Spatiotemporal aftershock evolution of the 2014 M 6.4 and 5.9 Kangding double shocks in Sichuan, southwestern China

We apply the graphics processing unit-based match and locate method to detect missing small aftershocks of the 2014 M 6.4 and 5.9 Kangding double shocks (hereafter KD1 and KD2, respectively) in Sichuan, southwestern China. KD1 and KD2 occurred on two adjacent branch faults of the Xianshuihe fault zone, respectively. We use the relocated routine catalog events as templates to scan through the continuous waveforms from KD1 to approximately 5 days after KD2. Approximately 7.8 times more aftershocks than the routine catalog are obtained. Our results show that the aftershocks occurring between the two mainshocks first expanded northwestward along the KD1 seismogenic fault and then migrated southeastward to the KD2 seismogenic fault. This feature may reflect the stress transfer process after KD1, which may represent the triggering process of KD1 to KD2. These aftershocks also show minor migration with logarithmic time in the along-strike and up-dip directions, which suggests that the afterslip may occur between the two mainshocks and improve the stress level around KD2 to some extent. Based on the improved aftershock catalog, we also calculate the b-values within different space-time windows in the aftershock zone. A relatively low b-value around the KD2 nucleation point after KD1 and before KD2 is observed, which may indicate that this area contained a high stress level and might be in the critical state of rupture, in which KD2 occurred at this particular moment. Our study suggests that after a moderate-to-large earthquake a more complete aftershock catalog can be obtained by detecting microearthquakes with which the triggering processes of subsequent mainshocks could be revealed; based on the detailed spatiotemporal distribution of aftershocks and b-values, the subsequent moderate-to-large earthquake hazard could be assessed to some extent.