Temporal variation of seepage water chemistry before and after the Hengchun Ms 7.2 earthquake in south Taiwan

A destructive magnitude (M) 7.2 earthquake (21.9° N, 120.6° E) occurred in Hengchun of Taiwan at 8:26 pm on 26 December 2006. Seepage water chemistry was monitored twice a month from four routinely monitored points in the Kengting Uplifted Coral Reef Nature Reserve (KUCRNR) around Hengchun from January 2002 to December 2007. The different variants were presented for pH, conductivity, Na, K, Ca, Mg, NH4, F, Cl, NO3, SO4, and HCO3. It was observed that the occurrence of the Hengchun Ms 7.2 earthquake caused both temporary and permanent modification in the water chemistry of the seepage water studied. The significant increase of Cl concentration in seepage water were found 2 years prior to the earthquake, and remained steady until the present. Cl-rich waters are therefore expected to have been introduced within the KUCRNR water systems. Ca showed nearly the same trend as HCO3 with high peak concentrations before the Hengchun Ms 7.2 earthquake, and then decreased with time after the earthquake. The high Ca and HCO3 contents may be due to intense interaction processes between the thermal waters and the carbonate reservoir. The Ca and HCO3 species would be added to the water during flow through the rocks. The effect of limestone may explain this result, because the sedimentary rock in this area majorly includes limestone.