Riedel shear structures in the co-seismic surface rupture zone produced by the 2001 Mw 7.8 Kunlun earthquake, northern Tibetan Plateau

We present a case study of Riedel shear structures related to the co-seismic surface ruptures produced during the 2001 Mw 7.8 Kunlun earthquake in northern Tibet, which are related to strike-slip movement along the Kunlun Fault. Field investigations and interpretations of high-resolution remote sensing images show that the 2001 co-seismic surface ruptures, striking WNW–ESE, are mainly characterized by Riedel shear structures, including T fractures, and R, Y, and P shears. A left-lateral shear sense is indicated. To assess the Riedel shear fabrics quantitatively, we measured the azimuth of 19,455 co-seismic surface rupture strands, the width of rupture zones at 474 profiles, and 336 fold axes of mole track structures, using 1-m-resolution IKONOS and 0.61-m-resolution QuickBird images acquired soon after the earthquake. The analytical results show that i) the co-seismic surface ruptures are generally concentrated in up to five subparallel sub-rupture zones, with individual sub-rupture zones varying in width from 3 to 350 m (generally <100 m); ii) the total width across all sub-rupture zones is generally <500 m (though locally >1–2 km); iii) T fractures are mainly developed within the alluvial deposits at counterclockwise angles of 15–40° relative to the general trend of the rupture zone; iv) P shears are also mainly found in the alluvial deposits, but at counterclockwise to clockwise angles of 5–10° relative to the general trend; and v) Y and R shears are developed within both the alluvial deposits and basement rocks, mainly representing the reactivation of pre-existing fault traces. The results demonstrate that the co-seismic Riedel shear structures are primarily controlled by the local geology during surface rupture formation, consistent with the idea that Riedel shear structures are common fault patterns within strike-slip shear zones and that their development is related to the early stages of fault evolution.