Abstract
Changes in streamflow have been observed at 23 stream gauges in central Taiwan after the 1999 MW 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.
Highlights
Hydrological changes associated with earthquakes have been observed at many places in the world
We presented the streamflow changes in the vicinity of the seismogenic fault before and after the Chi–Chi earthquake
The proximity of gauge CS64 to the epicenter suggests that the streamflow decrease is likely induced by the crustal extension in the hanging wall during the rupture of the thrust fault
Summary
Hydrological changes associated with earthquakes have been observed at many places in the world. Examination of available data recorded at monitoring wells in the footwall of the Chelungpu fault revealed widespread co-seismic groundwater-level changes (Wang et al 2001). Widespread sustained co-seismic groundwater-level changes were observed in monitoring wells in the footwall (Chia et al 2001, 2008a; Wang et al 2001). Groundwater-level falls were recorded in wells near the ruptured fault, while rises were recorded in most wells farther away to the west in the coastal plain consisting of unconsolidated deposits (Chia et al 2001). The distribution of the sustained changes in the footwall revealed that crustal extension dominated near the Chelungpu fault during the earthquake, while compression prevailed away from the fault
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