The 2007 Ning'er Mw 6.1 Earthquake: A Shallow Rupture in Southwest China Revealed by InSAR Measurements

The Ning'er Mw 6.1 earthquake on 3 June 2007 occurred in the southwest to the Sichuan-Yunnan rhombic block with complicated seismic structures. The focal depth determined by the U.S. Geological Survey is only 5 km, but the field investigation did not find obvious surface ruptures in the epicenter area. The magnitude obtained by inverting the broadband waveforms is Mw 6.4 that is much larger than the Mw 6.1 calculated by U.S. Geological Survey, indicating this event is complicated. The Interferometric Synthetic Aperture Radar is used to study the fault geometry, coseismic slip model, and Coulomb stress changes around the seismic region. It shows that this event is mainly dominated by a right-lateral strike-slip fault with a slight thrust component. The fault plane is 28-km long along the strike and 14-km wide along the dip. The maximum slip is up to 1.02 m at a depth of 3.6 km and the released moment is 2.57 x 10(18) Nm, corresponding to a magnitude of Mw 6.25. The shallow source of this event is related to inelastic off-fault deformations along the fault or magma chamber. The lack of obvious surface ruptures indicates the strain is not completely released. The coseismic Coulomb stress changes suggest this earthquake has enhanced the stress on the Puwen fault and Mohei fault. The future seismic risks of these two faults and its adjacent regions cannot be ignored since aftershocks are fewer and the residual strain unreleased by the Ning'er event.