河北省强震区内外深部S波速度结构特征研究

Study on the structure of deep S wave velocity inside and outside macroseismic region of Hebei province

利用短周期记录的 S波记录反演了沙城老震区及其邻区共 3个台站下方的剪切波速度结构 ,并进行了对比分析。结合以前得到的邢台、唐山震区及其附近 5个台站的速度结构 ,发现强震区具有以下几个大致相同的构造特征 ,即 :(1 )地壳厚度低于周围台站 ;(2 ) S波低速层速度低于周围地区 ,高低速层差异大 ;(3 )具有较低的上地幔 S波速度 ;(4)强震往往发生在低速层上面的高速层内或高低速层交界面上 ;(5 )易震层往往是两个低速层夹持的高速层等 ,并对这种结构易于发震的机理给出了分析。总之强震的发生是与低速层紧密相关的 ,而剪切波对于反映低速体构造非常敏感 ,因此 S波速度结构研究对于揭示强震的深部构造背景 ,深入研究震源机制都是很有意义的。而且 ,强震区所共同具有的这种特征为我们今后强震危险区的划分和强震地点的预测提供了较为科学的依据。

The shear velocity structure under 3 stations in-the old earthquake region of Shacheng and its neighboring areas is inversed with short-period S wave records.All the results are analyzed by comparison.Combined with the velocity structures under other 5 stations in Xingtai and Tangshan earthquake region,the commoncharacters of the velocity structures under macroseismic region are found,i.e.(1)the thickness of the crust is thinner than that of the stations around;(2)the velocity in low velocity layer of S wave is much lower than that of its vicinity,and the difference between high and low velocity layer is much bigger;(3)the S wave velocity in upper-mantle layer is lower under the region;(4)strong earthquakes usually took place in the high veliocity layer above a low velocity layer or on the interface of high and low velocity layer;(5)usually,the earthquake prone layer is the high velocity interlayer of two low velocity layers.(The earthquake prone mechanism of such structure is analyzed).In short,the occurrence of strong earthquake is closely related with the low velocity layer,which shear wave is sensitive to its structure.So the study on S wave velocity structure is significant to revealing the background of deep structures and focal mechanism under strong motion region.Simultaneously,these common characters under strong motion region provide a scientific basis for the division of macroseismic risk region and the place prediction of potential strong earthquake.