Streamflow Changes in the Vicinity of Seismogenic Fault After the 1999 Chi–Chi Earthquake

Changes in streamflow have been observed at 23 stream gauges in central Taiwan after the 1999 M_W 7.6 Chi–Chi earthquake. Post-earthquake increases, ranging from 58 to 833% in discharge, were recorded at 22 gauges on four rivers and their tributaries. The streamflow increase typically peaked in 2–3 days and followed by a slow decay for a month or more. An increased groundwater discharge to the river after the earthquake can be attributed to rock fracturing by seismic shaking as well as pore pressure rise due to compressive strain. A large decrease in discharge was recorded immediately after the earthquake at the gauge near the earthquake epicenter. Further analysis of long-term data indicates that the post-earthquake discharge at the gauge reduced to a level smaller than that at an upstream gauge for 8 months. Such a streamflow decrease might have been caused by a discharge to the streambed due to a co-seismic decrease in pore pressure induced by crustal extension during the rupture of the thrust fault.