Earthquake prediction on the basis of Vp/Vs variations — a case history

On November 7, 1976, an earthquake of the strike-slip fault type, and of magnitude 6.7, occurred at two mountainous localities, Yanyuan and Ninglang, in the border region of the Szechuan and Yunnan Provinces, China. One year before the earthquake, a prediction was made as to the magnitude and location of the impending earthquake by the present authors, on the basis of the results of a general survey of an area of about 20,000 km² for V_p/V_s ratio variations. The prediction of time of occurrence was made afterwards by the combined analysis of results of observations of some other precursory phenomena. The actual occurrence of the event was generally considered as being in agreement with the prediction.The present study may be taken as a new example for the detection of V_p/V_s ratio variations prior to an earthquake of the strike-slip fault type. By considering the difference between the shear rupture and stick-slip motion and the anisotropy induced by dilatancy, a preliminary discussion is made concerning the related results.