Co-seismic and post-seismic gravity variation associated with the 2008 M=8 Wenchuan earthquake : implication for crustal dynamics

Abstract

Longmen Shan Mountain Belt is a prominent orogeny along the eastern margin of the Tibetan plateau. Its current deformation has significant implications for the Cenozoic tectonics of the Tibetan plateau. The M=8 Wenchuan earthquake substantially ruptured the Longmen Shan mountain in 2008. Numerous tectonics and rheological implications are concluded by this event on crustal dynamics along the eastern margin of the Tibetan plateau. Several high-resolution gravity surveys were conducted before and after the event to investigate the regional isostasy and crustal dynamics. From 2008 to 2011, four gravimetric surveys were carried out along two profiles across southern and northern Longmen Shan from the Sichuan Basin to the Songpan-Graze Terrane. The Bouguer gravity anomalies drop from -162 mGal to -431 mGal in the Aba Basin with a steep gradient of 0.84 mGal/km. There is a significant increase of crustal thickness from 40 km in the Sichuan to more than 60 km in the Tibetan plateau. Negative isostatic anomaly of -30 mGal over 150 km of the Songpan-Graze Terrane infers an over-compensation of excess crustal thickness up to 20 km. Hence, upward isostatic rebound is resulted and coupled with on-going crustal movement. Gravity values change significantly before and after the Wenchuan earthquake, ranging from -1.2 mGal to 0.7 mGal near the epicentral area. Significant thrust slip of 7.5m and normal slip of 4.5 m were simulated along the Beichuan fault and Wenchuan fault by an elastic dislocation theory. The co-existence of thrusting and normal faulting implies both compressional and extensional settings along Longmen Shan. The normal slip corroborates a large-scale crustal extension, lending support to a model with the inflation of lower crustal flow. The two-year post-seismic gravity variations were more than 0.1 mGal near the epicentral area. About 25% of them could be attributed to viscoelastic mantle relaxation. The dynamics topography along the eastern margin of the plateau is proposed to be a consequence of lower crustal flow squeezed by isostatic rebound and topographic load. The strong Yangtze Block is thought to obstruct the crustal flow horizontally and direct it to flow upward beneath Longmen Shan. The steep topography and seismicity along Longmen Shan are then resulted probably from the vertical stress induced in this upward flow.