According to dispersion theory of coda of local events, using near-field data aftershocks recorded by two mobile digital stations at the Tianxin and Yunlong after the Wuding, Yunnan MS=6.5 main-shock and sampling at different central frequencies(1.5～20 Hz), spatial distribution features of coda attenuation ratio β(f) are studied. The results show that there exists obvious non-uniformity of coda attenuation ratio β(f) given by the two stations in small region. When f＜6.0 Hz the coda attenuation ratio β(f) observed on both sides of earthquake-generating fault are basically identical. Yunlong station is located on east side and Tianxin station on west side. The corresponding Qc(f) is in the range of 66～120. However, in the range of high freqency(f≥ 6.0 Hz) the coda in near field going through major fault encounters a strong absorption from the fracture zone. The β(f) going through the fault observed at Yunlong station is 30% lower than that in Tianxin station in which the seismic waves do not go through the fault. The reason for the difference in space distribution of coda attenuation is discussed. The results also show that factor A0(f) of waves source is not only related with the source strength, but also with frequence f. The mean free path L of S wave obtained using 1.5～20 Hz is respectively 30 km on east side of the fault and 40 km on west side, from which it is verified that there is a stronger dispersion body when seismic waves go through the fracture zone.%根据地方震尾波散射理论，利用云南武定MS6.5主震后近场田心和云龙两个流动数字化台站记录的余震序列地震资料, 以不同中心频率(1.5～20 Hz)取样分频研究了近场尾波振幅衰减率β(f)的空间分布特征．结果表明， 两个台站给出的尾波振幅衰减率β(f)存在明显的小区域范围非均匀性变化． 位于主发震断层两侧的云龙台(东侧)和田心台(西侧)测得的尾波振幅衰减率β(f) 值， 当f＜6.0 Hz时基本一致．相应的Qc(f)值为66～120的低值; 但在高频范围(f≥6.0 Hz)， 由于穿过主断层的近场尾波遭到断层破碎带强烈的吸收，因而云龙台测得的尾波振幅衰减率β(f)值， 比不穿过主断层的田心台给出的β(f)降低30%， 并讨论了产生近场尾波振幅衰减空间分布差异的原因. 本文的研究结果还表明，波源因子A0(f)不仅与震源强度相关， 也与频率f相关，用1.5～20 Hz求出的S波平均自由程L在主断层东侧为30 km、西侧为40 km ，由此证实了地震波穿过主断层破碎带时有更强的散射.
展开▼
