Relocation of the aftershock sequence of Milin M_S6.9 earthquake in 2007 and spatio-temporal variation characteristics of b-value

The aftershock sequence of the Milin M_S6.9 earthquake in Tibet in 2017 are relocated by double-difference algorithm in this paper. Based on the relocated results, the spatial distribution of b-value in the source region of Milin earthquake is obtained. Combining historical b-value data from previous researches, the temporal distribution of b-value in the studied area is also obtained. The relocation results show that the aftershock sequence of the Milin earthquake are distributed in NE−SE direction along the NE boundary of Indus-Yarlung Tsangpo big-turned suture and Bilu tectonic slice, most aftershocks are concentrated in 3−20 km depth. The temporal distribution of b-value shows before Milin earthquake b-value is lower than that in the year of 2008, indicating that the stress accumulation in the studied area is strong before Milin earthquake. After Milin earthquake, there was a step-rise of b-value, showing the releases of stress, and b-value tended to stable. b-value ranges in 0.52−1.35 in the source region of Milin earthquake, the b-value variation above the depth of 15 km is distributed along the east-west direction, and below 15 km along the NE−SW direction. Therefore, it is deduced that different b-value distribution represents different structure characteristics, i.e., the b-value variation of the shallow area is related to the degree of rupture in the source region, and the b-value variation of deep area reflects the lithology difference of different tectonic units.