Understanding the driving forces and nature of intraplate seismicity remains a major unsolved problem in seismology. In the western Pyrénées, seismicity is concentrated in a narrow region that follows the boundary between the Axial Zone and the North Pyrenean Zone. Despite the presence of a permanent network in the region, the geometry of active faults, and their relationship with crustal structures, remain elusive, owing to significant earthquake location uncertainties. Here, we exploit data recorded by a large-N nodal array deployed in the Chaînons Béarnais region during four weeks of 2022 in order to image crustal structures and characterize active faults. We automatically detected and picked P and S waves with PhaseNet, resulting in a catalog of over 500 events, half of which are located beneath the temporary deployment. Tomographic images obtained from the inversion of P and S arrival times provide detailed insight into the geometry of folds and thrusts in the sedimentary cover, as well as the presence of a main fault in the basement which dips northward with an angle of 65° (Chaînons Béarnais normal fault). Seismicity relocation within the 3D model obtained by tomography shows that earthquakes are concentrated along this main active fault, extending from the top of the basement to a depth of approximately 16 km. These results demonstrate that passive imaging approaches can offer cost-effective alternatives to traditional controlled source imaging for seismotectonic studies and natural resource exploration in regions with active seismicity.

中文翻译：

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具有大 N 节点部署的活动断层的地壳成像和特征 - 在 Chaînons Béarnais 地区（法国比利牛斯山脉山麓西部）的应用


了解板内地震活动的驱动力和性质仍然是地震学中一个未解决的主要问题。在比利牛斯山脉西部，地震活动集中在沿着轴向区和北比利牛斯带之间边界的狭窄区域。尽管该地区存在永久性网络，但由于地震位置的重大不确定性，活动断层的几何形状及其与地壳结构的关系仍然难以捉摸。在这里，我们利用了 2022 年四周内部署在 Chaînons Béarnais 地区的大 N 节点阵列记录的数据，以便对地壳结构进行成像并表征活动断层。我们使用 PhaseNet 自动检测并挑选 P 和 S 波，从而生成了包含 500 多个事件的目录，其中一半位于临时部署之下。从 P 和 S 到达时间的反演中获得的断层图像提供了对沉积覆盖层褶皱和逆冲几何形状的详细见解，以及基底中存在一个向北倾斜 65° 的主断层（Chaînons Béarnais 正断层）。通过层析成像获得的 3D 模型中的地震活动重新定位表明，地震集中在这个主要的活动断层上，从基底顶部延伸到大约 16 公里的深度。这些结果表明，被动成像方法可以为地震活跃地区的地震构造研究和自然资源勘探提供传统受控源成像的经济高效的替代方案。