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Postseismic decrease in groundwater temperature was documented on the upper rim of a large alluvial fan near the epicenter of the 1999 M w 7.5 Chi-Chi earthquake (Taiwan). We use a model of coupled heat transport and groundwater flow, constrained by documented water-level changes, to interpret this change. We show that groundwater temperature is sensitive to earthquake-induced flow and the observed temperature decrease may be explained by increased groundwater discharge due to earthquake-enhanced vertical permeability. The result implies that heat flow near active mountain fronts may be lowered by recurrent earthquakes.

Earthquake-induced increases in streamflow, producing ∼0.7 km 3 of total excess water, were documented after the 1999 (M w = 7.5) Chi-Chi earthquake in central Taiwan. Analysis of stream gauge data and well records suggests that the excess water originated in the mountains. We propose that the extensive high-angle fractures formed during the earthquake allow rapid release of water from mountains and that mountains in tectonically active areas may be repeatedly flushed by meteoric water at time intervals comparable to the recurrence time of large earthquakes.