Did the reservoir impoundment trigger the Wenchuan earthquake?

Abstract

January 16, 2009 issue of Science magazine published, in its “ News of the Week” section, an article entitled “A Human Trigger for the Great Quake of Sichuan?”. The article boldly proposed, “Zipingpu reservoir may have triggered the Wenchuan earthquake”. Incidentally, similar reports had appeared in the media such as Telegraph website in UK and New York Times, six months after the Wenchuan earthquake. About 6 km upstream of Dujiangyan and 60 km away from Chengdu, the Zipingpu reservoir is a large-scale water control project situated on the Minjiang River in Sichuan Province. The dam was constructed in March of 2001, the water impoundment started in September of 2005, and the project was completed in 2006. As one of the few high dams in China, the rock-fill dam with reinforced-concrete face has a maximum height of 156 meters and a storage capacity of 1.112 billion cubic meters. With just dozen kilometers away from the epicenter of the M8 Wenchuan earthquake on May 12, 2008, could Zipingpu reservoir have triggered the great quake? A reservoir-triggered earthquake, or reservoir earthquake, refers to substantially enhanced seismicity (frequency and magnitude of earthquakes) near the reservoir after impoundment. The first reported earthquake triggered by reservoir occurred in 1931 near the Marathon reservoir in Greece. Since then, it has been realized that construction activities such as water impoundment or dam construction could trigger earthquakes. However, most reservoir earthquakes are moderate or weak events, presenting no great damage to the dams. Only 18 reservoirs suffered damages from reservoir-triggered earthquakes, the strongest of which registered a magnitude of M6.4. Statistics on more than one hundred cases of reservoir-triggered earthquakes show that these events share the following characteristics: (1) Epicenters of reservoir earthquake are close to the reservoir, mostly within 5 km. Their focal depths are mostly shallower than 5 km and very few deeper than 10 km. In contrast, the distance between the reservoir and the hypocenter (where the rupture starts) of Wenchuan earthquake or ruptured faults is much greater than 5 km. Zipingpu reservoir is located in a seismically active region, and at least five moderate and strong earthquakes in history had occurred nearby: the M61⁄2 Wenchuan earthquake in 1657 was within 30 km from the reservoir; the M51⁄2 Wenchuan event in 1748 was also within 30 km; the M43⁄4 Guanxian event in December of 1787 was within 5 km; the M6.2 Dayi event in February of 1970 was within 55 km; and the Lushan M4.7 event in March of 1970 was within 51 km. A digital seismic network was installed to monitor seismicity during construction of the reservoir, and seismic observation started on August 16, 2004, prior to water impoundment. The network passed evaluation and was permitted for full-scale operation started on June 27, 2005. Based on statistics of earthquake catalogs provided by the Reservoir Earthquake Institute of Sichuan Earthquake Administration, frequency and magnitude of earthquakes near Zipingpu reservoir did not change substantially since impoundment, though water levels changed. Therefore, with comparison of seismic records before and after the impoundment (i.e., October 1, 2005―April 2008, a period of two years and seven months), we think that seismicity