Surface ruptures on the transverse Xiaoyudong fault: A significant segment boundary breached during the 2008 Wenchuan earthquake, China

The similar to 220 km-long rupture of the 2008 Mw 7.9 Wenchuan earthquake breached several km-scale geometric discontinuities along strike, including the previously un-mapped NW-trending Xiaoyudong fault, connecting between the two major, NE-trending rupture planes on the Beichuan and Pengguan Faults. In this paper, we present high-resolution mapping of the 8-km-long surface breaks and sinistral oblique thrusting coseismic slip on the Xiaoyudong fault. Scarp height is the largest at the NW end, reaching 3.5 m, and decreases southward in steps to less than 0.2 m, with an average slip gradient of 6 x 10(-3) at a few tens of meters length scale, but up to 50 x 10(-3) locally. Left-lateral offsets co-vary with the vertical component. The largest sinistral slip vector we observed is 2.2 m. Geological and geophysical evidence suggests that the Xiaoyudong fault is likely a similar to 30 degrees SW-dipping lateral ramp that soles into the Pengguan fault, and at its northwestern end intersects with the Beichuan fault, where the latter has a step in the fault plane. Kinematically, the Xiaoyudong fault functions as a tear and conjugate fault and coincides with significant coseismic slip rake rotations on both the Beichuan and Pengguan Faults. Similar correlation of fault bends with sharp changes in faulting style occurs at other steps along the Wenchuan rupture. The Xiaoyudong fault may have played a positive role in linking coseismic slip partitioning between parallel reverse fault planes, facilitating the growth of a longer and more destructive rupture. This highlights the role of tear faults in bridging ruptures between segments, such that reverse-type ruptures can breach steps wider than anticipated from strike-slip fault examples. Transfer faults are common, and perhaps poorly documented features in reverse fault systems and their roles in ruptures may increase the maximum potential earthquake magnitude for fold-and-thrust belts. (C) 2012 Elsevier B.V. All rights reserved.