Static stress interactions in extensional earthquake sequences: An example from the South Lunggar Rift, Tibet

Abstract

An extensional earthquake sequence occurred in 2004-8 across a graben in the South Lunggar Rift on the Tibetan Plateau. We use InSAR data to determine the location, fault geometry and slip distribution of these earthquakes and to test whether the sequence is compatible with static stress triggering. The Mw 6.2 and 6.3 earthquakes in 2004 and 2005 both ruptured west-dipping faults on the east side of a graben. In 2008, a Mw 6.7 earthquake ruptured a pair of east-dipping fault segments on the other side of the graben, offset from the earlier ruptures. We compute first-order dislocation models of stress change and demonstrate that the order and spatial configuration of this sequence of events is compatible with triggering by static stress transfer. A continuation of the sequence would be most likely to occur on the northern extension of the 2008 rupture, although variable slip rate along the rift may mean that the sequence has run its course. The InSAR data for the 2008 earthquake also reveal slip on a fault that cuts the graben at a highly oblique angle. We suggest that this is a release fault accommodating differential throw in the hanging wall, and associate the deformation with a Mw 6.0 aftershock. Activity on such a release fault has not been directly imaged before. The Zhongba sequence is one of several examples of recent clustered normal-fault earthquakes on the Plateau, and may be an example of phase-locking of similar faults.