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Abstract: Riedel shear structures are often developed within strike-slip fault zones. This study focuses on the Riedel shear structures within the co-seismic strike-slip surface rupture zone produced by the 2010 Mw 6.9 Yushu earthquake in the central Tibetan Plateau, China. Field surveys and structural analysis of the surface ruptures reveal that the co-seismic Riedel shear structures are characterized by (i) tension cracks (T fractures); (ii) compression structures, mainly mole tracks (P shears); and (iii) discrete shear faults (R and Y shears) developed in unconsolidated alluvial deposits along the pre-existing left-lateral strike-slip Ganzi–Yushu Fault Zone. The T fractures generally show a right-stepping en echelon pattern. In contrast, the P and R shears indicate a left-stepping en echelon pattern. The Riedel shears are inclined to the general trend of the co-seismic surface rupture zone at mean counterclockwise angles of 41° (T fractures) and 21° (R shears), and a mean clockwise angle of 35° for the P shears. The present results show that (i) the co-seismic Riedel shear structures indicate a left-lateral strike-slip sense for the seismogenic fault, and (ii) the T fractures, and P and R shears are the primary Riedel shear structures formed during the early stages of the evolution of a strike-slip fault constrained by pre-existing geologic structures. [Copyright &y& Elsevier]

Abstract: Field investigations reveal that the M w 7.9 Wenchuan (China) earthquake of 12 May 2008 produced a 285-km-long surface rupture zone, with dominantly thrusting slip accompanied by a right-lateral component along the central-northern segments of the zone, and left-lateral component along the southern segment, along the Longmen Shan Thrust Belt, eastern margin of the Tibetan Plateau. The co-seismic ruptures mainly occurred along the pre-existing Yingxiu–Beichuan, Guanxian–Anxian, and Qingchuan faults, which are the main faults of the Longmen Shan Thrust Belt. The displacements measured in the field are approximately 0.5–6.5 m in the vertical (typically 1–3 m), accompanied by an average left-lateral component of <2 m along the 50-km-long southernmost segment of the rupture zone and an average right-lateral component of <1 m along the 150-km-long central-northern segments. The maximum thrust slip amount is estimated to be ~10 m, accompanied by 9 m of shortening across the rupture zone; this finding is consistent with estimates based on seismic data. The rupture length and maximum vertical displacement are the largest among all intracontinental thrust-type earthquakes reported to date. Our findings demonstrate that i) the Wenchuan earthquake occurred upon pre-existing active faults of the Longmen Shan Thrust Belt, thereby controlling the spatial distribution of co-seismic surface rupture and displacement, and the rupture processes of the earthquake; ii) the long rupture length and large thrusting slip resulted from compressive stress associated with eastward extrusion of the Tibet Plateau as it accommodates the ongoing penetration of the Indian Plate into the Eurasian Plate; and iii) present-day shortening strain upon the eastern margin of the Tibetan Plateau is mostly released by seismic slip along thrust faults within the Longmen Shan Thrust Belt. [Copyright &y& Elsevier]