Detailed Spatial-Temporal Evolution of Aftershocks Following the 2014 Ms 6.3 and Ms 5.8 Kangding Double Shocks, Sichuan, China

The Ms 6.3 and Ms 5.8 Kangding earthquakes occurred near the southeast segment of the NW-trending left-lateral strike-slip Xianshuihe fault system in southwest China on 22 and 25 November 2014, respectively. In this study, we apply the double-difference earthquake location algorithm (hypoDD) to relocate earthquakes listed in routine catalog and then utilize the graphics processing unit-based match and locate method (GPU-M&L) to detect missing small aftershocks following the two mainshocks. Relocated events from the relocation step are used as templates to scan through the continuous waveform data based on only 39 regional seismic stations from 22 November 2014 to 31 December 2014 and 39 regional seismic stations as well as 4 temporary seismic stations deployed by Sichuan Earthquake Administration after the Ms 6.3 earthquake from 26 November 2014 to 31 May 2015, respectively. We obtain five times more aftershocks than listed in the routine catalog. The spatial-temporal evolution of newly detected aftershocks that occurred before the Ms 5.8 earthquake present that most of aftershocks expand northwest along the fault strike in the first 30 minutes, and then a small number of aftershocks start to expand southeast toward the subsequent Ms 5.8 earthquake. The aforementioned aftershocks expand along the fault strike with logarithmic time suggesting that they are likely driven by afterslip of the Ms 6.3 mainshock, and the Ms 6.3 earthquake may triggered the Ms 5.8 earthquake. In addition, the focal mechanism solutions of small aftershocks (i.e., 2≤M<3), which are investigated from P-wave first motions and S/P amplitude ratios, show that most of them are srike-slip mechanisms which are similar to the two mainshocks, while there are few normal and thrust mechanisms in northwest and southeast side of the Ms 6.3 mainshock, respectively, indicating a different stress state on the two sides.