Spatiotemporal variation of crustal deformation in northeastern Tibet following the 2008Mw 7.9 Wenchuan earthquake and its impact on fault activity

Abstract

SUMMARYThe 2008 Wenchuan earthquake, which occurred on the eastern edge of the Tibetan Plateau, produced significant time-dependent post-seismic deformation in the northeastern Tibetan Plateau. To explore the spatio-temporal evolution of crustal deformation and its impact on nearby active faults due to the Wenchuan earthquake, we first solve the velocity fields, strain rates and slip rates of the major faults at different stages of the earthquake cycle, based on GPS observations spanning approximately 10 yr before and after the Wenchuan earthquake. The results show that: (a) during the late interseismic phase, the GPS velocity fields relative to the Sichuan basin and strain rates near the Longmenshan fault (the seismogenic fault of the Wenchuan earthquake) are insignificant in magnitude, and the geodetic slip rates of the major faults are in good agreement with geological investigations; (b) After the Wenchuan earthquake, post-seismic deformation on both flanks of the causative Longmenshan fault is asymmetrically distributed, with significant deformation distributed between the Longmenshan fault and the Longriba fault. The post-seismic deformation decreases with increasing distance away from the Longmenshan fault. Furthermore, geodetic slip rates for the major active faults at the early post-seismic stage differ significantly from those at the late interseismic stage; (c) The crustal deformation in the northeastern Tibetan Plateau is generally small during the late interseismic phase, but it increases significantly during the early post-seismic stage and begins to decrease again as it progresses to the second post-seismic stage. We speculate that this kinematic evolution of crustal deformation is ascribed to that the Longmenshan fault was in a locking state during the late interseismic period, whereas the Longmenshan fault is no longer locked after the Wenchuan earthquake, and thus the marked post-seismic deformation occurred in the epicentral and its surrounding area. Then, the post-seismic deformation becomes weaker with time lapse, and generally the crustal deformation begins to slowly recover to the pre-seismic level. Using a forward model, we test whether post-seismic deformation caused by viscoelastic relaxation of the lower crust 4 yr after the Wenchuan earthquake can explain the observed spatio-temporal pattern of the crustal deformation. We find that this mechanism can account for the evolutionary pattern of crustal motion in the Songpan-Ganzi block east of the Longriba fault in central-east Tibet and central-eastern and southern regions of the West Qinling-Songpan Tectonic Syntaxis in east Tibet 4 yr after the Wenchuan earthquake. Moreover, the post-seismic viscoelastic relaxation of the Wenchuan earthquake is the main cause of loading rate variations of primary faults in the early post-seismic stage in the northeastern Tibetan Plateau.