Fault locking and dynamic deformation of the Longmenshan fault zone before the 2013 Lushan M_S7.0 earthquake

By using the negative dislocation model of DEFNODE, a Fortran program to model elastic lithospheric block rotations and strains, and locking or coseismic slip on block-bounding faults, we inverted the GPS horizontal velocity observations during 1999-2007 and 2009-2011 for a 3-D fault locking model of the Longmenshan fault zone before the Lushan earthquake, and analyzed characteristics of fault deformation at different depth in combination with GPS profiles. The inversion results indicate that during 1999-2007 the middle-northern segment of the Longmenshan fault was tightly locked (locked fraction is 0.99). Meanwhile the southern segment of the Longmenshan fault was tightly locked from the surface to the depth of 16 km, and the locked fraction decreases to about 0.62 at the depth of 16-21 km. The whole fault turns to creeping gradually between the depth of 21 km and 24 km. During 2009-2011, i.e., after the Wenchuan earthquake, the middle-northern segment of the Longmenshan fault was in a creeping state. Meanwhile the locked fraction of the southern segment of the Longmenshan fault decreased to about 0.45 at the depth of 16-21 km, and remained unchanged at other depths. In addition, the GPS profiles of 2009-2011 show that the middle-northern segment of the fault was thrust and dextral slip after the Wenchuan earthquake. The southern segment could not slip freely and the deformation width was large. Comprehensive analysis indicates that the southern segment of the Longmenshan fault has no rupture after the Wenchuan earthquake and is still tightly locked, and the Wenchuan earthquake speeds up the preparation process of the Lushan earthquake. Because the locked depth of the southern segment is lower than that of the middle-northern segment, therefore intensity, earthquake magnitude and rupture range of the Lushan earthquake are all less than those of the Wenchuan earthquake.