Two earthquake sequences along the Selaha and Zheduotang branches of the Xianshuihe fault, southwest China, and implication for aseismic transient slip on continental transform faults

English Abstract:

During one week since 22 November 2014, over 900 earthquakes of M<6 struck the northwest of Kangding within a distance of ~30 km on the Xianshuihe fault in southwest China. This intense seismicity are two earthquake sequences initiated by two large events: November 22 (UTC 08:55:25) Mw 5.9 and November 25 (UTC 15:19:07) Mw 5.6, along the Selaha branch and the Zheduotang branch respectively. The inversion results of slip distribution from InSAR images taken between September 26 and December 5, 2014, show the optimal dip angle of the seismogenic fault is ~84°, and the maximum slip of the first earthquake sequence is 0.5 m. Using a rigidity of 30 GPa, the geodetic moment is 2.45×10¹⁸ N m, which is about two-fold the total seismic moment (equivalently Mw 6.0) accumulated during the same time period. This implies that there was probably aseismic slip between these two earthquake sequences. The along-strike migration velocities of seismicity in the first ~5 hours after two large events are both ~ 5 km/hr, which is on the order of the propagating speed (km hr^-1) of seismicity driven by aseismic slip on tectonic transform faults [Roland and McGuire, 2009]. However, the occurrence of two earthquake sequences does not completely decrease the seismic risk of a big earthquake on this Kangding segment of Xianshuihe fault, which has accumulated seismic moment up to 7.15×10¹⁸ N m since the 1955 M7(1/2) event [Jiang et al., JGR, 2015; under review].