VERTICAL GROUND SURFACE DEFORMATION BEFORE AND AFTER THE 1976 SONGPAN-PINGWU EARTHQUAKE

Before and after the Songpan—Pingwu earthquake of 1976, results of repeated geodetic surveys show certain regularities in ground surface deformation around the epicentral area. The first survey carried out in July—September of 1975, indicated a slow vertical relative displacement of rate 3mm/year on both sides of the Minjiang fault, since 1960. In 1975, another survey revealed accelerated rise of the east side of the same fault and 2—3 months preceding the earthquake, anomalous ground surface deformation took place near certain sensitive points of some tectonic structure. At the same time, the result of repeated surveys of a short base-line, laid at Songpan also gave short-range anomalies.After the earthquake, the Zhangla Basin descended relative to the surroundings, while the area south of the Jiaochang Diexi arc structure uplifted. West of Nanping, along the extension of the aftershock belt, vertical surface deformation seems to show elastic recovery of the crust. Deformations in the regions around the epicenter, reflected the stages of the process of development and energy release of the earthquake.Based on certain discussions on the connection between crustal deformation and earthquake parameters, a preliminary relationship of vertical ground surface deformation and earthquake magnitudes has been established. Some numerical computations gave no conflicting results which perhaps suggest certain usefulness.