Preseismic and coseismic horizontal crustal movements of the 2013 MS7.0 Lushan earthquake in Sichuan

In this paper the coseismic horizontal displacements of the MS7.0 Lushan earthquake on April 20，2013 in Sichuan, China are obtained in the global and regional reference frames from continuous GPS observations at some stations in the Crustal Movement Observation Network of China (CMONOC) and other stations. Comparison of the two sets of solutions for the coseismic horizontal displacements shows that they are in good agreement and both of the reference frames can be taken as the dislocation reference frame for the coseismic horizontal displacements of this event. So the coseismic horizontal displacements in the global reference frame can be used as coseismic horizontal displacements in the regional reference frame. The relationship between the coseismic horizontal displacements and the preseismic horizontal displacements can be analyzed reasonably with the results in the regional reference frame so as to study the crustal movements precursory to this event.Time series of preseismic displacements and the coseismic displacements in the regional reference frame at continuous GPS observation stations show that the tectonic horizontal crustal movements were quite significant at some stations west to the station SCTQ near epicenter of the Lushan earthquake, while this station showed abnormal locking in the horizontal movement. The coseismic horizontal displacements at the station SCTQ and other stations were elastic rebound to their normal horizontal tectonic movements. Therefore the locked horizontal displacements at these stations were anomalous in the spatial distribution of the horizontal displacements at the GPS stations and were precursors to the earthquake. The horizontal displacement change in time and space show that the preseismic and coseismic displacements at the station SCTQ are in good agreement with Reid’s elastic rebound model. The combined studies on time series of the horizontal displacements and the coseismic horizontal displacements in the regional reference frame are important for the reasonable explanation on the crustal movements precursory to the earthquake. Though the data used in this study are limited, combination of the results in this paper with other papers suggests that the result obtained from data at the station SCTQ can be verified by the data at other GPS stations. Based on the studies on the preseismic and coeseismic horizontal displacements, some problems in the GPS monitoring networks and the data processing and analyses are discussed.