The 2014 M_W6.1 Ludian,Yunnan,earthquake:A complex conjugated ruptured earthquake

Both intensity and aftershock distributions have shown that the seismogenic structure of the 2014 MW6.1(MS6.5)Ludian,Yunnan,earthquake was quite complex.Especially,aftershocks distribute in a reverse L-shape on conjugated faults(i.e.,the NNW-SSE fault and the ENEWSW fault),making it difficult to determine the causative fault and rupture process.For better understanding the seismogenic structure of the Ludian earthquake,we invert local seismic(strongmotion and broadband data)as well as teleseismic data to investigate the rupture kinematics.Three cases of fault ruptures were tested.(1)Ruptures occur only on the NNW-SSE fault.(2)Ruptures occur only on the ENE-WSW fault.(3)Ruptures can propagate freely on the conjugated faults.A linear technique was used in these inversions.We have two steps in this work.First,local seismic data were inverted for rupture process to identify the rupture faults.Second,withthe determined rupture faults,the rupture model was renewed by jointly inverting local seismic and teleseismic waves.Local seismic data inversions show that data fitting of the NNW-SSE fault is better than that of the ENE-WSW fault,suggesting the NNW-SSE fault is the major rupture fault of the Ludian earthquake.Meanwhile,the best data fitting was found in inversion of conjugated faults,in which data fitting per seismic moment of the ENE-WSW fault was even slightly better than that of the NNW-SSE fault,indicating that both conjugated faults may have ruptured during the earthquake.From the joint inversion of local and teleseismic data,rupture initiated on the ENEWSW fault in the first two seconds,and after 2s,the NNW-SSE fault began to rupture.The source duration is about 17 s,but most seismic moments were released in the first 10 safter the rupture initiation.On the conjugated faults,ruptures propagated toward up-dip directions and mainly occurred in very shallow depth,which may account for the terrible earthquake damages.However,rupture extent of the NNW-SSE fault is much larger than that of the ENE-WSW fault.As a result,rupture area on the NNW-SSE fault is comparable with aftershocks along that direction,but on the ENE-WSW fault,the dimension of rupture area is much smaller than that of aftershock distribution.The above inversion results show that the Ludian earthquake is a complex conjugated ruptured earthquake.Ruptures firstly initiated on the ENE-WSW fault,and then triggered the NNW-SSE fault.Consecutive ruptures on conjugated faults constitute the complex rupture process of the earthquake.The left-lateral strike-slips in the south of the NNW-SSE fault could unlock the west part of the ENE-WSW fault,leading to aftershocks in that area where no significant ruptures were observed.