Sequence Evolutions Of Earthquakes With M_s?6.5 In Sichuan-Yunnan Region

The seismicity occurring before and after large earthquakes has long been of interest in seismology because they can be good indicators of how fault stores and releases tectonic stresses during an earthquake cycle.Many recent studies have reported that large earthquakes are accompanied by distinctive foreshocks or aftershocks that occur near the mainshock rupture zone.These studies suggested that fault slip phenomena such as the nucleation process before and postseismic slip after large earthquakes could be illuminated by the spatiotemporal evolution of foreshocks and aftershocks.We selected the 2013 Ms7.0 Lushan earthquake,2014 Ms6.5 Ludian earthquake,Ms6.6 Jinggu earthquake and 2017 Ms7.0 Jiuzhaigou earthquake as our target study events.By applying the matched filter technique to continuous waveform data during 90 days before and after the mainshock,we identified approximately 3?150 times earthquakes than listed in the standard China Earthquake Networks Center earthquake catalog.About 50 percent of detected events have been relocated by relocated catalog provided by recent researches excepting the Jinggu earthquake sequence,which laid a solid foundation for our spatiotemporal evolution investigation.Based on the improved earthquake catalog,we investigated the spatiotemporal evolution of the seismicity of four large mainshocks and obtain following results and conclusions.(1)In the four study areas,foreshock and aftershock seismicity distributed heterogeneously both in time and space.Most of events and cumulative seismic moment concentrated around the mainshock hypocenter and became less active with increasing epicentral distance.Seismicity of Lushan,Ludian and Jiuzhaigou earthquakes shared a same location pattern and located in the slower side where there were relatively high bodywave velocity contrasts,which suggested that the seismicity distribution may conducted by heterogeneity of underground material.As for the seismicity of Jinggu earthquake occurring in the relatively low bodywave velocity area,we considered it may be related to highly fractured crust or presence of fluid filled in faults and microfractures in this area.(2)The decreased variation in the time-varying b values suggest that the transient faulting stress increased?1 month before the Lushan,Jianggu and Ludian mainshocks.However,the time-varying b values rapidly increased without apparent seismicity variation before the Jiuzhaigou mainshock.We speculated this might be due to the unstable calculation of b value with relatively small foreshock amount.(3)The foreshock sequences along the strike and dipping of the seismogenic fault showed a concentration with logarithmic time before the four mainshocks.The foreshocks before the Lushan and Ludian mainshocks appeared to occur as earthquake swarms,in that they did not begin with the largest foreshock.We also found some foreshock couples had high waveform similarities around seismogenic fault of Jianggu and Jiuzhaigou mainshocks,implying that they occurred repeatly in the same area.These results suggested the foreshock concentration was driven by aseismic slip.(4)The aftershocks showed an expanded migration pattern with logarithmic time along the strike and the dipping of the seismogenic fault immediately after the mainshock rupture in the four study areas.The aftershock seismicity and magnitude followed the Omori-law-type decay with similar relaxation times.Based on these results,we suggested that aseismic slip play an important role in driving the aftershock sequences.In conclusion,we speculated the foreshock seismicity before the four target large earthquakes were likely consistent with the pre-slip model.The foreshock concentration indicates the stress transfer to the initiation of unstable dynamic rupture of the mainshock.The aftershock migration sequences suggested the aseismic afterslip around the mainshock rupture zone and the migration velocities were controlled by fault friction parameters and stress changes.