地震研究

云南盐津2个地震序列震源参数测定及发震构造探讨

顾慧冬^1,2,姜金钟^1,2,3,李姣¹,张演^1,3,杨跃文¹,王光明^1,3,张帅^1,3,姚远^1,3

(1.云南省地震局,云南昆明 650224; 2.云南大学地球科学学院,云南昆明 650504; 3.中国地震局昆明地震预报研究所,云南昆明 650224)

Focal Parameters and Seismogenic Structure of Two Earthquake Sequences in Yanjin,Yunnan

GU Huidong^1,2,JIANG Jinzhong^1,2,3,LI Jiao¹,ZHANG Yan^1,3,YANG Yuewen¹,WANG Guangming^1,3,ZHANG Shuai^1,3,YAO Yuan^1,3

(1.Yunnan Earthquake Agency,Kunming 650224,Yunnan,China;2.School of Earth Sciences,Yunnan University,Kunming 650504,Yunnan,China;3.Kunming Institute of Prediction,China Earthquake Administration,Kunming 650224,Yunnan,China)

Keywords：the Yanjin earthquake sequence; focal mechanism; relocation; sPn phase; seismogenic structure

DOI: 10.20015/j.cnki.ISSN1000-0666.2024.0040

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2021年1月23日云南盐津发生M_L5.2地震,在此次地震前后该区域已经发生了多次中小地震。为了更好地评估该地区的地震危险性及识别发震构造,利用云南和四川地震台网的近震波形资料,采用CAP方法和2种速度模型反演了盐津M_L≥3.5地震的震源机制解和震源深度,结合sPn深度震相测定了盐津M_L5.2地震的震源深度,最后采用双差定位法对盐津地震序列进行重定位。结果表明:①最大震级为M_L5.2的“孤立型”地震序列发震断层为逆断层,走向近SN向、倾向东、倾角约50°,是一次沿断层面自下而上破裂较为充分的浅源地震; ②最大震级为M_L3.9的“震群型”地震序列发震断层为走滑型断层,走向为NNW向、倾向北东、倾角约为80°,该序列内3次M_L≥3.5地震的震源机制解略有差异,可能是由于该区域复杂的断层结构和应力环境影响所致; ③最大震级为M_L3.9的“震群型”地震序列的发震断层可能为盐津—筠连断裂,而最大震级为M_L5.2的“孤立型”地震序列的发震断层可能为华蓥山断裂与盐津—筠连断裂共同作用产生的一条未知隐伏断裂。

Before and after the M_L5.2 earthquake in Yanjin,Yunnan Province on 23 January 2021,several medium and small earthquakes occurred in this region.In order to better assess the seismic hazard and identify the seismogenic structures in this region,we use the waveform data of the near earthquakes from the Yunnan Seismic Network and the Sichuan Seismic Network to invert the focal mechanisms and the depths of the M_L≥3.5 earthquakes in Yanjin by the cut-and-paste(CAP)method and two different velocity models.Then,we determine the depth of the M_L5.2 earthquake using the depth phase sPn.Finally,we employ the double-difference relocation method to relocate the earthquake sequences in Yanjin.We obtain that seismogenic fault of the isolated M_L5.2 earthquake sequence belongs to the thrust type.It strikes near the N-S direction and dips eastward at 50°.The M_L5.2 earthquake has a shallow source,the rupture occurred from the bottom to the top along the fault plane.The seismogenic fault of the swarm-type M_L3.9 earthquake sequence belongs to the strike-slip type.It strikes from NNW and dips northeastward at about 80°.The focal mechanisms of the three M_L≥3.5 earthquakes in this swarm-type sequence are slightly different,this may be due to the region's complex fault structure and stress environment.Based on this analysis,we believe that the seismogenic fault of the M_L3.9 earthquake sequence is the Yanjin-Junlian fault.The seismogenic fault of the M_L5.2 earthquake sequence is an unknown hidden fault in the near N-S direction and with a dip angle of about 50°,which is generated by the Huayingshan fault and the Yanjin-Junlian fault.

引言
1 研究数据和方法
2 CAP方法反演震源机制解
3 利用sPn深度震相测定盐津M_L5.2地震震源深度
4 盐津地震序列重定位
5 讨论
6 结论

表1 云南地震台网给出的2019年7月—2022年2月盐津ML≥3.0地震目录<br/>Tab.1 The catalogue of earthquakes(ML≥3.0)in Yanjin area from the Yunnan Seismic Network(July 2019-February 2022)

表1 云南地震台网给出的2019年7月—2022年2月盐津ML≥3.0地震目录
Tab.1 The catalogue of earthquakes(ML≥3.0)in Yanjin area from the Yunnan Seismic Network(July 2019-February 2022)

图1 盐津地区2019年7月—2022年2月地震震中位置(a)及周边台站分布(b)<br/>Fig.1 Distribution of the faults,epicenters(a),and the surrounding stations(b) in Yanjin area during July 2019-February 2022

图1 盐津地区2019年7月—2022年2月地震震中位置(a)及周边台站分布(b)
Fig.1 Distribution of the faults,epicenters(a),and the surrounding stations(b) in Yanjin area during July 2019-February 2022

图2 盐津2个地震序列M-T图<br/>Fig.2 The M-T map of two earthquake sequences in Yanjin during July 2019-February 2022

图2 盐津2个地震序列M-T图
Fig.2 The M-T map of two earthquake sequences in Yanjin during July 2019-February 2022

图3 CAP反演中用到的两种一维速度模型<br/>Fig.3 Two 1-D velocity models used in CAP inversions

图3 CAP反演中用到的两种一维速度模型
Fig.3 Two 1-D velocity models used in CAP inversions

图4 利用模型一(a)、二(b)反演盐津ML≥3.5地震震源深度拟合误差图<br/>Fig.4 Fitting errors of the focal depths of the earthquakes(ML≥3.5)in Yanjin inverted by using Model Ⅰ(a)and Model Ⅱ(b)

图4 利用模型一(a)、二(b)反演盐津ML≥3.5地震震源深度拟合误差图
Fig.4 Fitting errors of the focal depths of the earthquakes(ML≥3.5)in Yanjin inverted by using Model Ⅰ(a)and Model Ⅱ(b)

图5 利用模型一(a)、二(b)反演得到盐津ML5.2地震在震源深度7 km的理论波形(红线)和观测波形(黑线)对比图<br/>Fig.5 Comparison between the synthetic(red line)and the observed(black line)seismograms of the Yanjin ML5.2 earthquake at a source depth of 7 km by inversion using Model Ⅰ(a)and Model Ⅱ(b)

图5 利用模型一(a)、二(b)反演得到盐津ML5.2地震在震源深度7 km的理论波形(红线)和观测波形(黑线)对比图
Fig.5 Comparison between the synthetic(red line)and the observed(black line)seismograms of the Yanjin ML5.2 earthquake at a source depth of 7 km by inversion using Model Ⅰ(a)and Model Ⅱ(b)

表2 利用CAP方法反演盐津ML≥3.5地震的震源机制解<br/>Tab.2 Focal mechanisms of the earthquakes(ML≥3.5)in Yanjin inverted by using CAP method

表2 利用CAP方法反演盐津ML≥3.5地震的震源机制解
Tab.2 Focal mechanisms of the earthquakes(ML≥3.5)in Yanjin inverted by using CAP method

图6 利用速度模型一计算得到的不同震源深度和震中距的sPn及Pn震相理论走时(a)及理论走时差(tsPn-Pn)变化(b)<br/>Fig.6 The theoretical travel time of sPn and Pn phases(a)and their difference values tsPn-Pn(b)calculated by using Model I

图6 利用速度模型一计算得到的不同震源深度和震中距的sPn及Pn震相理论走时(a)及理论走时差(tsPn-Pn)变化(b)
Fig.6 The theoretical travel time of sPn and Pn phases(a)and their difference values tsPn-Pn(b)calculated by using Model I

表3 利用sPn震相测定盐津ML5.2地震震源深度的台站震中距和方位角<br/>Tab.3 Epicentral distances and azimuths of the seismic stations determining the focal depth of the Yanjin ML5.2 earthquake using sPn phase

表3 利用sPn震相测定盐津ML5.2地震震源深度的台站震中距和方位角
Tab.3 Epicentral distances and azimuths of the seismic stations determining the focal depth of the Yanjin ML5.2 earthquake using sPn phase

图7 不同震中距的台站记录到的盐津ML5.2地震Pn及sPn震相<br/>Fig.7 The Pn and sPn phases of the Yanjin ML5.2 earthquake recorded by stations

图7 不同震中距的台站记录到的盐津ML5.2地震Pn及sPn震相
Fig.7 The Pn and sPn phases of the Yanjin ML5.2 earthquake recorded by stations

图8 2019年7月—2022年2月M&L检测后的地震目录在双差重定位前(a)、后(b)的震中分布及地震事件沿AA'(c)和BB'(d)剖面的震源深度分布
Fig.8 Distribution of the epicenters of the earthquakes from July 2019 to February 2022 before(a)and after(b)DD relocation of the earthquake catalog through M&L detection,and the earthquakes' hypocenters along profile AA'(c)and BB'(d)