Seismicity in the Tsengwen reservoir area, Taiwan

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Abstract
The Tsengwen reservoir, with a maximum depth of 128 m, and a storage volume of 708 × 106 m3, is located over an active thrust fault, the Chuko Fault. The Chuko Fault was evidently the causative fault of a magnitude 634 (Pasadena, MS earthquake in 1964. Filling of the reservoir started in April 1973 and water reached the designed level in September of that same year; since then, the water level has undergone yearly cycles with 40 to 50 m amplitude. An earthquake swarm occurred near the dam in December 1972, before the filling of the reservoir, and microearthquakes in the area have been monitored for various periods since that time. Before filling, there were some very shallow events, with depths less than 2.5 km, but these disappeared shortly after the water level rose to the maximum. The majority of hypocenters after reservoir loading lie in a layer between depths of 2.5 and 8 km; the seismicity under the reservoir is noticeably lower than that in the surrounding area. There is no obvious correlation of seismicity with water level, based on available data. The seismicity in the Tsengwen area can be described as a response of the over-pressured and fractured sedimentary strata to the tectonic stress accumulation, modified by the loading effects of the reservoir. A ts/tp versus time study revealed anomalies, but these are not precursory to large earthquakes, as the duration of the anomalies would imply.