To explore a local relationship between geological structures and the occurrence of very-low-frequency earthquakes (VLFEs), a particular class of slow earthquakes with characteristic periods of 10–100 s, we investigated three-dimensional (3D) structural features using reprocessed 3D seismic data from the Nankai Trough off Kumano, southwestern Japan. In this region, VLFEs have been observed along the subducting Philippine Sea Plate. Although the detailed source distribution of VLFEs was estimated by means of recent land-based and offshore seismic networks, the relation with geological features is not well understood. First, we reprocessed the 3D seismic data with advanced techniques and reinterpreted the fault distribution in the sediment layer of the accretionary prism and tracked two key horizons: a décollement and the oceanic crust surface. In the accretionary prism sediments, multiple continuous reflectors of basal detachments in the underthrust sequence and conjugate faults cutting the shallow imbricated thrust sequence were identified. In contrast to the gentle variation in the décollement surface, the topographic relief of the oceanic crust was prominent, with ridges and surface displacement due to faults in the oceanic crust. Then, we compared the structural features with the VLFE source locations. Most VLFEs were located deep in the underthrust sediments where the sediments may consist of underconsolidated muds. Furthermore, a high spatial correlation was observed between the VLFE distribution and the oceanic crust topographic relief. The maximum stress direction, which was inferred from the conjugate faults in the imbricated thrust zone, was consistent with the spatial relation between the VLFE localization and the oceanic crust central ridge. Oceanic crust ridges may cause strain accumulation in the underthrust sediments on the landward sides of the ridges, and low-angle slow thrust movements might be caused using weak slip planes in the underthrust muddy sediments. That is, the topographic relief of the oceanic crust may control the occurrence of shallow VLFEs in the Nankai Trough.

中文翻译：

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大洋地壳三维地形起伏可能控制熊野附近南海海槽浅层极低频地震的发生

为了探索地质结构与极低频地震 (VLFE)（一种特征周期为 10-100 秒的特定类型的慢地震）发生之间的局部关系，我们使用重新处理的 3D 研究了三维 (3D) 结构特征。来自日本西南部熊野附近南海海槽的地震数据。在该地区，沿着俯冲的菲律宾海板块观测到了 VLFE。尽管通过最近的陆基和海上地震网络估计了 VLFE 的详细震源分布，但其与地质特征的关系尚不清楚。首先，我们用先进的技术重新处理了 3D 地震数据，重新解释了增生棱柱沉积层中的断层分布，并追踪了两个关键层位：脱层和洋壳表面。在增生棱柱沉积物中，识别出多个连续的下冲断层反射层和切割浅层叠瓦状逆冲断层的共轭断层。与 décollement 表面的温和变化相反，大洋地壳的地形起伏显着，由于大洋地壳中的断层导致脊和地表位移。然后，我们将结构特征与 VLFE 源位置进行了比较。大多数 VLFE 位于逆冲沉积物的深处，其中沉积物可能由未固结的泥浆组成。此外，在 VLFE 分布和海洋地壳地形起伏之间观察到高度的空间相关性。从叠瓦状逆冲带共轭断层推导出的最大应力方向，与 VLFE 定位与洋壳中央脊的空间关系一致。洋壳脊可能导致海脊向陆侧下冲断层沉积物的应变积累，下冲断层泥质沉积物中弱滑移面可能引起低角度缓慢逆冲运动。即大洋地壳的地形起伏可能控制南海海槽浅层VLFEs的发生。