EARTHQUAKES AND HEAT TRANSPORT NEAR ACTIVE MOUNTAIN FRONT

Craig Forster was a pioneer in investigating the role of groundwater flow in transporting geothermal heat from mountains to the surrounding lowlands. As a supplement, we show here that such heat transport near an active mountain front may be episodic, responding to recurrent large earthquakes.

Following the 1999 Mw7.5 Chi-Chi earthquake, Taiwan, nearly 0.8 km³ of groundwater was released in the form of increased streamflow (Wang et al., 2004). In addition, a network of hydrological stations on an alluvial fan near the epicenter documented widespread changes in groundwater temperature several months after the earthquake. Most stations showed an increase in temperature, suggesting an increase in post-seismic discharge of groundwater from depth, while stations near the higher rim of the alluvial fan showed a decrease in temperature, suggesting an increase in post-seismic recharge. We use coupled modeling of heat transport by groundwater flow to show that the observed temperature changes may be explained by a change in groundwater flow induced by earthquake-enhanced vertical permeability. Post-seismic draining of groundwater from the foothills of the mountains not only increases streamflow but also recharges the fractured basement aquifers beneath the foothills. In the lowlands, the increased pore pressure in the fractured basement greatly enhances the permeability of faults, allowing rapid upward transport of hot water to raise temperature in the overlying alluvial fan. After the earthquake, permeability recovers to the pre-earthquake value and temperature also returns gradually to the pre-earthquake state. Recurrent large earthquakes repeatedly release groundwater from active mountains and episodically flush geothermal heat to the nearby lowlands.