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Title:: Study on Temporal and Spatial Features of Stress Drop for Low-to-moderate Earthquakes in Sichuan and Yunnan Region

Author(s):: LIU Li-fang¹; SU You-jin¹; LIU Jie²; YANG Jing-qiong¹; (1.Earthquake Administration of Yunnan Province,Kunming 650224,Yunnan,China)(2.China Earthquake Networks Center,Beijing 100045,China)

Keywords:: stress drop; generalized inversion technique; low-to-moderate earthquake; Longmenshan Mountain Fault Belt; Yunnan; Sichuan

摘要:: 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M_L2.0～5.3地震和四川地区323次M_L3.0～5.1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。

Abstract:: 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 M_L2.0-5.3 in Yunnan region and those of 323 earthquakes with M_L3.0-5.1 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 M_S8.0 Wenchuan,Sichuan earthquake.