Temporal and spatial variation of b-value before and after Wenchuan earthquake and its tectonic implication

Seismic b-value shows close relationship with stress, with high stress corresponding to low b-values. Research on temporal and spatial variation of b-value in Longmenshan area will provide useful information to understand stress evolution of Wenchuan earthquake. In this paper, we relocated the earthquakes in Longmen Shan region before and after the Wenchuan earthquake (M(S)8. 0) by both joint inversion of hypocenter location and velocity structure and the double difference earthquake location method, and then calculated temporal variation of the b-value in entire region and its spatial distribution in different time periods before and after Wenchuan earthquake. The results show: (1) The b-values show a significant boundary at depth of similar to 15 km, above which the b-value is much higher than that blow 15 km. This boundary indicates the division between brittle and ductile layers in the upper crust. (2) Temporal variation of average b-value shows clear decrease after the Wenchuan Earthquake, and then gradually increases till Lushan earthquake, however decreases again after the Lushan Earthquake. This variation trend represents not only the co-seismic change but also healing process of the Longmenshan fault. (3) Spatial variation of b-value shows increase (i.e. decreased stress) in Wenchuan rupture zone after Lushan earthquake that reveals possible ductile deformation in the gap between the Lushan and the Wenchuan earthquake and the Lushan earthquake promoted the healing of the rupture zone of the Wenchuan earthquake. (4) The significant low b-value anomaly in the Pengguan complex rock area near Yingxiu before Wenchuan earthquake suggests high differential stress in this region and therefore maximum dislocation occurred here after the Wenchuan Earthquake.