Sharp midcrustal shear-wave velocity contrast beneath the lushan segment of the longmen shan revealed by ambient-noise tomography

The 2013 M_W6.6 Lushan earthquake ruptured the Longmen Shan (LMS) thrust belt five years after the 2008 M_W7.9 Wenchuan event, yet the two ruptures did not link across a 50–80 km gap. We use Rayleigh-wave ambient-noise tomography from a dense seismic array across the southern LMS and western Sichuan Basin (ScB) to image upper- and mid- crustal structure. Rayleigh-wave dispersion between 3 and 18 s is inverted for phase-velocity maps and a three-dimensional shear-wave velocity (Vs) model. The model reveals a sharp midcrustal low-Vs wedge between ~8 and 20 km depth beneath the frontal LMS that ends abruptly against high velocities beneath the ScB. This boundary coincides with steep Bouguer gravity and topographic gradients along the plateau margin. We interpret the wedge as the shallow expression of a weak, eastward-flowing midcrustal channel impinging on a rigid foreland buttress. The resulting rheological contrast likely concentrates stress near the downdip edge of the wedge and may help explain localization of the Lushan rupture and persistence of the ductile Wenchuan–Lushan gap.