Co-seismic changes of Wenchuan M8.0 earthquake and six strong aftershocks were recorded by 4 digital deformation instruments at Xuzhou seismic station at an epicentral distance of 1392 km.The result shows that the strain-step changes and wave motions are caused by the arrival of the corresponding surface waves.The shape and size of the step changes and the response time recorded by different instruments were different

even they were located in the same rock body only 7.65 m to 10.57 m apart.This difference is probably a reflection of different instrument properties

such as sensitivity and frequency response.The earthquake-causing stress changes

which were mainly compression in Xuzhou

had an important triggering effect on far-field strain changes.