【摘要】 1976-07-28唐山地区发生了震惊中外的7.8级大地震。为什么在华北古老克拉通内部的唐山地区能够发生如此的大地震一直是一个令人费解的问题。是否会在唐山地区再次出现同样的破坏性地震值得认真研究。利用流动地震观测台阵数据和接收函数反演方法,我们研究了唐山地区60 km深度范围内的三维地壳上地幔速度结构。结果表明:(1)由活动断裂切割的唐山断块与周围介质存在明显差异,围限唐山断块的断裂均为超壳的活动断裂;(2)唐山大震区中上地壳具有明显的非均匀壳内低速体;(3)该地区壳幔界面表现为明显的断块式隆升,与两侧相比,唐山菱形地块下方的上地幔顶部异常隆起的高度达到10 km左右,下伏的上地幔具有异常的非均匀结构;(4)唐山大震区可能有幔源物质较大规模的侵入,形成了中、上地壳内的低速体。由于较已往的研究结果有更高的空间分辨率,我们得到了一些以往尚未发现的有关唐山地区深部结构的异常特征;(5)首都圈地区内破坏性地震发生的地点绝非偶然,它们均与其相应的深部构造背景密切有关,这为强震发生地点的预测提供了可能。根据本文结果,我们认为,1976年唐山大地震的主因源于上地幔的垂向运动变形及壳幔之间物质及能量的交换,区域水平向应力场为次要作用。这有助于解释为什么能够在我国华北古老克拉通地区发生7级以上强震,在唐山地区再次发生7级以上大地震的可能性值得给予进一步的研究和关注。更多还原

【Abstract】 In July 28,1976,the great Tangshan earthquake shocked the whole world and occurred in the Tangshan area of Hebei Province.An important question is why such a large earthquake occurred within the ancient North China craton.It is worth considering whether a similar event will reoccur in the Tangshan region.In this study,using the receiver-function inversion technique and teleseismic P waveform data recorded at the Capital Circle Seismic Network and our movable seismic array,we investigated the 3D S-wave velocity structure of the crust and upper mantle down to 60 km beneath the Tangshan area.Our results show that(1) the media beneath the Tangshan block cut by active faults are very different from the adjacent areas and all of the active faults surrounding the Tangshan block was through the whole crust;(2) in the upper and middle crust,there exist obvious heterogeneous low-velocity media beneath the Tangshan earthquake region;(3) the crust-mantle boundary has an obvious block uplift and,in comparison with both sides,the top anomalous uplift of the upper mantle beneath the Tangshan block reaches to 10 km,and the upper mantle beneath has an anomalous heterogeneous structure;(4) beneath the Tangshan earthquake region,there are probably massive intrusions derived from the upper mantle,which form the low-velocity body in the upper and middle crust.Because of our results having much higher resolution than previous results,some new features of the crust and upper mantle velocity structure could be shown in this study;(5) The locations of destructive earthquakes are not random and are related closely to their deep structure of the crust and upper mantle.This provides a possibility of correctly estimating the location of destructive earthquakes.Based on our results,we discuss the genesis of the Tangshan earthquake.We consider that the main dynamic source for the Tangshan earthquake is the vertical movement of the upper mantle,which leads to material and energy exchange between the crust and upper mantle.The horizontal stress field plays a subordinate role.This observation facilitates our understanding of why the Tangshan earthquake with the magnitude larger than 7.0 occurred within the North China craton.A problem worth-considering is whether there is a possibility that same magnitude events will reoccur in the Tangshan region.更多还原