Abstract:

Since 2014, seismic activity along the southern margin of the Sichuan Basin has shown a significant increase in both frequency and magnitude. In particular, the Rongxian-Weiyuan-Zizhong region in southern Sichuan has experienced five destructive earthquakes with magnitudes of M_S≥4.7, as well as numerous events with M_S≥4.0, in recent years. However, the unique geological characteristics of this region, including its compartmentalized fold structures and the absence of Late Cenozoic sedimentary deposits, have hindered understanding of the seismogenic mechanisms and spatiotemporal evolution of earthquakes in the area. This limited understanding further constrains the accurate assessment of future seismic trends and associated seismic hazards. In response to the increasing occurrence of destructive earthquakes, constructing a detailed deep structural model of southern Sichuan is essential for identifying the geometry and properties of seismogenic faults, investigating earthquake generation mechanisms, and evaluating regional seismic risk.

To investigate the deep tectonic background and seismogenic processes of earthquakes in this region and to better assess seismic activity patterns and potential hazards, a dense short-period seismic array consisting of 75 stations with an average spacing of approximately 5km was deployed across the Rongxian-Weiyuan-Zizhong area. Continuous waveform data recorded by the array were processed using the ambient noise cross-correlation method to obtain empirical Green’s functions of surface waves between station pairs. Fundamental-mode Rayleigh-wave phase velocity dispersion curves within the period range of 1-8 s were extracted. Subsequently, the Fast Marching Surface-wave Tomography(FMST)method and CPS3.30 software were employed to invert the dispersion data and construct a high-resolution three-dimensional S-wave velocity model down to a depth of 6km. For microearthquake detection and location, the LOC-FLOW workflow was adopted. Specifically, a Lightweight Phase Picking Network(LPPN)was used for seismic phase identification, the Rapid Earthquake Association and Location(REAL)method was applied for phase association, and hypoinverse together with hypoDD was employed for precise earthquake relocation. In total, 1, 255 seismic events were identified, significantly exceeding the number reported in the regional seismic catalog during the same period. The dense seismic array provided excellent monitoring capability, enabling comprehensive analysis of the three-dimensional S-wave velocity structure, seismicity distribution, and deep tectonic environment of the study area.

The results show that most earthquakes recorded by the dense array in the Rongxian-Weiyuan-Zizhong region have magnitudes below M_L3.0, with a completeness magnitude of approximately M_L0.6. Their spatiotemporal distribution exhibits strong clustering characteristics, forming a ring-like pattern surrounding the urban area of Weiyuan. Most earthquakes occurred within the sedimentary cover overlying the crystalline basement, with focal depths primarily concentrated between 1 and 5km. The shallow three-dimensional S-wave velocity structure exhibits pronounced lateral heterogeneity. Within the upper 2km, S-wave velocities in most areas range from 2.2 to 2.6km/s, whereas some areas near the Lijiachang fold exhibit velocities exceeding 2.8km/s. The velocity structure generally increases with depth. At depths between 3 and 6km, velocities within the northwestern anticlinal structures exceed 2.8km/s, whereas the southeastern sedimentary cover exhibits velocities ranging from 2.6 to 2.8km/s. The spatial distribution of velocity anomalies correlates strongly with regional topography, with low-velocity anomalies corresponding to the southeastern sedimentary basin and high-velocity anomalies associated with the Lijiachang fold in the northwest.

Significant lateral variations in shallow subsurface properties are observed between the Rongxian and Weiyuan-Zizhong seismic zones. The epicenters of the Rongxian earthquake sequence, the Weiyuan M_S5.4 earthquake, and the Zizhong M_S5.2 earthquake are mainly concentrated near the boundaries between high- and low-velocity anomalies. The spatial distribution of the Rongxian-Weiyuan-Zizhong earthquake sequences is closely related to the velocity structure of the crustal medium, indicating that seismicity is strongly controlled by heterogeneous subsurface structures. A pronounced low-velocity anomaly is observed at depths of 3~5km on the southeastern side of the Rongxian seismic source region, suggesting the possible presence of deep fluids. It is therefore inferred that earthquakes in this region are jointly controlled by regional tectonic stress and fluid-induced perturbations, which may reactivate pre-existing buried faults.

Key words: earthquakes in southern Sichuan, ambient tmography, S-wave velocity structure, earthquake distribution, seismogenic environment

摘要：

四川盆地南缘近年来发生的地震频次增加、强度增大, 然而目前对川南地震的孕震机理及时空演化特征的研究和了解都不够充分, 这也限制了对该区未来地震活动趋势研判和地震风险的准确评估。文中利用布设在川南荣县—威远—资中地区的短周期密集台阵观测数据, 对该地区的微小地震进行检测识别和精确定位, 并反演获取该区的三维S波精细速度结构, 综合分析三维S波速度结构、区内地震展布及深部构造环境等科学问题。结果表明, 密集台阵记录获取的荣县—威远—资中地区的地震分布呈现较强的时空丛集性, 且绝大多数地震分布在结晶基底上的覆盖层内部, 震源深度优势分布层位主要集中在1~5km。研究区浅层三维S波速度结构表现出明显的不均匀分布特征, 且速度结构随深度呈趋势性增加。在深度2.0km以内, 研究区大部分范围内的S波速度为2.2~2.6km/s, 在3.0~6.0km深度范围, 探测区北西侧背斜构造内大部分区域的S波速>2.8km/s, 南东侧沉积盖层的S波速度为2.6~2.8km/s, 速度特征分布与研究区域西北高、东南低的地形地貌对应较好。荣县震区和威远—资中震区浅部存在显著的横向介质分布差异, 震中主要位于S波速度的高、低速异常分界带附近, 地震序列的空间展布特征与震区介质速度结构存在密切关系。荣县震区震源体东南侧3.0~5.0km深度范围内存在较为显著的低速异常分布, 指示着该处可能赋存流体, 推测该区地震可能是区内先存隐伏断层在区域构造应力及震区流体扰动共同作用下发生挤压错动所致。

关键词: 川南地震, 背景噪声成像, S波速度结构, 地震分布, 孕震环境