Boso Peninsula, Japan, was formed by the interaction of the Philippine Sea, Eurasian and Pacific plates around the trench–trench–trench Boso triple junction. Normal-type earthquakes are persistently observed in the subducting Philippine Sea slab under the peninsula at a depth of ~ 30 km, including a recent (2019) M w 4.9 earthquake which caused shaking throughout the Kanto region (greater Tokyo). Such shallow intraplate earthquakes are potentially hazardous to this heavily populated region, yet their mechanism is poorly understood, especially in the context of a three-plate system. Here, we calculate stress rates in the Philippine Sea slab and the surrounding area, using a subduction model constructed in a previous study, to explain the generation of the regional stress field and its effect on earthquake occurrence. In general, the calculated stress rates under Boso Peninsula are horizontally extensional both above and below the Eurasian–Philippine Sea plate interface. We apply our calculated stress rates to the nodal planes of the observed earthquakes to calculate the Coulomb failure function (ΔCFF). These calculated ΔCFFs are generally positive on normal-type earthquakes under Boso. The ΔCFFs are also consistent with earthquakes in adjacent areas that are seismically active, for example, in the Philippine Sea plate to the south, in the collision zone around Izu Peninsula, and in the cluster in the Eurasian plate northeast of Boso Peninsula, which further supports our stress loading model. Calculation of the individual contributions of Philippine Sea plate and Pacific plate subduction shows that the development of the stress field around Boso is dependent upon contributions from both subducting plates. In contrast, the arc–arc collision at Izu Peninsula has little influence.

中文翻译：

---

日本房总半岛应力载荷与正常型地震的发生

日本房总半岛由菲律宾海、欧亚和太平洋板块围绕海沟-海沟-海沟房总三重交界处相互作用而形成。在半岛下方俯冲菲律宾海板块持续观察到约 30 公里深度的正常型地震，包括最近（2019 年）M w 4.9 地震，该地震在整个关东地区（大东京）引起了震动。这种浅板内地震对这个人口稠密的地区具有潜在的危险，但其机制却知之甚少，尤其是在三板系统的背景下。在这里，我们使用先前研究中构建的俯冲模型计算菲律宾海板块及周边地区的应力率，以解释区域应力场的产生及其对地震发生的影响。一般来说，计算的房总半岛下的应力率在欧亚-菲律宾海板块界面的上方和下方都是水平伸展的。我们将计算出的应力率应用于观测到的地震的节点平面来计算库仑破坏函数 (ΔCFF)。这些计算出的 ΔCFF 对房总下的正常型地震通常是正的。ΔCFFs 也与地震活跃的相邻地区的地震一致，例如在南部的菲律宾海板块、伊豆半岛周围的碰撞带以及房总半岛东北部的欧亚板块群中，这进一步支持我们的应力加载模型。菲律宾海板块和太平洋板块俯冲各自贡献的计算表明，博索周围应力场的发展取决于两个俯冲板块的贡献。相比之下，伊豆半岛的弧-弧碰撞影响不大。