Abstract:On the basis of research reported during the past few decades and data from the national underground fluid precursory network,this study examines the coseismic changes of water level caused by the Minxian-Zhangxian MS6.6 earthquake.Coseismic change was observed in only one water well level in the static strain field with an epicentral distance of≤1.5the fault rupture length,and its amplitude was only 1.5cm.Such static strain is not likely to trigger further earthquake activity.The total water level changes in observation wells indicated that the distance between the water level changes to the epicenter was distributed from 230km to 1470km.In the far field,in which the epicentral distance was larger than 1 000km,coseismic changes were observed in two observation well,both of which showed abrupt water level changes.The mechanism and characteristics of this result could indicate the dominant factor between the elastic-plastic spread and shutting of the rock fracture shown by the influence of quaking around the wells.When the elastic-plastic spread was less than the shutting,the decreased rock porosity resulted in ajump in observation well water level.Moreover,eight observation wells showed coseismic changes of water level in the entire Chinese mainland,which is less than the number of the earthquake coseismic changes.The distribution of the coseismic changes of water level in the wells was large and irregular with geologic structure.From the perspective of all the coseismic changes in observation wells,six observation wells showed abrupt changes in water level.Such increases were the main type of change and were attributed to 75%of the total coseismic changes in water level.Regional tectonic stress should have been released by the Minxian-Zhangxian MS6.6 earthquake.In addition,the structure and hydrogeological conditions of the observation wells were key to the coseismic changes in the water level,and the lithology effect of the observation wells on the coseismic changes was far less than the distance effect.