通过消除S波观测谱中的传播路径、场地响应、仪器响应、噪声等的影响,得到中小地震的震源谱。根据Brune震源模型,运用遗传算法计算了云南地区2 121次ML2.0～5.3地震和四川地区323次ML3.0～5.1地震的应力降参数Δσ,分析了云南地区和四川地区中小地震应力降时空变化特征。结果 表明:云南地区的地震应力降范围为0.03～47.55 MPa,其中Δσ≥10.0 MPa(高应力降)的地震占4.8%,Δσ的年均值在2.30～5.05 MPa之间; 四川地区应力降范围为0.19～47.47 MPa,其中Δσ≥10.0 MPa(高应力降)的地震占21.7%,Δσ的年均值在5.11～8.72 MPa之间。Δσ≥10.0 MPa(高应力降)的地震主要分布在滇西北至滇东北及川滇交界一带,尤其是在龙门山断裂带有较明显分布。龙门山断裂带Δσ≥10.0 MPa的地震占35.8%,Δσ的年均值在5.20～12.09 MPa之间,2004～2007年高应力降地震事件明显增多,反映了汶川8.0级大震前的高应力变化过程。
The observational S-waveform data are corrected by removing influential factors including the propagation route,site effects,instrument response and noise.Then,we obtained source spectra for the medium-small earthquakes.Based on Brune source model and by means of generalized inversion technique,we determined the stress drop(Δσ)of 2 121 earthquakes with ML2.0-5.3 in Yunnan region and those of 323 earthquakes with ML3.0-5.1 in Sichuan region,and we analyzed temporal and spatial features of the medium-small earthquakes in Yunnan and Sichuan Region.The results show that in Yunnan region the stress drop(Δσ)is 0.03-57.55 MPa.The earthquakes with Δσ≥10.0 MPa(higher stress drop)is 4.8% of the total,and the annual average of stress drop is 2.30-5.05 MPa.In Sichuan region,the stress drop is 0.19-47.47 MPa.The earthquakes with Δσ≥10.0MPa(higher stress drop)is 21.7%,and the annual average of stress drop is 5.11-8.72MPa.The earthquakes with Δσ≥10.0MPa(higher stress drop)mainly distribute from the Northwest to Northeast of Yunnnan region and along the boundary of Yunnan and Sichuan,especially in Longmenshan Mountain Fault Belt.The earthquakes with Δσ≥10.0 MPa in Longmenshan Mountain Fault Belt is 35.8%,and the annual average varies from 5.20 MPa to 12.09 MPa.The increase in the events with high stress drop from 2004 to 2007 reflects the high tectonic stress before the MS8.0 Wenchuan,Sichuan earthquake.
