Seismic detection of the untransformed ‘basaltic’ oceanic crust subducting into the mantle
Seismic detection of the untransformed ‘basaltic’ oceanic crust subducting into the mantle
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地震探测未变形的“玄武岩”洋壳俯冲到地幔中
DOI：
10.1111/j.1365-246x.1985.tb05162.x
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发表时间：
1985
影响因子：
2.8
通讯作者：
M. Ukawa
中科院分区：
文献类型：
--
作者：
S. Hori;H. Inoue;Y. Fukao;M. Ukawa
摘要壳下地震发生在日本西南部的板状带，与菲律宾海板块的俯冲有关。它们产生了一对明显的P和S晚期相，其视速度非常低，约为7和4 km s-1，这不能用横向均匀的速度结构来解释。这些后期阶段是针对特定的震源-接收器几何形状观察到的：震源深度浅于50-60公里，接收器在其下地壳下地震带与大陆地壳底部接触。我们使用两种方法研究了这些后期阶段的性质。一种是确定两种类型的视速度，“事件共同”视速度和“台站共同”视速度，这可能是彼此不同的横向不均匀的速度结构。另一种是对横向非均匀速度模型进行地震射线数值追踪。使用这两种方法进行的广泛分析得出的结论是，板状地震带构成了上地幔中的低速通道，其P和S速度与日本西南部下陆壳的速度相当。被困在低速通道内的波通过与大陆莫霍面的接触而逃逸，并被观察为一对不同的后期相位。由低速通道底边界引导的首波被观测为初相的弱波至。壳下地震带的厚度（<10 km）和速度（VP=7 km s-1，VS=4 km s-1）表明，它是由与下伏岩石圈地幔俯冲的“未转化的玄武质洋壳”组成的，至少在50-60 km深处的俯冲板片中，玄武岩不发生向榴辉岩的转化。我们提出了横向不均匀结构模型的地壳和上地幔沿着两个剖面在日本西南部。这些模型的特点是，由于俯冲洋壳与大陆莫霍面的直接接触，在特定地区下陆壳明显增厚。这些特征在定性上与爆炸地震学结果以及相关地区的布格异常模式一致。在俯冲后，顶部有未变形的“轻”洋壳的海洋岩石圈可能仍然是浮力的，这是在考虑板块构造的驱动机制、板片几何形状和与地震有关的构造过程时必须考虑的一个因素。
Summary. Subcrustal earthquakes occur in a slab-like zone beneath SW Japan in association with the subduction of the Philippine Sea plate. They generate a distinct pair of P and S later phases with anomalously low apparent velocities of about 7 and 4 km s−1, which cannot be explained by a laterally homogeneous velocity structure. These later phases are observed for particular source–receiver geometries: the sources at depths shallower than 50–60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust. We examined the nature of these later phases using two methods. One is to determine two types of apparent velocities, an ‘event-common’ apparent velocity and a ‘station-common’ apparent velocity, which may be different from one another for a laterally inhomogeneous velocity structure. The other is to trace seismic rays numerically for a laterally heterogeneous velocity model. An extensive analysis using these two methods leads to a conclusion that the slab-like seismic zone constitutes a low-velocity channel in the uppermost mantle with P and S velocities comparable to those of the lower continental crust beneath SW Japan. The waves trapped within the low-velocity channel escape from it through its contact with the continental Moho and are observed as a distinct pair of later phases. The head waves guided by the bottom boundary of the low-velocity channel are observed as weak arrivals of initial phases. The thickness (<10 km) and the velocities (VP=7 km s−1, VS=4 km s−1) of the subcrustal seismic zone indicates that it is made up of the ‘untransformed basaltic oceanic crust’ subducted with the underlying lithospheric mantle and that the transformation of basalt to eclogite does not take place in the subducting slab at least down to depths of 50–60 km. We present laterally heterogeneous structural models of the crust and the uppermost mantle along two profiles in SW Japan. The models are characterized by an apparent thickening of the lower continental crust in particular regions due to a direct contact of the subducting oceanic crust to the continental Moho. Such features are qualitatively consistent with the results of explosion seismology and with the Bouguer anomaly pattern in the relevant regions. The oceanic lithosphere with the untransformed ‘light’ oceanic crust at its top might still be buoyant after subduction, a factor which must be taken into account in considering the driving mechanisms of plate tectonics, the slab geometry and the earthquake-related tectonic process.