Co-seismic changes of Wenchuan Ms8.0 earthquake and six aftershocks recorded by four deformation instruments at Xuzhou seismostation

Co-seismic changes of Wenchuan Ms8.0 earthquake and six strong aftershocks were recorded by 4 digital deformation instruments at Xuzhou seismostation 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 were different for different instruments,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-caused stress changes,which were mainly compression in Xuzhou,had an important triggering effect on far-field strain changes.