甘肃地区地壳速度的非均匀分布

THE INHOMOGENEITY OF CRUSTAL VELOCITY IN GANSU REGION

用天然地方震以及近震Ｐ、Ｓ波到时资料反演了甘肃７个分区的地壳结构。其速度非均匀分布表明：就垂向非均匀性而言，河西大于河东，海原、古浪两个８级大震区大于周围地区，横向非均匀性正好相反，但无论垂向还是横向非均匀性都是上地壳（大约以１９ｋｍ深度为界）大于中下地壳；青藏块体东北缘速度自ＳＳＷ向ＮＮＥ的东北角规则变化，上中地壳（约以３０ｋｍ深度为界）内沿该方向递减，下地壳及上地幔顶部递增；横向非均匀性以低速区带为主，大致沿祁连山断裂系向ＳＳＥ直到兰州附近，再沿近ＮＳ向到舟曲武都，中间与近ＥＷ向直到康乐和政的西秦岭北缘断裂低速带交接，高速区主要在嘉峪关、玉门以北，景泰附近及兰州以南等处；中强以上地震不仅在低速区带发生，而且也在高速区及其它地方发生，两个８级大震震源区速度在研究区中地壳内为最低，要注意兰州附近及张掖民乐地区可能潜在的强震危险区。

Using P and S wave arrivals of local and near earthquakes,the crustal structures' of 7subregions in Gansu were .inverted. The results show that:(1)vertical heterogeneity of velocity for Hexi region is greater than that for Hedong region, and those of focal regions ofHaiyuan and Gulang MS events are greater than those for other regions,but horizontal heterogeneity is contrary. Vertical and horizontal inhomogeneity of upper crust(bounded atabout 19 km depth) is greater than that of middle and lower crust;(2) velocity along thenortheast edge of the Qinghai-Xizang block decreases along SSW-NNE direction in a regularly manner for upper and middle crust(bounded at about 30 km depth),but increases for lowercrust; (3)horizontal inhomogeneity is characterized by outstanding lower velocity zone,whichstrikes along Qilian fracture system of SSE direction up to near Lanzhou, then extendssouthward to Zhouqu-Wudu,in middle section crosses lower velocity zone of the north edgefracture in western Qinling, but higher velocity areas locate mainly in north of Jiayuguanyumen, near Jingtai and south of Lanzhou etc.; (4) moderate and greater earthquakes occurred in not only lower velocity zones,but also higher velocity and other areas. The focalvelocity of the two M8 events mentioned above is the lowest in middle crust of studied region. Thus,future strong earthquake possibly will occur near Lanzhou and in Zhangye-Minleregion.