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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 M 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. [ABSTRACT FROM AUTHOR]

The temporal and spatial characteristics of 1924 post-impoundment earthquakes (ML -0.3 to 3.1) recorded by 26 temporary stations from 16 March 2009 to 14 July 2010 in the Three Gorges reservoir area, Hubei Province, China, are investigated in this paper. The epicenters are mainly concentrated in three clusters and located along the Yangtze river within the range of 10 km from the reservoir water-front. The hypocentral depths range from 0 to 15 km and appear to increase with the epicenters approaching the dam. Source parameters for 97 selected reservoir-induced earthquakes of ML> 1.5 were estimated after applying corrections for geometrical spreading, frequency-dependent Q, and site effects. The results show that the seismic moments (M0) are between 3.96 x 10n and 4.07 x 10° N • m, whereas static stress drops (A

High-resolution three-dimensional V and V/ V images in the Longtan reservoir area were obtained from local earthquake data by using 3,178 events with total 24,153 P-wave and 23,987 S-wave arrivals collected from 23 seismic stations. The tomographic images show that significant V heterogeneities can be seen at layers of different depth in the Longtan reservoir area. Low- V anomalies both beneath and around the main rivers in the reservoir area may be related to the composition of rocks which are mainly deposit carbonate and arenaceous shale, which contributes to water saturation. We deduced that the high porosity rocks beneath the main rivers may be fully saturated with water. The phenomenon that V is relatively high in the area which is 10-20 km away from the rivers indicates that horizontal saturation of water is limited within a small range of area that is about 10-20 km from the main rivers. The characteristic is significant that seismicity in the Longtan reservoir area is coincident with the distribution of the low- V area. V/ V tomographic images show that V/ V ranges from 1.8 to 2.05 in shallow layers above 4 km depth beneath the Longtan reservoir, suggesting the properties of the rocks are limestone and shale. At the depth of 7 km, the distribution of V/ V image varies quite remarkably, especially in the dam area. This demonstrates that the range of influence by the saturation of water in the media below the reservoir surface can reach 4-7 km depth in the dam area. [ABSTRACT FROM AUTHOR]

SUMMARY High-resolution 3-D QP and QS models at depths of 0∼14 km in the Longtan reservoir area, which were used to further infer the water infiltration around the reservoir, were constructed by the local earthquake tomography technique. The inversion used t* values measured from velocity spectral amplitudes of 16 134 P-wave and 17 027 S-wave observations of 2828 events recorded by 23 digital seismic stations in the Longtan reservoir area. 3-D QP and QS tomographic images suggest a close relation between the attenuation models and water infiltration. Low QP and QS exist down to 4∼7 km depths beneath the main rivers in the reservoir area, whereas high QP and QS are found both at deep layers and at shallow layers in regions other than the main river area. We inferred that the rocks beneath the main rivers are infiltrated by substantial water and water infiltration reaches 4∼7 km deep, and the ranges of water infiltration vary by region. Specifically, the zone of high water content is estimated to extend up to 10 km away from the Buliu river in the reservoir dam area where water depth in the reservoir reaches maximum, and the extent of water infiltration in rocks can only be about 5 km around from the Hongshui river and the Youla river. Low QP and high QS indicate that the carbonate rocks in the east of the Bala fault area may be saturated with water. Earthquake clusters are mainly concentrated on the low- QP and low- QS zones, indicating that water infiltration beneath the main rivers possibly plays an important role in inducing earthquakes in the Longtan reservoir area. [ABSTRACT FROM AUTHOR]

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 M ≥ 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. [ABSTRACT FROM AUTHOR]