Si‐Hua Zheng

Source Parameters and Scaling Relations for Reservoir Induced Seismicity in the Longtan Reservoir Area

We investigate the relationship between the impoundment and seismicity in the Longtan reservoir, southwestern China and find evidence that the seismicity was reservoir induced. After the reservoir impoundment, a pronounced increase in seismicity was observed in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling. The observed induced seismicity shows a strong correlation with the filling cycles. The activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults. Source parameters for 1,616 earthquakes with ML 0.1–4.2 recorded by 24 fixed and temporary stations deployed around the reservoir were calculated after applying corrections for geometrical spreading, frequency-dependent Q, and site effects. The static stress drop and apparent stress in this area both appear to increase with increasing seismic moment over the entire magnitude range. Our results show that reservoir induced earthquakes have ten times lower average stress drop than natural tectonic earthquakes. These results may indicate that the reservoir induced seismicity can occur with a lower tectonic stress due to the high pore pressures of the underground medium, and that the effect of the water decreases the coefficient of friction.

Reservoir-induced seismicity in high seismicity region—a case study of the Xiaowan reservoir in Yunnan province, China

This paper discusses the spatiotemporal pattern of seismicity during the impoundment of the Xiaowan reservoir area, Yunnan Province, China, in the period from May 21, 2005 to December 31, 2012. The filling operations took place in five phases, starting on December 16, 2008. A notable increase in seismicity was only observed during the third filling phase, starting on July 15, 2010. Seismicity increase was mostly localized within two clusters, located to the northwest and west of the dam. Seismicity rates in these clusters showed a significant correlation with the water level increase, with the seismicity starting to increase when the water level reached the area covered by the two clusters, which additionally support they were induced by the reservoir impoundment. We further investigate source parameters for 44 pre-impoundment earthquakes and 164 post-impoundment earthquakes with ML ≥ 2.0. Corner frequencies, seismic moments, and stress drops are obtained based on the spectral analysis of regional data, upon corrections for geometrical spreading, frequency-dependent Q, and site effects. Our results show that during the post-impoundment phase reservoir-induced seismicity (RIS) inside the two clusters have systematically lower stress drops with respect to those occurring at further distance, and surrounding natural tectonic earthquakes, by a factor of about two to three times, suggesting a possible source characteristic that differentiate reservoir-induced seismicity from natural tectonic earthquakes. However, temporal stress drops changes within the clusters cannot be resolved. To compare the difference in stress drop between RIS and natural tectonic earthquakes, not only the difference in stress drop due to different region, but also the temporal change of background stress level even in the same region should be taken into account.