【摘要】 利用中国区域台网地震波形记录,采用CAP方法反演了香格里拉德钦(位于云南省)—得荣(属于四川省)2013年8月28日MS5.1、8月31日MS5.9地震及8次MS>4余震的震源双力偶断层面解和震源质心深度.结合震区地质构造、余震分布、烈度分布、动力学背景等资料,分析了此次地震序列的震源机制和应力场特征.反演结果表明,此次地震序列为节面倾角倾斜的正断层型地震,发震断层为NWW向活动构造带.序列中最大地震MS5.9和次大地震MS5.1地震的破裂节面分别为走向299°、倾角53°、滑动角-73°;走向290°、倾角55°、滑动角-72°.震源区受到强烈的水平拉张力、垂直挤压力作用.MS5.9地震后续余震T、P轴方位角随时间变化强烈,表明MS5.9地震后震源区应力调整作用明显.震源区应力场反演结果显示,地震发生的构造带上最大主拉应力为NNE-SSW向,最大主压应力为NW-SE向,与GPS观测所反映的地表最大主应力分布方向基本一致,表明震源区的应力状态可能主要受到背景大尺度构造应力场的控制.此次地震序列填充了川滇地区震源机制及应力场的空间分布图像,1976年以来可靠的震源机制解资料表明香格里拉次级块体是川滇块体及周边区域显著的拉张作用区域.香格里拉次级块体和保山次级块体正断层地震的断层节面及震源应力轴分布的空间变化,与GPS观测反映的地表最大主拉应力分布较一致,其空间分布特征反映了在青藏高原物质挤出背景下,块体之间相互作用、地势差异等作用对构造活动的影响.更多还原

【Abstract】 No MS>5 earthquakes have been observed in the northwestern boundary of the Sichuan-Yunnan Block,the Jinshajiang fault and the Deqen-Zhongdian-Daju fault systems,since1976.The MS5.1earthquake and the MS5.9earthquake,which sequentially took place in Shangri-LaDeqen,Yunnan province-Derong Sichuan province on April 28 th,2013and April 31 th,2013,supplement apparent gaps of the focal mechanisms spatial distributions image of Sichuan-Yunnan Block boundary fault systems and also provide important date for further researches in terms of both faulting activities and tectonic stress field characteristics.Using digitally broadband seismic records,we determined focal mechanisms of 10 moderate earthquakes(MS>4)in this MS5.9and MS5.1earthquake sequence.The focal mechanism solutions then supply data for the analyses of seismogenic fault activities and investigations of focal mechanisms and stress field characteristics in the Sichuan-Yunnan region.Regional broadband seismic records of China Seismograph Network were used to determine the focal mechanisms and focal depths of the MS5.9earthquake,MS5.1earthquake and other MS>4events by the whole wave inversion method called the Cut-and-Paste method.The Focal Mechanism Stress Inversion method was introduced to estimate the local crust stress field of the source region.Combining our researches of focal mechanisms of moderate and large earthquakes in recent years with the geologic structures,aftershock distributions,intensity features and geodynamic backgrounds,we analyzed the characteristics of focal mechanisms and stress field of this earthquake sequence.The following results are obtained from focal mechanisms and stress field solutions of the MS5.9,MS5.1earthquake sequence:(1)The MS5.9earthquake and MS5.1earthquake are normal faulting events,so are other eight MS>4aftershocks.Results show seismogenic fault is a northwest-west striking active fault.The fault solutions of the MS5.9and MS5.1earthquakes are strike 299°,dip 53°,rake-73°and strike 290°,dip 55°,rake-72°respectively.(2)Spatial distributions of T-axes and P-axes of the MS5.9 and MS5.1 events are exhibited similar,whereas,the orientations of these two axes of the Ms5.9aftershocks deviate from the ones of the main event in the following two days and relocate to the ones that are similar with the main event later,indicating an obvious stress readjustment process in the seismic source region.(3)The maximum principal axes(maximum compression)are perpendicularly oriented in northwestsoutheast and the minimum principal axes(minimum compression)are horizontally in northnortheast-south-southwest,leading to an obvious normal-faulting active region.These distributions are consistent with the trending of the surface maximum principal tensile stress field derived from the geodetic observations.(4)Combined our previous focal mechanisms determination,it can be concluded that the Shangri-La sub-block and the Baoshan sub-block are two interior regions in the Sichuan-Yunnan Block which are obviously under the normal-faulting regimes.However,the fault nodal planes and P-axes and T-axes strike differently in each boundary fault system.The orientations of nodal planes exhibit a clockwise rotation,with a gradual change from trending east-west in the northern section of the Jinshajiang fault and the middle section of the Litang fault to trending northwest-west in the Deqen-Zhongdian-Daju fault then to trending nearly northsouth in the southwestern section of the Lijiang-Xiaojinhe fault and the northern margin of the Baoshan sub-block.The T-axes are correspondingly oriented in north-south or north-northwest-southsoutheast further to northeast-southwest then to east-west or northeast-east-southwest-west,suggesting that stress states in these boundary faults are not consistent under the normal faulting regimes.In sum,this MS5.9,MS5.1earthquake sequence is an apparent normal-faulting sequence.The stress field of seismic source experienced distinct stress readjustments during the development process of the whole sequence.Solutions of focal mechanisms of the Sichuan-Yunnan region in recent years show that the Shangri-La sub-block and the Baoshan sub-block are primarily under normal faulting stress patterns which are dominated by the horizontally tensile stress field.Whilethe changes of the nodal planes and T-axes orientations in each boundary fault system indicate the existed influences of the sub-blocks interactions,terrain diversities under the background tectonic stress field.更多还原