Seismotectonics beneath the Tokyo metropolitan area, Japan: Effect of slab-slab contact and overlap on seismicity
Seismotectonics beneath the Tokyo metropolitan area, Japan: Effect of slab-slab contact and overlap on seismicity
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DOI：
10.1029/2008jb006101
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发表时间：
2009-08-28
影响因子：
3.9
通讯作者：
Hasegawa, Akira
中科院分区：
文献类型：
--
作者：
Nakajima, Junichi;Hirose, Fuyuki;Hasegawa, Akira
我们首先根据由适当的一维速度模型重新定位的板间地震的分布来确定日本关东下太平洋(PAC)板块的上表面的形状。然后，利用关东地区6508次地震的735,520个P波和444,049个S波的到达时间，进行走时层析成像，估计关东地区的三维地震速度结构。结果表明，菲律宾海板块俯冲至130-140千米深度，没有空隙，甚至在伊豆碰撞带西北部。我们随后定义了PHS板底部和PAC板上表面之间接触区的横向范围(PHS-PAC界面)，并揭示了板接触区位于关东平原下方更大的区域之下。PAC板上板间(逆冲型)地震的下倾极限在板接触带下局部加深了约30公里。这种加深可能是由于PAC板块中温度较低的环境造成的，这是由于PHS板块与上方俯冲的PHS板块重叠，从而导致PHS板块与热地幔楔子的热屏蔽。我们在PHS板块的最东端发现了一个极低速度的异常，这可能是由于地幔橄榄岩的蛇纹岩化作用所致。PHS-PAC界面上的蛇纹化地幔覆盖的PHS-PAC界面几乎不发生板间地震，表明由于蛇纹岩的低粘度，沿该界面发生了韧性变形。
We first determine the configuration of the upper surface of the Pacific (PAC) slab beneath Kanto, Japan, from the distribution of interplate earthquakes relocated by an appropriate 1-D velocity model. Then, traveltime tomography is carried out to estimate three-dimensional seismic velocity structures around Kanto using 735,520 P wave and 444,049 S wave arrival times from 6508 local earthquakes. The obtained results suggest that the Philippine Sea (PHS) slab is subducting to depths of 130-140 km without a gap, even to the northwest of the Izu collision zone. We subsequently define the lateral extent of the contact zone between the bottom of the PHS slab and the upper surface of the PAC slab (PHS-PAC interface) and reveal that the slab contact zone underlies a wider area beneath Kanto in harmony with the Kanto plain. The downdip limit of interplate (thrust-type) earthquakes on the PAC slab is deepened by similar to 30 km locally under the slab contact zone. This deepening is probably caused by a lower-temperature environment in the PAC slab, resulting from the overlap with the PHS slab subducting above and consequent thermal shielding by the PHS slab from the hot mantle wedge. We detect an extremely low-velocity anomaly in the easternmost portion of the PHS slab, which is probably attributable to serpentinization of mantle peridotite. Interplate earthquakes are almost absent along the PHS-PAC interface overlain by the serpentinized mantle in the PHS slab, suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine.