易桂喜,
龙锋,
梁明剑,
赵敏,
王思维
四川省地震局, 成都 610041

基金项目: 国家自然科学基金（41574047，41974066），国家重点研发项目（2018YFC150330501）及2020年地震预测预报运维专项和2020年地震大形势项目共同资助


详细信息

作者简介: 易桂喜, 博士, 研究员, 主要从事地震预报方法研究及壳幔速度结构反演等.E-mail:yigx64@163.com

中图分类号: P315

收稿日期:2020-03-10
修回日期:2020-07-02
上线日期:2020-09-05

Geometry and tectonic deformation of seismogenic structures in the Rongxian-Weiyuan-Zizhong region, Sichuan Basin: insights from focal mechanism solutions

YI GuiXi,
LONG Feng,
LIANG MingJian,
ZHAO Min,
WANG SiWei
Sichuan Earthquake Agency, Chengdu 610041, China



MSC: P315

--> Received Date: 10 March 2020
Revised Date: 02 July 2020
Available Online: 05 September 2020


摘要
摘要:四川盆地荣县—威远—资中地区属于历史弱震区，然而2019年相继发生多次破坏性地震事件.本文基于四川区域地震台网宽频带地震仪记录波形资料，利用CAP（Cut and Paste）波形反演方法，获得了2016年以来发生在荣县—威远—资中地区的26个M_S≥3.0地震的震源机制解、震源矩心深度和矩震级，对该区域发震构造几何结构与变形特征及构造应力场特征进行了初步分析.主要获得如下认识：（1）26个M_S≥3.0地震的震源矩心深度在1.5~5 km之间，平均深度3.4 km，表明事件发生在上地壳浅部沉积层内；震源深度分布揭示发震断层面倾向SE、缓倾角.（2）26个地震的震源机制全部为逆冲型，表明发震构造整体为逆断层性质.节面优势方位NNE-NE，结合走向与倾角统计结果，本文推测发震构造可能为威远背斜南翼一系列倾向SE、走向NNE-NE的缓倾角盲冲断层.（3）P、T、B轴优势方位单一，表明研究区域处于相对简单的构造应力环境.区域应力场反演获得的最大主压应力轴σ₁方位NW-SE，近水平，与目前已知的该区域构造应力场水平主压应力方向一致，反映区内构造活动主要受区域构造应力场控制；其明显有别于四川盆地南缘2019年6月17日长宁M_S6.0地震余震区NE-SW向的最大主压应力轴方位也揭示出四川盆地构造应力场具有明显的分区特征.（4）26个地震整体的应变花表现为NW-SE向挤压白瓣形态，表明区内发震构造整体呈NW-SE向纯挤压变形模式，明显有别于2019年长宁M_S6.0地震序列NE-SW向挤压兼具小量NW-SE向拉张分量的构造变形模式，进一步表明四川盆地构造变形模式也具有明显的分区特征.
关键词: 荣县—威远—资中地区/
震源机制/
发震构造/
构造应力场/
构造变形模式

Abstract:The Rongxian-Weiyuan-Zizhong (RWZ) region in central-western Sichuan Basin has been recognized to be a place with low historical seismicity, however, several destructive earthquakes occurred in 2019. Based on the seismic data from Sichuan Regional Seismic Network, our present study calculated the focal mechanism solutions, centroid depths and moment magnitudes of 26 M_S≥3.0 earthquakes since 2016 in the RWZ region by the CAP (Cut and Paste) waveform inversion method. The characteristics of the geometry and tectonic deformation of the seismogenic structures are then analyzed preliminarily. The results revealed that the centroid depths of 26 M_S≥3.0 earthquakes range from 1.5 to 5 km, and the mean depth is about 3.4 km. This finding indicates all of the M_S≥3.0 earthquakes occurred in the sedimentary layers of the shallow part of the upper crust. The focal mechanism solutions are all of thrust faulting mode, and with similar strike of NNE-NE direction and low dip angle toward SE. Based upon the statistics of strike and dip of two nodal planes, together with the vertical cross-section of hypocenters, we suggest that the seismogenic structures are a series of blind thrust faults along the south flank of the Weiyuan anticline. Dominant unique orientations of the P, T and B axes for the 26 earthquakes demonstrate a relatively simple tectonic stress settting. Our stress tensor inversion from focal mechanism solutions indicates that the maximum principal stress axis (σ₁) in the RWZ region is horizontal and orientated in NW-SE direction, which coincides with the regional horizontal compressional stress field. But this maximum principal stress axis is different from that in NE-SW direction for the 17 June 2019 M_S6.0 Changning aftershock area in the southern Sichuan Basin. This observation indicates that tectonic activity in the RWZ region is controlled by the regional stress field. The tectonic deformation of the seismogenic structures here is generally featured by NW-SE pure compression from the strain rosette of the entire 26 M_S≥3.0 earthquakes, and it is contrast to the NE-SW compression with a small NW-SE extensional component in the 2019 M_S6.0 Changning aftershock area. All of these above findings reveal that both of the tectonic stress field and tectonic deformation mode in Sichuan Basin are of an obvious domain feature.
Key words:Rongxian-Weiyuan-Zizhong region/
Focal mechanism/
Seismogenic structure/
Tectonic stress field/
Tectonic deformation mode



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