关键词: 布格重力异常, 小波分解, 断裂, 深部构造

Abstract:

This paper applies multi-scale wavelet decomposition to high-precision Bouguer gravity anomalies in the study area, yielding Bouguer gravity anomaly fields of different scales within the same spatial domain. We investigate the relationships between these multi-scale Bouguer gravity anomaly fields and fault structures. By integrating previous geological and geophysical exploration results, this study further examines the major faults and deep structural characteristics of the study area. The main findings are as follows:

(1) As the wavelet detail order increases from low to high, the closed contours of the Bouguer gravity anomaly field evolve from small to large, and from scattered and complex to continuous and simple. The 1st- and 2nd-order detail fields mainly reflect geological structures and density inhomogeneities in the upper crust. The 3rd-order detail field mainly reflects density differences of geological bodies in the middle and upper crust, whereas the 4th-order detail field reflects density differences of geological bodies in the lower crust. The strikes of the gravity anomaly axes are predominantly NNE and NE, consistent with the regional structural trend. Gravity anomalies change sharply at block boundaries. All four lower-crustal faults are located along block boundaries, indicating that deep large faults at these boundaries control the distribution of gravity anomalies.

(2) Based on the wavelet detail fields of gravity anomalies at different orders and the fault analysis results, a total of 21 faults were identified in the study area, including 13 upper-crustal faults, 4 middle-crustal faults, and 4 lower-crustal faults. However, owing to factors such as fault scale, crustal structure, and data resolution, the Bouguer gravity anomaly field could not resolve all faults. To verify the reliability of the results, seismic reflection profiles and previous regional tectonic studies were compiled to compare the fault cutting depths interpreted from the gravity field with those derived from seismic profiles. Except for the Tianzhen-Yanggao Fault and the Sangganhe Fault, which were not identified in the gravity field, the fault cutting depths inferred from the gravity data show good agreement with those from the seismic profiles for the remaining faults, thereby supporting the credibility and reliability of gravity-based fault identification.

(3) The Moho depth ranges from 29.0 to 42.5km and generally deepens from southeast to northwest. In the Taihang Mountains, the Moho rapidly shoals and the crust becomes thinner. Crustal thickness varies markedly among different blocks: it is the greatest in the Yinshan-Yanshan fault block, followed by the Ordos fault block and the Shanxi rift zone. The overall trend of Bouguer gravity anomalies broadly reflects variations in crustal thickness. The Moho discontinuity zone represents not only a zone of crustal thickness variation, but also a deep large-fault zone and a boundary between fault blocks.

(4) The four deep large faults identified in the study area—the Kouquan Fault(F₇), the northern margin fault of the Yanfan Basin(F₁₅), the northern piedmont fault of the Liulengshan Mountains(F₁₆), and the Yanhecheng-Zijingguan Fault(F₁₉)—are all located at block boundaries. Moreover, except for the northern piedmont fault of the Liulengshan Mountains, the other three faults also coincide with zones of marked Moho variation. Thus, the Moho discontinuity zone is both a crustal thickness transition zone and a deep large-fault zone, while also serving as a boundary between fault blocks.

(5) All three earthquakes with magnitudes of 7 or greater in the study area occurred on or near active fault zones. Earthquake occurrence requires the accumulation and release of stress and strain energy. Because fault zones are relatively weak regions within the crust, they are the primary sites of stress concentration and release, and their activity directly generates earthquakes. In addition, two of these earthquakes occurred where the Moho interface changes significantly. Abrupt changes in the Moho may trigger deep stress perturbations, which couple with fault activity to induce seismic events. In other words, beneath locally uplifted or subsided segments of the Moho surface controlled by faults lies a deep tectonic setting favorable for major earthquakes.

(6) Owing to the influence of the grid size and spatial extent of the Bouguer gravity anomaly data, the downward cutting depth of faults inferred from the power spectrum does not represent the true burial depth and shows some deviation from the actual fault depth. It therefore reflects only the approximate downward extent of the faults.

Through a comprehensive analysis and systematic review of the spatial distribution of major faults in the study area, combined with investigation of the Moho surface, deep large faults, and historical strong earthquakes, this paper improves our understanding of the seismogenic environment and deep structural characteristics of the region. These results are of practical significance for understanding earthquake mechanisms and constraining potential earthquake locations, and can provide a basis for future earthquake prevention and disaster mitigation.

Key words: Bouguer gravity anomaly, wavelet decomposition, faults, deep structure