Geometry of the Philippine Sea Slab and the Convergent Tectonics in the Tokai District, Japan
Geometry of the Philippine Sea Slab and the Convergent Tectonics in the Tokai District, Japan
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菲律宾海板的几何形状和日本东海地区的聚合构造
DOI：
10.4294/zisin1948.49.3_295
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
S. Noguchi
中科院分区：
文献类型：
--
作者：
S. Noguchi
根据近14年来观测到的11，340个重新定位的震源和重新计算的约20%的震源机制解，我们确定了日本东海地区菲律宾海板块的上边界。利用不同方位的震源剖面和P、T轴分布来描述PHS板块的结构。Wadati-Benioff带的厚度约为5至7公里，几乎相当于洋壳。由此产生的板片上边界的总体几何形状是向海凸起的，并从Pt下方约20 km深度处形成NW至NNW走向的“板片谷”。从御前崎到本州岛中部70公里深处的最深锥形末端。板块的区域性凹陷形状合理地解释了板块边界沿着的地震活动性和震源机制的特征，即PHS板块俯冲球壳的守恒，以及过去PHS板块和上覆板块之间相对板块运动方向的变化。从滨名湖下到1891年Nobi大地震震源区南端40 km深度处，沿相对板块运动方向的沿着缓倾板块边界存在强耦合。骏河海槽的板片俯冲在垂直于海槽轴线的方向上具有较陡的倾角。一个显着的向陆倾斜集群上覆的Wadati-Benioff区在中低地壳深度下骏河湾西海岸。丛集地震的震源机制表现为P轴近水平、T轴近垂直的逆滑断层。一个薄的非磁性层之间的集群和下面的Wadati-Benioff区夹层。从南马格纳窝的增生构造历史和其他地球物理和地质证据，我们推断该集群是由底侵的海洋物质，提供了一个海洋到大陆的过渡地震层。根据自1854年安西东海地震以来持续的倾斜群上方的大规模地壳沉降，底侵物质可能与下面的海洋板块一起通过薄的地震层向下拖曳。我们解释说，薄的，可能是部分强夹层的海砂层是下一个“东海地震”的核的网站。另一个重要特征是，较深的Wadati-Benioff带似乎延续到骏河海槽和伊豆半岛下方较浅的缓倾地震带。这表明一个替代的构造过程，一个新的会聚板块边界正在骏河海槽和伊豆半岛下方产生并向海延伸。
We determine the upper boundaries of the Philippine Sea (PHS) plate in the Tokai District, Japan, based on a total of 11,340 relocated hypocenters and recalculated mechanism solutions of about twenty percent of hypocenters, for earthquakes observed during recent about 14 years. Variously oriented cross sections of hypocenters and P and T axes distributions are used to delineate the configuration of the PHS slab. The thickness of the Wadati-Benioff zone which nearly corresponds to the oceanic crust is about 5 to 7 km. The resultant overall geometry of the upper boundary of the slab is convex seaward and forms NW-to-NNW trending `slab valley' from the point of about 20 km depth beneath Pt. Omaezaki to the deepest tapered end of 70 km depth beneath central Honshu. Regional sinus shapes of the slab reasonably explain the features of seismicity and focal mechanisms along the plate boundary, in terms of the conservation of the subducted spherical shell of the PHS plate, and the change in the direction of relative plate motion between the PHS and the overriding plates in the past. A strong coupling is suggested along gently-dipping plate boundary in the direction of relative plate motion extending from the point beneath Lake Hamana to the point of 40 km depth beneath the southern end of the source area of the great 1891 Nobi intraplate earthquake. The slab subducting at the Suruga trough has rather steeply-dipping angle perpendicular to the trough axis. A remarkable landward-dipping cluster overlies the Wadati-Benioff zone at the mid to lower crustal depths beneath the western coast of Suruga Bay. The focal mechanisms of the clustered events show reverse-slip faultings with subhorizontal NW-SE oriented P axes and near-vertical T axes. A thin aseismic layer is intercalated between the cluster and the underlying Wadati-Benioff zone. From the history of accretion tectonics and the other geophysical and geological evidences in the South Fossa Magna, we infer the cluster is composed of underplated oceanic materials presenting an ocean-to-continent transitional seismic layer. The underplated materials may probably be dragging downward coupled with the underlying oceanic plate through the thin aseismic layer, in accordance with the large crustal subsidence just above the dipping cluster continuing since the 1854 Ansei Tokai earthquake. We interpret that the thin and probably partially-strong intercalated aseismic layer is the site of nucleation of the next `Tokai earthquake'. Another important feature is that the deeper Wadati-Benioff zone seems to continue to the shallower gently dipping seismic zone beneath the Suruga trough and the Izu Peninsula. This suggests an alternative tectonic process that a new convergent plate boundary is creating and extending seaward underneath the Suruga trough and the Izu Peninsula.