Heterogeneous strain regime in the eastern margin of Tibetan Plateau and its tectonic implications

摘要

The eastern margin of Tibetan Plateau is one of the most active zones of tectonic deformation and seismicity in China.To monitor strain buildup and benefit seismic risk assessment,we constructed 14 survey-mode global position system (GPS) stations throughout the northwest of Longmenshan fault.A new GPS field over 1999-2011 is derived from measurements of the newly built and pre-existing stations in this region.Sequentially,two strain rate fields,one preceding and the other following the 2008 Mw7.9 Wenchuan earthquake,are obtained using the Gausian weighting approach.Strain field over 1999-2007 shows distinct strain partitioning prior to the 2008 Mw7.9 Wenchuan earthquake,with compression spreading over around Longmenshan area.Strain field derived from the two measurements in 2009 and 2011 shows that the area around Longmenshan continues to be under striking compression,as the pattern preceding the Wenchuan earthquake,implying a causative factor of the sequent of 2013 Mw6.7 Lushan earthquake.Our GPSderived dilatation shows that both the Wenchuan and Lushan earthquakes occurred within the domain of pronounced contraction.The GPS velocities demonstrate that the Longriba fault underwent slight motion with the faultnormal and-parallel rates at 1.0 ± 2.5 mm and 0.3 ± 2.2 mrn/a;the Longmenshan fault displayed slow activity,with a fault-normal rate at 0.8 ± 2.5 mm/a,and a fault-parallel rate at 1.8 ± 1.7 mm/a.Longriba fault is on a par with Longmenshan fault in strain partitioning to accommodate the southeastward motion of eastern margin of the Tibetan Plateau.Integrated analysis of principal strain tensors,mean principal stress,and fast directions of mantle anisotropy shows that west of Sichuan is characterized as mechanically strong crust-mantle coupling.