Evidence for low-angle normal faulting in the Pumqu-Xianza Rift, Tibet

Abstract

SUMMARY Low-angle normal faulting is widely discussed as a possible mechanism for continental extension, however, unambiguous evidence for seismogenic low-angle normal faulting is lacking. Here, we investigate seismicity along a short segment of the Pumqu-Xianza Rift (PXR) in southern Tibet, where the HiCLIMB seismic array recorded over 500 earthquakes between 2004 July and 2005 August. Hypocentres of the 40 best recorded earthquakes are approximately 20–25 km west of the rift and tightly clustered at about 10 km depth, consistent with moment tensor depths of the 11 largest (3.4 ≤Mw≤ 4.5) earthquakes. Events in this group have N–S striking normal faulting mechanisms with low-angle (29°) west dipping fault planes. Rupture along a west dipping, low-angle, planar normal fault (the eastern PXR boundary fault) is consistent with event hypocentres, fault dip from moment tensors, and prominent surface morphology. The dip of 29° is at the low end of physically possible values assuming normal frictional behaviour and state of stress. We suggest it is possible for a planar, low-angle fault to nucleate seismically at a low angle at depth in the presence of basal shear and work its way aseismically through the brittle crust to the surface with the aid of lubricating minerals.