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

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