关键词: 威远地区, LOC-FLOW流程, 双差定位, b值, 时空分布特征

Abstract:

Seismic activity across the Sichuan Basin has historically remained low. Since 2014, however, seismicity has increased dramatically in the Weiyuan region of the southern Sichuan Basin, where large-scale shale gas hydraulic fracturing(HF)operations have been conducted. In particular, three successive earthquakes with M≥4.0 struck Rongxian during February 24-25, 2019. Just six months later, a moderate M5.4 earthquake occurred in the vicinity of Weiyuan County, the largest event recorded in the study area to date.

To enhance local seismic monitoring capabilities, a temporary seismic network was deployed across the Weiyuan region by the Institute of Geophysics, China Earthquake Administration(CEA) in 2019. The network consists of 40 broadband stations equipped with CMG-3ESPC/40T sensors with an average interstation spacing of ~8km. Using waveform data recorded by this network, we first constructed a high-precision earthquake catalog using a machine-learning workflow. The b-value for the entire catalog was then estimated with the maximum-likelihood method. Robust and timely estimates of b-values are essential for quantitative constraints on the evolution of earthquake sequences.

In this study, we detected and relocated a total of 49 982 events using the machine-learning-based LOC-FLOW from continuous waveforms recorded by the temporary network between October 2019 and December 2020. Cross-correlation differential travel times of these events were incorporated to constrain the relative location. We further applied a local magnitude calibration to calculate magnitudes for small-to-moderate earthquakes with hypocentral distances of ≤50km. The resulting high-resolution earthquake catalog has a completeness magnitude(M_c) of 0.4, and the largest event corresponds to the M_L5.2 Zizhong mainshock. HypoDD relocation results show that the majority of earthquakes are confined to depths less than 7km, with an average depth of 3.5km, and generally align with the burial depth of the shale gas reservoir in the study area. Specifically, the hypocenter of the Zizhong M_L5.2 earthquake lies at a focal depth of 4.43km. In map view, seismicity in the Weiyuan region delineates linear clusters adjacent to known faults, predominantly striking NW or nearly NS to NNE.

The b-value from the Gutenberg-Richter frequency-magnitude law is a critical parameter for characterizing regional seismicity. Based on the enhanced earthquake catalog, we further estimate b-values with the maximum-likelihood estimation technique to investigate their spatiotemporal variations within the study region. The spatial distribution of b-values shows clear lateral heterogeneity across the region. The estimated b-values are 0.92, 0.93, and 0.81 for southwestern(Region I), southeastern (Region Ⅱ), and northeastern(Region Ⅲ) Weiyuan regions, respectively, with corresponding completeness magnitudes of 0.6, 0.4, and 0.5. Particularly, Region III(northeastern Weiyuan) has a lower b-value than that estimated from the entire catalog(0.93), suggesting a locally elevated crustal stress within this zone.

Moreover, we detect a systematic decrease in b-values with increasing depth, likely reflecting the increase in rock confining pressure. An exception appears at the southern segment of the Molin fault, where the events at shallow depth generally correspond to low b-values, indicating concentrated elastic stress accumulating near the fault termination. Additionally, we identify a near-vertical seismic cluster characterized by high b-values. Some local earthquakes with relatively high b-values are also concentrated around the burial depth of the shale gas reservoir, which are likely triggered by shear failure via high-pressure fluid diffusion.

Time-dependent b-values reveal that decreases in b-values generally coincide with elevated seismicity, implying that small-to-moderate earthquakes mainly release stress accumulation within the shallow crust. Before the 2019 Zizhong M5.2 earthquake, the b-values decreased rapidly over a short period, followed by gradual recovery after the mainshock. Such temporal evolution is likely associated with the deformation process within the source region, including preseismic stress accumulation, coseismic stress relaxation, and post-seismic stress adjustment. The high-completeness catalog and detailed spatiotemporal b-value distributions in our study provide a fundamental dataset for traffic light monitoring frameworks and assessing seismic hazard in the Weiyuan region.

Key words: Weiyuan gas field, LOC-FLOW, HypoDD relocation, b-value, spatio-temporal variation