首都圈地区的地壳厚度及泊松比

利用2002~2003年中国地震局地质研究所台阵实验室以唐山大震区为中心布设的40个流动宽频带地震台站和首都圈数字台网的107个固定台站的远震数据，采用接收函数叠加搜索方法测定了首都圈地区地壳厚度和平均泊松比.综合利用首都圈数字地震台网的宽频带和短周期台站，以及流动地震台阵的观测数据，使我们的结果具有较前人更好的空间分辨率，为我们研究首都圈地壳的变形及其与地震的关系提供了重要的数据平台.结果表明: (1)首都圈地区地壳厚度和泊松比具有明显的横向分块特征，并与断层切割的地质块体有较好的相关性，地壳厚度变化形成了由涵盖北京—三河—唐山的NE向地壳厚度变化过渡带与张家口—北京—天津的NW向壳幔界面凹陷带构成的交汇构造，后者与所谓的张家口—渤海地震带基本吻合，而且本文给出的深部构造背景与首都圈地区的NE向和NW向地震带的交汇特征相吻合；(2)首都圈地区地壳厚度具有较大的差异，区域内地壳厚度的变化达15 km.其中，台站SZJ下方地壳厚度达到43.8 km，而台站BDH下方地壳厚度仅为28.8 km，总体上研究区西北侧的张家口—怀来地区的地壳厚度较大(~40 km)，而唐山以东地区地壳较薄(28~32 km)；(3) 研究区地壳的平均泊松比值为0.26左右，其最大值偏离泊松介质（σ=0.25）21%以上，而最小值偏离标准泊松比值9.6%，北京周边地区被高泊松比的介质环绕，而唐山东侧为低泊松比介质，地壳泊松比的分布特征反映了华北克拉通裂解过程中地幔物质的侵入；(4)研究区中强地震大多发生在地壳厚度和泊松比变化的陡变带，且偏于低泊松比的一侧，首都圈地震的成因仅考虑由于板块作用引起的水平应力场是不够的，有必要充分重视由于上地幔变形引起的地壳垂直变形和上地幔物质侵入造成的地壳变形运动与热效应.

The crustal thickness and Poisson's ratio beneath the Capital Circle Region

Simultaneously using the teleseismic waveform data recorded by 40 movable stations deployed by Laboratory of Lithospheric Seismic Array, Institute of Geology, China Earthquake Administration and 107 permanent stations of the Digital Network of the Capital Circle, we investigate the crustal thickness and averaged Poisson′s ratios beneath the stations by means of the grid-stacking-search method of receiver functions. Simultaneously using the data recorded by the broadband stations and short period stations makes our results have much higher spatial resolution than those given by the previous studies. Our results provide a data base for investigating the relationship between the earthquakes and crustal deformation. Our results show that: (1) the distribution of the crustal thickness and Poisson′s ratio over the Capital Circle region has an obvious laterally blocked feature related closely to the geological blocks incised by the faults. The variations of the crust thickness reveal a structural intersection of the transition zone along the NE direction through Beijing-Sanhe-Tangshan area and the depressed Moho belt along the NW direction across Zhangjiakou-Beijing-Tianjin region, which is roughly in accordance with the so-called Zhangjiakou-Bohai seismic belt, and the deep structure background given by this study is in accordance with the characteristic of NE and NW earthquake zones in the Capital Circle region. (2) The crustal thickness has a large variation of 15 km over the Capital Circle region. The crustal thickness is 43.8 km beneath the station SZJ, and the crust thickness beneath the station BDH is only 28.8 km. In general, the crustal thickness is ~40 km in the Zhangjiakou-Huailai region located at the northwestern side of the Capital Circle, and it becomes thinner (28~32 km) in the area east to Tangshan city. (3) The averaged Poisson′s ratio is about 0.26 over the area investigated. In comparison with the Poisson medium (σ=0.25), its maximum deviates 21%, and the minimum deviates 9.6%. The adjacent area surrounding the Beijing city has a high Poisson′s ratio, and the Poisson′s ratio of the area east to Tangshan is lower. The Poisson′s ratio distribution reveals the mantle material intrusions in the splitting process of the North China craton. (4) The middle and large earthquakes mostly took place in the abrupt variation belt of the crustal thickness and the Poisson′s ratio, and locate in the region with a lower Poisson′s ratio. Only considering the horizontal stress field caused by plate movement is not adequate for earthquake genesis in the Capital Circle and it will be necessary simultaneously considering the vertical crustal deformation caused by the upper mantle movement as well as the thermal effects due to the upper mantle material intrusion.