2013年芦山地震序列震源机制与震源区构造变形特征分析

Focal mechanism and tectonic deformation in the seismogenic area of the 2013 Lushan earthquake sequence,southwestern China

基于四川区域地震台网记录的波形资料,利用CAP波形反演方法,同时获取了2013年4月20日芦山M7.0级地震序列中88个M≥3.0级地震的震源机制解、震源矩心深度与矩震级,进而利用应变花（strain rosette）和面应变（areal strain）As值,分析了芦山地震序列震源机制和震源区构造运动与变形特征.获得的主要结果有：（1）芦山M7.0级主震破裂面参数为走向219°/倾角43°/滑动角101°,矩震级为MW6.55,震源矩心深度15km.芦山地震余震区沿龙门山断裂带走向长约37km、垂直断裂带走向宽约16km.主震两侧余震呈不对称分布,主震南西侧余震区长约27km、北东侧长约10km.余震分布在7~22km深度区间,优势分布深度为9~14km,序列平均深度约13km,多数余震分布在主震上部.粗略估计的芦山地震震源体体积为37km×16km×16km.（2）面应变As值统计显示,芦山地震序列以逆冲型地震占绝对优势,所占比例超过93%.序列主要受倾向NW、倾角约45°的近NESW向逆冲断层控制;部分余震发生在与上述主发震断层近乎垂直的倾向SE的反冲断层上;龙门山断裂带前山断裂可能参与了部分余震活动.P轴近水平且优势方位单一,呈NW-SE向,与龙门山断裂带南段所处区域构造应力场方向一致,反映芦山地震震源区主要受区域构造应力场控制,芦山地震是近NE-SW向断层在近水平的NW-SE向主压应力挤压作用下发生逆冲运动的结果.序列中6次非逆冲型地震均发生在主震震中附近,且主震震中附近P轴仰角变化明显,表明主震对其震中附近局部区域存在明显的应力扰动.（3）序列整体及不同震级段的应变花均呈NW向挤压白瓣形态,显示芦山地震震源区深部构造呈逆冲运动、NW向纯挤压变形.各震级段的应变花方位与形状一致,具有震级自相似性特征,揭示震源区深部构造运动和变形模式与震级无关.（4）不同深度的应变花形态以NW-NWW向挤压白瓣为优势,显示震源区构造无论是总体还是分段均以NW-NWW向挤压变形为特征.但应变花方位与形状随深度仍具有较明显的变化,可能反映了震源区构造变形在深度方向上存在分段差异.（5）芦山地震震源体尺度较小,且主震未发生在龙门山断裂带南段主干断裂上,南段长期积累的应变能未能得到充分释放,南段仍存在发生强震的危险.

The April 20 th,2013 Lushan M7.0earthquake sequence occurred on the southern segment of the Longmen Shan fault zone,southwestern China.Utilizing the waveform data of M≥3.0earthquakes between 20 Apr.2013 and 31 Dec.2015 from Sichuan Regional Seismic Network,our present study determined the focal mechanism solutions,centroid depths and moment magnitudes for the Lushan mainshock and 87 M≥3.0aftershocks by CAP waveform inversion method.Along with analysis on the strain rosette and areal strain（As）,we aimed to discuss the focal mechanism of the sequence and its implications to the tectonic deformation in the seismogenic area.The major findings are as follows：（1）The parameters of the ruptured plane are of strike219°,dip 43°and slip 101°for the Lushan M7.0mainshock with a moment magnitude of MW6.55 and centroid depth of 15 km.The 87 M≥3.0aftershocks are distributed asymmetrically near the mainshock in a region ~37km long along the strike of the Longmen Shan fault zone,and ~16km wide perpendicular to the fault zone.All the aftershocks are located within a depth range of 7~22km.Most of the aftershocks are above the mainshock,and the average depth of the sequence is about 13 km.No aftershocks were found shallower than 7km,indicating the seismogenic source was relatively deep.The estimated seismogenic source of the Lushan earthquake is 37km×16km×16km.（2）The values of areal strain（As）show that reverse faulting is dominant for the sequence.The plots of the hypocenters of the sequence on the vertical cross-sections indicate that a NE-SW-striking thrust fault dipping about 45°to NW is the main seismogenic fault of the Lushan earthquake sequence,aportion of aftershocks occurred on the SE-dipping back thrust fault nearly perpendicular to the NW-dipping main fault,and the range-front fault of Longmen Shan fault zone may be responsible for some aftershocks.P-axis is nearly horizontal and orientated in NW-SE direction,coinciding with the regional tectonic stress field.This finding indicates that the seismogenic area is controlled by the stress field,and the Lushan earthquake sequence was resulted from the reverse faulting of the NE-SW-trending faults under a nearly horizontal principle stress with NW-SE orientation.Existence of non-thrusting type earthquakes and obvious variation of the plunge angle of P-axis surrounding the mainshock together indicate the stress disturbance in the local area was influenced by the Lushan mainshock.（3）The strain rosettes for the entire sequence and different classes of magnitudes all display NW-trending compressional white lobe,therefore,we infer that the geological structures for the entire seismogenic area are of thrust faulting under NW-oriented compressional deformation.The strain rosettes exhibit self-similarity in terms of orientation and shape for all classes of magnitudes,reflecting that the deformation pattern of the seismogenic faults is independent of magnitude.（4）The shape of the strain rosette at each depth is dominated by compressional white lobe in NWNWW direction,indicating all the geological structures for the entire seismogenic area orindividual segments within different depths are under NW-NWW-oriented compressional deformation.However,variation of both the shapes and orientations of the strain rosettes with depth is observed,indicating existence of segmentation of tectonic deformation in the vertical direction.（5）The dimension of the seismogenic source of the Lushan earthquake is relatively small,and the mainshock was not on the main faults along the southern segment of the Longmen Shan fault zone,and further we suggest that the cumulated strain energy was not released thoroughly,posing strong earthquake risk on the southern segment.