Inferring water infiltration in the Longtan reservoir area by three-dimensional attenuation tomography.

SUMMARY High-resolution 3-D Q_P and Q_S 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 Q_P and Q_S tomographic images suggest a close relation between the attenuation models and water infiltration. Low Q_P and Q_S exist down to 4∼7 km depths beneath the main rivers in the reservoir area, whereas high Q_P and Q_S 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 Q_P and high Q_S 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- Q_P and low- Q_S 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]