Zhiming Sun

Fast slip‐rate along the northern end of the Karakorum fault system, western Tibet

[1]xa0The exact location of the northern Karakorum fault (KF) in western Tibet is unclear and its current activity is debated. Here, we investigate the possible northern extension of the KF, the Muji fault, located in the Chinese Pamir, which belongs to the Kongur Shan extensional system, and provide the first quantitative estimate of its Holocene slip-rate. The fault cuts and offsets a series of 6 fluvial terraces, yielding a minimum slip-rate of 4.5 ± 0.2 mm/yr, by matching the largest terrace riser offset with its upper surface age (10Be, n = 24). Field evidences of right-lateral movement along the Kongur Shan fault, as well as geometry and kinematic similarities with the southern half of the KF attest that the Muji fault belongs to the KF system. Therefore, its fast slip-rate combined with the slow slip-rates along minor splays of the northern KF (maybe up to 4 mm/yr) southwest of the Tashkorgan basin agrees with the late Pleistocene southern KF slip-rate (>8 mm/yr).

Lithological and structural characterization of the Longmen Shan fault belt from the 3rd hole of the Wenchuan Earthquake Fault Scientific Drilling project (WFSD-3)

Drilling in an active fault quickly after a large earthquake is an effective way to study earthquake mechanisms. In order to better understand the mechanical, physical, and chemical characteristics of the faults that ruptured during the 2008 Wenchuan earthquake (Mw 7.9), six boreholes were drilled on the two main strands (Yingxiu–Beichuan and Guanxian–Anxian faults) by the Wenchuan earthquake Fault Scientific Drilling project (WFSD). This paper focuses on the cores from the WFSD-3 borehole which drilled across the Guanxian–Anxian fault. A detailed petrological study shows that fault gouge and fault breccia are developed in the WFSD-3 cores in the Late Triassic Xujiahe Formation. The thicknesses of fault gouge range from ~1xa0mm to ~2.3xa0m. According to the characteristics of the fault rock combinations and their distribution, at least 22 subsidiary fault zones were recognized in the WFSD-3 cores. The Guanxian–Anxian fault zone is composed of fault rocks from 1192 to 1250.09xa0m depth, with a real thickness of ~50xa0m (~60xa0m thick in the WFSD-3 cores), and an actual damage zone of ~160xa0m (~980–1192xa0m depth in the WFSD-3 cores), and shows characteristics of multiple high-strain fault cores. The damage zone is only present in the hanging wall. The actual total thickness of the Guanxian–Anxian fault zone is ~210xa0m. Based on the analyses of comprehensive logging data, characteristics of the fault gouge, and seismic fault structures, the principal slip zone for the Wenchuan earthquake is identified in the black fault gouge at 1249.95xa0m depth in the cores, which lies almost at the bottom of the Guanxian–Anxian fault zone, and is also confirmed by surface rupture zone observations. The slip plane of the Wenchuan earthquake is a low-angle thrust fault with a dip angle of ~38° as estimated from the results of the WFSD-3 core analyses. The results from WFSD-1 showed that the Yingxiu–Beichuan segment is a high-angle thrust fault striking NW with a dip angle of ~65°. These two fault segments have different thicknesses and fault structures, which may suggest different faulting mechanisms and evolution history.

Coseismic Surface Ruptures Associated with the 2014 Mw 6.9 Yutian Earthquake on the Altyn Tagh Fault, Tibetan Plateau

Field investigations reveal that the 2014 M wxa06.9 Yutian earthquake on the left‐lateral strike‐slip Altyn Tagh fault (ATF) system, Tibetan Plateau, produced an ∼25‐km‐long surface rupture zone that contains conjugate Riedel shear faults. The coseismic surface ruptures occurred mainly along two parallel east‐northeast‐trending active left‐lateral strike‐slip faults. Rupture also occurred in a conjugate, west‐northwest‐trending zone along an active right‐lateral strike‐slip fault. The east‐northeast‐trending ruptures are concentrated in a zone of <500‐m wide and ∼25‐km long, and are characterized by Riedel shear structures including distinct shear faults (Y) with a maximum sinistral displacement of ∼1u2009u2009m, right‐stepping en echelon cracks, and mole tracks. In contrast, the west‐northwest‐trending ruptures occur within a zone of up to 1.5‐km wide and ∼4‐km long in the jog area between the two parallel east‐northeast‐trending faults, and this zone is characterized by discontinuous shear faults with dextral displacements of <0.5u2009u2009m, left‐stepping en echelon cracks, and mole tracks, all oriented oblique to the east‐northeast‐trending rupture zones at an angle of 30°–40°. The lengths and displacements of the coseismic surface ruptures measured in the field are comparable with those obtained from the empirical relationships between magnitude and coseismic surface rupture length and displacement. Our findings demonstrate that the coseismic conjugate Riedel faulting was controlled mainly by preexisting active faults of the ATF system, reflecting the present‐day tectonic stress field associated with the ongoing penetration of the Indian Plate into the Eurasian Plate.