EARTHQUAKES INDUCED BY RESERVOIR IMPOUNDING AND THEIR EFFECT ON HSINFENGKIANG DAM

Abstract

Earthquakes occurred at the Hsinfengkiang Reservoir region following the commencement of water impounding in October, 1959. Measures were promptly taken for the strengthening of the dam, while different agencies concerned were organized to carry out in close collaboration the extensive investigation on such problcms as the features and origin of seismicity in the reservoir rcgion, the characteristics of the ground motions, and their effects on the dam and so on.This paper first dcals with, on the basis of the obscrvations and research results conducted over the past years, the geological conditions for the reservoir-induccd earthquakes and the effect of the reservoir impounding on the toctonic stress fields. It points out that the carthquakes which occurred during the past 13 years were mainly distributed along the narrow valleys of the rcservoir region where the weak zoncs in the fissured granite body were situated, and that there is a ccrtain correspondence between the focal mechanism and the tcctonic stress fields. Since the state of stress at the focus is formed by the combiued elements of the earth stresscs and the seepage pressures of the rcscrvoir water, the sequcnces of the earthquakes with respect to the proccss of their origin and development have some features distinct from those of the tectonic carthquakcs.This paper also briefly presents some results of research concerning the earthquake resistance of the dam, including the design considerations and construction practices of the strengthening of the dam in two stages, as well as a description of the dam having withstood a violent earthquake of intensity Ⅷ. The causes of cracking at the top portions of some dam sections during this violent earthquake are analyscd by carrying out vibration tests simulating the irregular waves of the accelerograms and the dynamic analyses applying the finite element method with step-by-step integration. It has also been madc clear that the high value of dynamic magnification factor at the dam crest, the influence of the vibration mode of higher order and the strcss concentration and so on are the important factors rendcring the top portion of the dam to become the vulnerable part during the earthquake.