The Chi-Chi earthquake(� = 7.3 and 12 km focal depth) occurred on 21 September 1999 in central Taiwan resulting in deaths totaling over 2300. After the earthquake we collected commercialized bottled water (namely Chingjing water), pumped from wells at Pnli, Nanton County, central Taiwan, about 10 km northeast of the epicenter of this earthquake. The anions, e.g., Cl, SO , '" and NO , •, of the bottled water prepared from 1 De cember 1998 until after the shocks for duration of about two and a half years were analyzed. The analytical data display that the concentrations of both sulfate and nitrate have increased steadily after March 1999, as compared to the con stant level measured since December 1998. They reached an excess of 129.9 and 94.7% in April, then dropped in July 1999, and decreased to about 10 and 20% below the constant values, respectively, till after the earthquake. These precursory chemical changes may be attributed to the stress/strain induced pressure changes in the subsurface water systems, followed by lim ited precursory geochemical discharges generated by limited changes in the levels of the subsurface reservoirs, thus finally leading to the mixing of deeper aquifer. Then, the groundwater was diluted by the injection of sur face water into the subsurface system. This result, therefore, confirms that groundwater chemistry is promising for monitoring earthquakes.

The silica concentrations from 130 hot spring measurements were calculated and used to indicate the heat flow distribution in the active Taiwan orgenic belt. The heat flow spatial distribution seems to be controlled by the Taiwan arc-continent collision zone tectonic development. Two abnormally high silica heat flow regions, 160 and 190 mW m -2 , are identified in northeastern and southeastern Taiwan, respectively. In northeastern Taiwan, the Chingshui area, the data suggests that the anomalous heat flow distribution may be controlled mainly by hot fluids and faults. The highest heat flow in southeastern Taiwan may be generated by heat advection caused by a rapidly uplifting hot crust with a decreasing mature arc-continent collision zone exhumation rate. In addition, we estimated the exhumation rate in different tectonic zones in Southern Taiwan by applying fission track ages and silica heat flow values. The exhumation rates are 1.33, 1.72 -3.87, and 0.51 -1.74 mm yr -1 in the tectonic zones around Taiwan for advanced, initial arc-continent collision and accretionary wedge deformation, respectively.

Hydrothermal experiments on an andesite have been carried out under the condition of 110 °C, autogeneous pressure, 0.05 M H2SO4 solution and renewal of acid solution every 6 hours. The experimental results indicate that the plagioclase and pyroxenes in the treated samples show micropits and microfractures. Small crystals readily suffered from alteration, as compared with large ones, and the susceptibility of the minerals to the acid solution is decreased in the order of plagioclase, augite, hypersthene and opaque minerals. Plagioclase, which is the most reactive mineral in the experiments, shows an increase of SiO2, but a decrease of Al2O3, CaO and Na2O as the experiments proceeded. The chemical change of the treated andesite, it indicates that the relative amounts of SiO2, TiO2, MgO, ΣFeO, MnO and K2O increase, while those of Al2O3, CaO, Na2O and P2O5 decrease with an increase of the experimental duration. The relative mobility of chemical elements listed in decreasing order is P, Ca, Al, Na, Si, K, Mg, Fe, Mn and Ti in terms of the K value. It is no wonder that Ca, Al and Na are more mobile than others because these elements are readily released into solution from the more reactive plagioclase, while Mg, Fe etc. still stay in the less reactive pyroxenes and opaque minerals.

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