.3 (CHENGet al., 1999) and its epicenter near the Kuantzeling area. Here, therefore, our attention was focused on the hot and artesian springs in this area, and the cations and anions in the water samples were regularly analyzed.

3. Sampling and Analysis

From July 15 to September 19, 1999 and for a period of about two months before the Chi-Chi earthquake, students from a local high school for a national science competition collected 9 samples from the Kuantzeling hot springs at different time intervals. Twenty days after the earthquake, the present researchers joined in and intensely collected one hot spring sample per day during two months and then decreased the sampling interval to one sample every three days and subsequently to one a week until the end of July 2001. Thus, a total of over 200 samples of hot spring water were collected for analyses. Meanwhile, one sample was also collected from the beginning of January 2000 to December 2000 every three days from an artesian spring located about 1 km southwest of the Kuantzeling hot springs. Later the sampling intervals decreased to one per week until the end of August 2001 for a total of about 170 samples of artesian spring water for analyses.

Dissolved anions and cations in both sets of samples were measured with an ion chromatographer (IC, Type Dionex DX-100) and an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Type Jobin-Yvon JY-38plus), respectively. A sample from the same spring was measured after each sample analysis in order to enhance the precision of the measurements. Analytical uncertainties in the absolute concentrations were less than 3% for all of the anions and less than 5% for all of the cations. This study also analyzed the oxygen and hydrogen isotopes of the Kuantzeling hot springs from September 1999 to September 2000 using a Finnigan Delta Plus-Mass Spectrometer with precisions of about 0:1& for the oxygen isotopes and 1& for the hydrogen isotopes.

4. Results and Discussions 1. Temporal Variations in the Chemical Compositions

The temporal variations in the Cl)and SO24 concentrations of the water samples from the Kuantzeling hot springs from July 1999 to July 2001 are shown in Figs 2A and 2B, respectively. Chloride ion is the major anion in the hot springs and its average concentration reaches 2201 ppm, whereas that of the sulfate ion is about 33.6 ppm. Generally, the concentrations of chloride of in samples are almost constant, except on a few dates, but this is unlike those of sulfates, which are more

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fluctuant in two periods, i.e., from March 2000 to June 2000 and from December 2000 to February 2001. Two-sigma relative standard deviations (2r) were calculated for those samples and are 12.0% (Cl)) and 36.9% (SO2

4 ). We have considered the 2r

domains as representative of spring water background values, which may have Figure 2

Temporal variations in (A) Cl)and (B) SO42)concentrations in the Kuantzeling hot springs. The average

concentration (solid lines) and 2r variation range (dashed lines) are also shown. The vertical lines represent the earthquakes with magnitudes and intensities greater than 4 that occurred in this area. (C) Daily amounts

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resulted from water-rock interactions in the deep circulations of the hot spring reservoirs, sampling heterogeneity and analytical uncertainties. Except for the chloride and sulfate ions, all of the cations and anions vary within the 2r domains during the entire sampling period. Figure 2A shows that the Cl) concentrations increased abruptly on September 19, October 1, October 31, 1999 and November 1, 2000, reaching their maximum values of 2970 ppm, 2988 ppm, 3107 ppm and 2987 ppm, which are, respectively, about 34.9%, 35.8%, 41.2% and 35.7% above the mean value. It is important to note that these variations are very sudden. The sulfate contents during the same period seem to show no variations, but they do show high fluctuations from March to June 2000 and December 2000 to February 2001, when the respective concentrations reached their peaks at 126.6 ppm and 111.8 ppm, or about 276.8% and 232.7% above the mean. Figures 3A and 3B show the temporal variations in the hydrogen and oxygen isotopic ratios of the water samples from the Kuantzeling hot springs from September 1999 to September 2000, respectively. The d18O and dD ratios of all samples remain fairly constant during the sampling period, firmly indicating that the hot spring waters have come from a stable homogeneous subsurface water body.

The temporal variations in the Cl)and SO24 concentrations of the water samples from the Kuantzeling artesian spring from January 2000 to August 2001 are shown in Figures 4A and 4B, respectively. The sulfate ion with an average concentration of about 25.4 ppm is the major anion in the spring waters, if we compare with an average of about 2.70 ppm for the chloride ion. The chloride and sulfate concentrations of all samples are fairly constant during the sampling period. Two-sigma relative standard deviations (2r) were calculated for those samples and are 30.4% (Cl)) and 17.9% (SO42)). These 2r domains can be assumed as representative of the spring water

background values. They may be attributed to annual fluctuations in groundwater chemistry, which are themselves mainly as a result of rainfall and other superficial phenomena, such as heterogeneity in the sampling and analytical uncertainties (TOUTAINet al., 1997). Except for the chloride and sulfate ions, all cations and anions vary within the 2r domains during the entire sampling period. Figure 4A shows that the Cl) concentrations increase sharply on April 12, June 13 and July 16, 2000 reaching their maximum values (6.43 ppm), (5.55 ppm) and (4.63 ppm), which are, respectively, about 138.1%, 105.6% and 71.5% above the mean value. Again, of significance is these variations are very abrupt. Except for July 15, 2000 when little change is found, the variations in the sulfate content (Fig. 4B) are the same as those for chloride, with concentrations reaching their maximum values of 50.8 ppm and 64.8 ppm, which are, respectively, about 100.0% and 155.1% above the mean value.

2. Mechanism for the Chemical Changes

Changes in the chemical compositions of hot and artesian springs have previously been attributed to several factors. Different compositions of groundwater recharge,

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petrologic and mineralogical compositions of subsurface rocks, water-rock interac-tions (DOMENICOand SCHWARTZ, 1990; LANGMUIR, 1997), mixing of different water compositions and artificial pollutants, etc. To evaluate which mechanisms are responsible for the observed chemical changes, two facts must be kept in mind. Firstly, chloride ion is considered chemically stable, and the concentration level is high enough to measure reliably. The second, in contrast to the chloride ion, sulfate is not so stable in groundwater conditions and can be affected by sulfide mineral oxidation, precipitation-dissolution of gypsum in an unsaturated zone, dissolution of anhydrite or gypsum or redox reactions in a saturated zone (DOMENICO and SCHWARTZ, 1990). Such reactions, however, are not quick enough to cause such abrupt changes in SO42)concentrations in a stable subsurface water system. Among

all those factors that are capable of causing the observed temporal variations in both Cl)and SO42)concentrations in a short duration (Figs. 2 and 4), mixing of different

water compositions (KINGet al., 1981; THOMAS, 1988) and artificial pollutants are the only two factors that cannot be ruled out.

Figure 3

Temporal variations in (A) dD and (B) d18O in the Kuantzeling hot springs. Heavy bars are daily amounts

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Kuantzeling is located in an industry-free, sparsely populated mountainous area. Accordingly, pollutant solutes require media, i.e., meteoric water, in order to be transported down into the groundwater system. However, no correlated relationships among the anomalies of the ions in the temporal variations and daily amounts of

Figure 4 Temporal variations in (A) Cl)and (B) SO2

4 concentrations in the Kuantzeling artesian springs. The

average concentration (solid lines) and 2r variation range (dashed lines) are also shown. The vertical lines represent the earthquakes with magnitudes and intensities greater than 4 occurred in this area. (C) Daily amounts of precipitation obtained at the Kuantzeling area (Data from Central Weather Bureau of

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precipitation are found during the sampling period (Figs. 2 and 4), highly suggesting that the observed temporal variations in the Cl)and SO24 concentrations are not, in fact, induced by recent meteoric water flowing down into the circulation system of the subsurface water system. Meanwhile, the analysis results of Tritium (3H) concentration in the hot springs are less than 0.2 (TU < 0.2) (CHENet al., 2004), and almost no variations in the oxygen and hydrogen isotopic ratios are found (Fig. 3). This again strongly supports the notion that no recent meteoric water flows down into the circulation system of the hot spring waters. Given these lines of evidence, the abrupt change in the temporal variations of Cl)and SO42)concentrations cannot be

attributed to artifact pollutants. It follows then that the most probable factor controlling the temporal chemical variations in the hot and artesian springs of the Chiayi area is the mixing of water bodies with different compositions.

Several mechanisms can bring about the mixing of different water compositions in a subsurface water body, and these include the mixing of meteoric and formation waters, groundwater and brines or pore waters, and the mixing of different aquifers or reservoirs with different chemical compositions (DOMENICO and SCHWARTZ, 1990). Among these, the formers may change chemical compositions gradually and eventually lead to complete changes or at least changes that last a long period, while the mixing of different aquifers or reservoirs with different chemical compositions can quickly occur and the chemical changes can disappear in a short duration. What is particularly salient is that Figures 2 and 4 clearly illustrate that such chemical changes are abrupt and that the dates correlate well with the occurrence of earthquakes, near the hot and artesian springs. Thus, it is reasonable to attribute the mechanism of the rapid temporal chemical variations in the hot and artesian springs in the Chiayi area to the mixing of different aquifers or reservoirs. It may be equally justifiable to make the claim that the mechanism of the rapid temporal chemical variations may have been induced by an earthquake. Such an interpretation is strongly supported by the observation of the large 1.0- to 11.1-m changes in the groundwater levels induced by the Chi-Chi earthquake on 21 September 1999, as recorded at 157 out of 179 monitoring wells in the Choshui River alluvial fan, which is located about 10–20 km northwest of the Chiayi area (CHIAet al., 2001).

Two different earthquake-induced mechanisms for the mixing of different water systems of different compositions have been proposed in previous studies. The first involves permeability enhancement due to a breaking in the crust (KINGet al., 1981; THOMAS, 1988; ROJSTACZERand WOLF, 1992; Rojstaczer et al., 1995; KING et al., 1999), while the second encompasses the changing of pressure in aquifer systems because of elastic compression (MUIR-WOODand KING, 1993; TOUTAINet al., 1997), before and after a major earthquake. The former necessitates cracking water barriers, i.e., fault gauge zones or aquicludes and, subsequently, inducing the mixing of initially isolated aquifers. Chemical changes induced by such a mechanism, however, would require large-scale mixing processes (TSUNOGAIand WAKITA, 1996) and would not be compatible with short-term reversible anomalies, such as those observed in the

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Table 1

TheClandSO24 concentrationsðppmÞ of the Kuantzeling hot spring

Date Cl) SO24 Date Cl- SO2 4 Date Cl ) SO2 4 7/15/1999 2093 28.1 11/8/1999 2200 11.8 2/19/2000 2132 46.6 8/8/1999 1983 21.7 11/9/1999 2233 20.5 2/22/2000 2272 72.8 8/18/1999 1875 23.5 11/10/1999 2193 44.5 2/25/2000 2102 45.7 8/20/1999 2123 25.4 11/11/1999 2237 40.3 2/28/2000 2123 31.1 8/27/1999 2178 25.8 11/12/1999 2043 41.0 3/1/2000 2385 80.3 9/3/1999 2219 23.7 11/13/1999 2242 40.5 3/2/2000 2272 78.6 9/12/1999 2153 14.1 11/14/1999 2054 27.5 3/5/2000 2055 63.6 9/14/1999 2215 24.7 11/15/1999 2263 41.0 3/8/2000 2141 48.2 9/19/1999 2970 11.7 11/16/1999 2388 39.9 3/11/2000 2057 59.0 9/26/1999 2268 34.1 11/17/1999 2406 37.7 3/17/2000 1994 40.3 10/1/1999 2988 13.5 11/18/1999 2260 31.9 3/20/2000 2022 38.1 10/11/1999 2284 34.4 11/19/1999 2259 38.0 3/23/2000 2089 84.6 10/12/1999 2306 34.6 11/20/1999 2425 33.1 3/26/2000 2133 38.3 10/13/1999 2287 35.5 11/21/1999 2237 33.2 3/29/2000 2220 41.3 10/14/1999 2285 33.9 11/22/1999 2382 35.6 4/1/2000 2258 83.8 10/15/1999 2136 33.8 11/23/1999 2145 30.7 4/4/2000 2359 126.6 10/16/1999 2234 29.3 11/24/1999 2226 32.5 4/7/2000 2398 119.7 10/17/1999 2184 28.3 11/25/1999 2000 32.9 4/10/2000 2320 38.8 10/18/1999 2236 34.8 11/26/1999 2112 33.7 4/13/2000 2266 38.1 10/19/1999 2211 35.4 11/27/1999 2263 27.5 4/16/2000 1764 88.4 10/20/1999 2593 29.8 1/6/2000 2388 41.0 4/19/2000 2168 67.2 10/21/1999 2635 39.4 1/9/2000 2406 39.9 4/25/2000 2196 44.8 10/22/1999 2646 39.9 1/12/2000 2260 44.5 4/28/2000 2164 36.5 10/24/1999 2399 38.6 1/15/2000 2259 27.4 5/1/2000 2340 61.7 10/25/1999 2627 47.4 1/18/2000 2085 38.0 5/4/2000 1714 65.3 10/26/1999 2489 43.8 1/21/2000 2177 33.1 5/7/2000 2299 55.0 10/27/1999 2585 40.8 1/24/2000 2123 20.3 5/10/2000 2534 48.6 10/28/1999 2361 37.5 1/27/2000 2200 35.6 5/13/2000 2485 17.9 10/29/1999 2497 40.9 1/29/2000 2150 30.7 5/16/2000 2413 43.0 10/30/1999 2553 39.1 2/1/2000 2150 32.5 5/19/2000 2426 20.6 10/31/1999 3107 32.4 2/4/2000 2204 32.9 5/25/2000 2464 36.1 11/4/1999 2617 36.5 2/7/2000 2177 33.7 5/28/2000 2183 33.8 11/5/1999 1851 49.6 2/10/2000 2166 40.6 5/31/2000 2317 37.0 11/6/1999 2623 36.5 2/13/2000 2188 32.5 6/3/2000 2290 34.3 11/7/1999 2558 36.4 2/17/2000 2080 48.1 6/6/2000 2306 21.8 6/9/2000 1886 49.7 10/20/2000 2334 40.7 3/4/2001 2243 30.6 6/12/2000 2299 33.9 10/23/2000 2266 62.4 3/7/2001 2262 57.6 6/15/2000 2306 34.5 10/26/2000 2380 29.3 3/10/2001 2314 41.0 6/18/2000 2313 36.0 10/29/2000 2987 18.0 3/13/2001 2300 24.7 6/20/2000 2311 36.3 11/1/2000 2437 42.9 3/21/2001 2376 36.6 6/21/2000 2312 44.1 11/4/2000 2734 20.7 3/27/2001 2620 35.7 6/24/2000 2321 36.5 11/7/2000 2461 35.7 3/30/2001 2295 54.2 6/30/2000 2329 34.9 11/10/2000 2291 15.9 4/2/2001 2022 36.1 7/6/2000 2345 35.7 11/13/2000 2304 37.4 4/8/2001 2173 42.4 7/9/2000 2339 35.5 11/16/2000 2373 20.7 4/14/2001 2289 39.9 7/12/2000 2361 36.9 11/19/2000 2342 26.2 4/17/2001 2385 30.5 7/18/2000 2366 36.5 11/22/2000 2444 26.2 4/20/2001 2259 38.6 7/21/2000 2386 35.3 11/25/2000 2388 79.3 4/23/2001 2108 16.9

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Kuantzeling hot and artesian springs, where the chloride and sulfate contents returned to pre-seismic levels within a few days after the onset of the anomaly. The latter mechanism, on the other hand, is induced by elastic compression on aquifers, which generates pressure variations among them, enough to generate reversible changes in hydraulic levels (MUIR-WOOD and KING, 1993) and, therefore, lead to subsequent limited geochemical discharge effects (THOMAS, 1988). It is clear that a limited mixing of aquifers with different compositions, not unlike that in the case of the rapid geochemical changes in the Kuantzeling hot and artesian springs (Figs. 2 and 4), is the most likely model for the generation of the drastic ion concentration changes. Aside from this, the mechanism is supported by the significant changes in groundwater levels induced by the Chi-Chi earthquake (CHIA et al., 2001). It is highly expected, therefore, that the chloride- and sulfate-rich spring waters were introduced into the Kuantzeling subsurface water systems. Although it is difficult to precisely identify the water introduced in the subsurface system, a hot spring with high chloride and sulfate concentrations, located nearby the Kuantzeling hot springs (KU, 2001) is a potential source. Table 1 (Contd.) Date Cl) SO24 Date Cl- SO2 4 Date Cl ) SO2 4 7/24/2000 2418 36.9 11/28/2000 2259 53.9 4/26/2001 2171 34.8 8/2/2000 2365 36.2 12/1/2000 2449 39.2 4/29/2001 2189 43.8 8/5/2000 2305 37.5 12/4/2000 2361 73.0 5/2/2001 2363 39.6 8/8/2000 2382 39.1 12/7/2000 2365 28.2 5/9/2001 1997 24.4 8/11/2000 2299 38.7 12/10/2000 2249 59.6 5/18/2001 2216 6.5 8/14/2000 2188 40.3 12/13/2000 2269 33.3 6/2/2001 1786 5.4 8/17/2000 2307 38.3 12/16/2000 2310 14.0 6/11/2001 2001 24.2 8/20/2000 2542 37.9 12/19/2000 2196 54.5 6/17/2001 2006 22.4 8/23/2000 2508 34.9 12/22/2000 2219 32.3 6/23/2001 2140 42.2 8/26/2000 2538 35.6 12/25/2000 2310 59.3 6/29/2001 2034 67.5 8/30/2000 2406 35.2 12/31/2000 2091 74.4 7/2/2001 2256 20.7 9/4/2000 2329 37.2 1/3/2001 2123 54.1 7/11/2001 2367 52.3 9/7/2000 2320 37.7 1/6/2001 2365 58.1 7/17/2001 2372 54.9 9/10/2000 2267 34.4 1/9/2001 2051 51.4 7/23/2001 1974 24.6 9/13/2000 2222 33.3 1/12/2001 2184 50.9 7/29/2001 2491 17.7 9/17/2000 2182 35.5 1/15/2001 2323 59.9 8/1/2001 2522 22.4 9/20/2000 2212 34.4 1/18/2001 2466 73.1 9/26/2000 2231 33.4 1/21/2001 2421 111.8 9/29/2000 2225 33.7 1/27/2001 2518 102.9 10/2/2000 2178 34.9 2/2/2001 2319 55.9 10/5/2000 1904 32.3 2/4/2001 1900 28.0 10/8/2000 2486 31.8 2/5/2001 2331 38.4 10/11/2000 2346 32.4 2/17/2001 2324 63.6 10/14/2000 2460 30.4 2/20/2001 2451 31.5 10/17/2000 2023 40.4 2/26/2001 2297 29.9

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Table 2

TheClandSO24 concentrationsðppmÞ of the Kuantzelingartesian spring

Date Cl) SO24 Date Cl- SO2 4 Date Cl ) SO2 4 1/7/2000 3.17 26.6 4/24/2000 2.96 26.6 7/31/2000 2.57 23.4 1/10/2000 2.58 27.4 4/27/2000 2.97 24.2 8/2/2000 2.20 24.3 1/13/2000 2.60 28.4 4/30/2000 2.66 23.7 8/4/2000 2.09 24.6 1/16/2000 2.71 27.8 5/3/2000 3.14 24.1 8/6/2000 2.08 21.1 1/19/2000 2.40 23.8 5/6/2000 3.27 24.4 8/8/2000 2.20 24.1 1/22/2000 2.48 27.8 5/9/2000 2.99 23.9 8/10/2000 2.07 21.4 1/25/2000 2.19 25.0 5/12/2000 3.15 24.1 8/12/2000 2.03 21.5 1/28/2000 2.71 28.4 5/15/2000 2.91 24.9 8/14/2000 2.17 21.3 1/31/2000 2.72 28.4 5/18/2000 3.23 24.5 8/16/2000 2.21 21.6 2/3/2000 3.11 26.9 5/21/2000 3.23 24.6 8/18/2000 2.48 26.0 2/6/2000 2.94 28.4 5/24/2000 3.15 24.9 8/20/2000 2.50 27.6 2/9/2000 2.79 26.5 5/27/2000 2.91 24.4 8/22/2000 1.96 21.2 2/12/2000 2.78 27.9 5/30/2000 3.52 24.2 8/24/2000 2.11 22.4 2/15/2000 2.14 24.2 6/1/2000 4.21 24.8 8/26/2000 2.11 22.2 2/18/2000 2.54 28.3 6/4/2000 3.30 25.0 8/28/2000 1.98 21.6 2/21/2000 2.20 25.6 6/7/2000 2.87 23.7 8/30/2000 2.08 21.3 2/24/2000 2.84 27.8 6/10/2000 5.24 64.8 9/1/2000 2.20 24.6 2/27/2000 2.44 26.6 6/13/2000 5.55 62.4 9/4/2000 2.33 27.3 3/1/2000 2.60 27.7 6/16/2000 4.29 24.1 9/7/2000 2.28 27.1 3/4/2000 2.28 25.2 6/18/2000 2.91 24.7 9/10/2000 2.54 28.3 3/7/2000 2.43 27.0 6/19/2000 4.12 24.1 9/13/2000 2.44 28.8 3/10/2000 2.57 25.8 6/20/2000 4.09 25.2 9/16/2000 2.33 25.8 3/13/2000 2.11 25.2 6/22/2000 3.48 27.0 9/19/2000 2.51 28.6 3/16/2000 2.46 24.2 6/25/2000 3.27 25.5 9/22/2000 2.61 28.9 3/19/2000 2.98 28.1 6/28/2000 3.53 27.6 9/25/2000 2.78 29.5 3/22/2000 2.98 28.8 7/1/2000 3.59 27.2 9/28/2000 2.29 28.1 3/25/2000 3.05 27.7 7/4/2000 3.27 26.6 9/30/2000 2.38 28.9 3/28/2000 2.95 28.3 7/7/2000 3.35 27.7 10/1/2000 2.48 28.2 3/31/2000 2.44 26.2 7/10/2000 3.48 28.8 10/4/2000 2.45 27.8 4/3/2000 2.20 27.6 7/13/2000 3.38 28.8 10/7/2000 2.27 27.5 4/6/2000 2.93 29.2 7/16/2000 4.63 26.9 10/10/2000 2.39 27.0 4/9/2000 2.90 25.1 7/19/2000 2.62 24.6 10/13/2000 2.26 24.3 4/12/2000 6.14 50.8 7/22/2000 2.95 27.7 10/16/2000 2.32 26.7 4/15/2000 6.43 50.1 7/25/2000 2.90 27.5 10/19/2000 2.50 26.7 4/21/2000 2.96 26.6 7/28/2000 3.18 27.8 10/22/2000 2.59 26.9 10/25/2000 2.71 26.0 2/5/2001 2.95 24.7 7/7/2001 1.63 26.6 10/28/2000 2.53 26.0 2/13/2001 2.96 24.2 7/11/2001 1.60 26.2 10/31/2000 2.63 26.3 2/15/2001 2.80 24.5 7/21/2001 1.53 21.9 11/3/2000 2.49 26.0 2/18/2001 2.90 24.1 8/3/2001 1.87 23.4 11/6/2000 2.92 28.2 3/3/2001 3.23 24.2 8/8/2001 1.65 24.9 11/9/2000 2.73 25.9 3/6/2001 3.33 24.1 8/15/2001 1.34 24.4 11/12/2000 2.86 26.2 3/8/2001 3.04 24.1 8/20/2001 1.89 24.9 11/15/2000 2.62 26.9 3/12/2001 3.17 23.0 8/26/2001 1.50 19.2 11/18/2000 2.64 27.4 3/19/2001 2.88 21.8 8/29/2001 1.16 19.6 11/21/2000 2.58 26.5 3/21/2001 3.16 21.4 11/24/2000 3.08 27.7 3/24/2001 3.26 24.6 11/27/2000 2.79 28.0 3/29/2001 2.90 21.9 11/30/2000 2.95 26.3 4/1/2001 2.90 21.6

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The Cl) and SO42), especially the SO42) variations, which responded to the

earthquakes in the Kuantzeling hot and artesian springs, seem to be so different. The variations in the hot springs were more fluctuant and lasted longer than those in the artesian springs (Figs. 2 and 4). This may have been a result of the different depths of the aquifers or reservoirs. In other words, the shallower the artesian springs are, the faster are the responses from the earthquake-induced stresses.

Although the chemical anomalies that occurred in the Kuantzeling hot and artesian springs can be explained by the earthquakes, there still remain several open questions like why some earthquakes do not cause chemical anomalies in the same subsurface water systems (Figs. 2 and 4). The answer surely must lie in the fact that such anomalies are likely related to the unknown characteristics of some subsurface structures and water systems, the complexities of the processes of earthquake-induced stresses on the crust and aquifers or reservoirs, the wide-ranging chemical compositions of subsurface waters, varying water-rock interactions, and so on. Obviously, more data from geochemical monitoring and further investigations into earthquake precursors are required to enhance our understanding vis-a`-vis the origin of earthquakes in the future.

5. Conclusions

Located in an orogenic belt with highly active seismicity, Taiwan has often been struck by major devastating earthquakes, which have caused huge numbers of

Table 2 (Contd.) Date Cl) SO24 Date Cl- SO2 4 Date Cl ) SO2 4 12/1/2000 2.78 27.8 4/5/2001 2.76 23.1 12/4/2000 2.78 27.5 4/9/2001 2.92 21.1 12/7/2000 3.25 26.9 4/14/2001 3.07 21.0 12/10/2000 2.99 26.8 4/16/2001 2.70 20.7 12/16/2000 2.95 26.0 4/21/2001 2.84 20.9 12/19/2000 2.71 25.0 4/24/2001 2.73 21.3 12/22/2000 2.80 26.1 4/29/2001 2.59 21.5 12/25/2000 3.06 25.9 5/5/2001 2.72 20.8 12/28/2000 2.95 26.3 5/10/2001 1.76 21.1 12/31/2000 2.88 27.1 5/16/2001 2.15 25.6 1/3/2001 3.02 26.3 5/29/2001 2.88 24.5 1/9/2001 3.35 28.0 6/3/2001 1.56 22.0 1/12/2001 3.03 28.6 6/9/2001 2.06 22.3 1/23/2001 3.29 26.0 6/15/2001 2.19 24.1 1/25/2001 2.97 25.8 6/19/2001 2.08 25.1 1/27/2001 3.05 24.4 6/26/2001 1.41 24.1 2/2/2001 2.90 25.0 7/1/2001 1.74 26.1

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fatalities and casualties as well as the destruction of countless buildings. Nonetheless, in spite of this, it has given rise to numerous opportunities to investigate the potentially hydrological geochemical precursors of the earthquakes. Here, short-term, reversible precursory geochemical anomalies have been recorded in hot and artesian springs prior and subsequent to the major earthquakes occurred September 1999 in the Kuantzeling area of west-central Taiwan. The anomalies were sharp sudden increases in chloride and sulfate ions. These are interpreted here as stress/strain-induced pressure changes in the subsurface water system, followed by limited precursory geochemical discharges generated by limited changes in the levels of the subsurface reservoirs, finally leading to the mixing of previously isolated subsurface water bodies. This strongly suggests that both the hot and artesian springs in the Kuantzeling area may be ideal sites for recording strain changes and that therefore, they should serve well in earthquake precursor research.

Acknowledgements

The authors appreciate the assistance of Mr. Yu, W.Y. for the field samplings and partial IC and ICP-AES analyses. This research was supported by the Central Geological Survey, Ministry of Economic Affairs and partly by the National Science Council, Republic of China under grants NSC 89-2116-M-002-046.

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(Received: January 30, 2003, revised: November 28, 2005, accepted: November 30, 2005)

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