Hydrochemical changes in spring waters in Taiwan: Implications for evaluating sites for earthquake precursory monitoring

To evaluate potential monitoring sites as well as useful ions which are capable of serving as earthquake precursors, ten subsurface water bodies in different tectonic domains in southwestern, northern and northeastern Taiwan were selected. They included the deep circulation of hot springs, shallower artesian springs and groundwater. Most of the hot springs clearly show chemical anomalies which correlate with earthquake events during the monitoring periods. Against this, the groundwater does not correspond to any events. Hot springs from deeper reservoirs are superior to artesian springs and groundwater from shallower reservoirs. The artesian spring from the smaller subsurface water body is superior to the groundwater from larger reservoirs. Aside from this, anions, especially chloride, outperform cations as geochemical precursors for earthquake monitoring. It is unambiguous that the major factors that determine the usefulness of chemical anomalies in the waters for earthquake precursory monitoring are the kinds, the depths and the size of reservoirs and the ion species of the subsurface water bodies.