Crustal P-wave velocity structure of the Longmenshan region and its tectonic implications for the 2008 Wenchuan earthquake

Abstract

The P-wave velocity structure of the crust in the Longmenshan region has been imaged by seismic travel time tomography using local and regional first P-wave arrivals recorded from 2000 to 2008. The tomographic model provides a way to analyze the deep tectonics of the Longmenshan fault belt and the tectonic implications for the 2008 M s 8.0 Wenchuan earthquake. The P-wave velocity images indicate that the initial rupture site and focal depth of the Wenchuan earthquake, together with the direction of rupture propagation, closely relate to the crustal structure of the Longmenshan region. The Pengguan massif to the west of the Longmenshan fault belt is characterized by high velocity anomalies, suggesting that the crust has a strong strain strength that can accumulate large stresses over a long period. The M s 8.0 Wenchuan earthquake is located at the southwestern end of the Pengguan massif and the western edge of the Sichuan Basin. The collision between the Pengguan massif and the Sichuan Basin becomes the primary reason for the occurrence of the M s 8.0 Wenchuan earthquake. To the north of Wenchuan, the occurrence and propagation of rupture benefit from low velocity anomalies along the Longmenshan fault belt; whereas to the south of Wenchuan, the brittle rupture can occur with more difficulty in relatively weak crust with low velocities. This may be one of the reasons for the absence of aftershocks to the south of Wenchuan, and the rupture induced by the M s 8.0 Wenchuan earthquake propagating from the north to the south along the Longmenshan fault belt. The deep geodynamics of the M s 8.0 Wenchuan earthquake may occur due to the discrepancy of crustal structures on the two sides of the Longmenshan fault belt. Ductile deformation and crustal flow can easily occur in the weak middle-lower crust beneath the Songpan-Garze orogenic belt. The eastward movement of the Tibetan Plateau is obstructed by the rigid lithosphere of the Sichuan Basin, and then the thickening of the middle-lower crust and vertical deformation occur in the crust of the Longmenshan fault belt. In addition, the down-warping of the Moho and the basement thrusting onto the range front induced crustal deformation and strain accumulation, which provided the potential energy to trigger the occurrence of the M s 8.0 Wenchuan earthquake.Longmenshan orogenic belt, Wenchuan earthquake, P-wave velocity, crustal structure, travel time tomography Citation:Li Z W, Xu Y, Huang R Q, et al. Crustal P-wave velocity structure of the Longmenshan region and its tectonic implications for the

Seismic P-wave tomography in eastern Tibet: Formation of the rifts

“…Comparing our results with those of other earlier studies, there is no such high-velocity anomaly existing around 98°E [26]. In the east of southeastern Tibet, in the vicinity of the Longmen Fault, travel time tomography [34,35] did not support the subduction of the Asian lithosphere directly to eastern Tibet. Besides these, there are no other obvious high-velocity anomalies to the west of the low-velocity zone [22,26,36].…”

Fault behavior and lower crustal rheology inferred from the first seven years of postseismic GPS data after the 2008 Wenchuan earthquake

“…2a). Based on seismic tomography results (Li et al, 2011), layered structures are defined for either side of the fault (Fig. 2b), which consists of an elastic upper crust and two underlying viscoelastic layers representing the lower crust and upper mantle, and for both, a Maxwell rheology is used (e.g., Freed et al, 2017).…”

High-resolution seismic tomography of the upper crust in the source region of the April 20, 2013 Lushan earthquake and its seismotectonic implications

“…We prefer a one-dimensional model as the starting initial model. In western Sichuan and eastern Tibet, many previous studies have provided reliable starting models in the recent years, which have afforded background models for this study (Lei and Zhao 2009;Guo et al 2009;Liu et al 2008Liu et al , 2009Xu et al 2010;Li et al 2011Li et al , 2012Wu et al 2009;Zheng et al 2013). However, the large scale regional model in the western Sichuan and eastern Tibet may not be very proper for the relative smaller area of the Lushan earthquake.…”

“…The India plate has been moving northward and subducting beneath central Asia, which has thickened the Tibetan plateau crust, tectonically forcing it eastward. Moreover, along the LMSFZ, the continental collision has formed the steepest topography gradient in the world (Li et al 2011(Li et al , 2012, which rises steeply westward from *500 m in the Sichuan basin to[4,000 m elevation in the eastern Tibet plateau over a horizontal distance of *50 km. The continental collision between India and Eurasian plates is believed to be the cause of both the Lushan and Wenchuan earthquakes (Zhang et al 2010;Zheng et al 2009).…”

“…The focal mechanisms, source rupture processes, stress changes, among other topics, are of great interest to geophysicists studying this region (Liu et al 2013a, b;Zeng et al 2013;Xie et al 2013;Wang et al 2013;Zhang et al 2013;Shan et al 2013). Many tomographic results from different methods and datasets near the LMSFT also provide helpful velocity models to study the background of the Lushan earthquake Liu et al 2009;Wu et al 2009;Lei and Zhao 2009;Xu et al 2010;Li et al 2011Li et al , 2012Zheng et al 2013). However, the tomographic resolution of most previous results is usually around several tens of kilometers or larger, which is not enough to analyze the velocity variation at the source region and determine seismotectonic implications of the Lushan earthquake.…”