Precursors to ScS Phases and dipping interface in the upper mantle beneath southwestern Japan
Precursors to ScS Phases and dipping interface in the upper mantle beneath southwestern Japan
复制标题
日本西南部上地幔中的 ScS 相前体和倾斜界面
DOI：
10.1016/0040-1951(80)90125-0
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发表时间：
1980
期刊：
影响因子：
2.9
通讯作者：
I. Nakanishi
中科院分区：
文献类型：
--
作者：
I. Nakanishi
在日本西南部四国和中部地区观测到的ScS相的纵向极化前兆被解释为ScSp到达，这是上地幔倾斜界面上ScSto-P转换的结果。文中还研究了日本东北部东北地区记录的一个ScSp位相。根据ScSp观测确定的四国地区下方转换界面的位置与根据地壳下地震活动模式推断的菲律宾海板块下降的上边界一致。在上地幔无地震活动的基础上，提出了菲律宾海板块下沉的前缘没有到达楚戈库地区下的上地幔的观点。然而，SCSP观测表明，菲律宾海板块在Chugoku地区下方的上地幔存在无地震延续的可能性。转换界面的另一种解释可能是可能的。本研究推断的界面可能对应于软流层与上一次板块会聚周期从南开海槽落下的无地震死板之间的边界。这种硫磷转换界面可能与中部地区晚第四纪火山活动密切相关。为了解释观测到的ScSp和ScS相之间的相对极性，在日本西南部和东北部的上地幔倾斜的ScS-P转换界面附近需要有低速带。岩石圈和上覆软流圈之间约为6%的速度差的一阶不连续，不能同时解释ScSp相的所有观测到的幅度、周期和极性。解释这些观测的最简单的模型是低速带，上面有尖锐的上界，下界有过渡的下界。这种低速带可能与洋壳的熔融或脱水有关，洋壳已经被下面的岩石圈板块俯冲。日本西南部的SCS-P转换界面比日本东北部的更具过渡性。这种转换界面性质的差异必须与两个地区地质历史的差异相对应。菲律宾海板块在九州地区的高倾角和四国地区的低倾角表明，本戈海峡和塞沃内海西部Iyonada之下的正断层次地壳地震可以用不同倾角的两个板块交界处的撕裂和铰断层来解释。
Longitudinally polarized precursors to ScS phases observed in the Shikoku and Chugoku districts, southwestern Japan, are interpreted as ScSp arrivals, resulting from ScSto-P conversions at a dipping interface in the upper mantle. An ScSp phase recorded in the Tohoku district, northeastern Japan, also is examined. The location of the conversion interface, beneath the Shikoku district, determined from the ScSp observations agrees with the upper boundary of the descending Philippine Sea plate inferred from the seismicity pattern of subcrustal earthquakes. It has been proposed on the basis of no seismic activity in the upper mantle that the leading edge of the downgoing Philippine Sea plate has not reached the upper mantle beneath the Chugoku district. The ScSp observations, however, present a possibility of the existence of an aseismic continuation of the Philippine Sea plate in the upper mantle beneath the Chugoku district. An alternative interpretation of the conversion interface may be possible. The interface inferred in the present study may correspond to a boundary between the asthenosphere and an aseismic dead slab which had descended from the Nankai trough at the previous cycle of plate convergence. This ScS-to-P conversion interface may be closely related to the late Quaternary volcanism in the Chugoku district. Low-velocity zones are required in the vicinity of the inclined ScS-to-P conversion interfaces in the upper mantle beneath southwestern and northeastern Japan in order to explain the observed relative polarity between the ScSp and ScS phases. The first-order discontinuity of the velocity contrast of about 6%, which has been suggested between the lithosphere and overlying asthenosphere, cannot simultaneously explain all of the observed amplitudes, periods, and polarity of the ScSp phases. The simplest model which explains these observations is the low-velocity zone with the sharp upper and transitional lower boundaries. This low-velocity zone may be related to the melting or dehydration of the oceanic crust which has been subducted with the underlying lithospheric plate. The ScS-to-P conversion interface inferred beneath southwestern Japan is more transitional than that beneath northeastern Japan. This difference in nature of the conversion interfaces must correspond to the difference in geological histories of both the regions. A high dip angle of the Philippine Sea plate in the Kyushu district and a low dip angle for the Shikoku district suggest that the normal-faulting subcrustal earthquakes beneath the Bungo channel and Iyonada, the western part of the Sevo Inland Sea, can be explained in terms of tearing and hinge faulting at the junction of the two slabs with different dip angles.