甘肃东南部地壳速度结构的区域地震波形反演

Crustal structure in southeastern Gansu from regional seismic waveform inversion

利用2007年完成扩建的甘肃东南部及邻近地区的24个宽频带固定地震台站记录到的2008年8月1日汶川地震余震的三分量地震全波形资料,采用小生境遗传算法和反射率法结合的波形反演方法,给出了甘肃东南部两个分区的地壳速度模型.西区和东区分别对应青藏高原块体和它与鄂尔多斯块体之间的过渡带,反演给出的平均模型显示,两个区上、中地壳的平均P波速度非常接近,由酸性岩和中性岩组成,下地壳P波速度差别较大,东区为6.41km/s,西区为6.96km/s,厚度相差也较大,东区为9.3km,西区为19.8km,地壳厚度由西向东减小,分别为54.6km和47.9km.显示西区下地壳由基性岩组成,而东区下地壳由中性岩组成,支持研究区内青藏高原东北缘地壳增厚主要发生在下地壳的观点.西区的上地幔顶部P波速度为7.73km/s,对应年轻的构造活动区,而东区为8.05km/s,对应稳定的古老地块.东区在上地壳上部存在厚约6.5km的沉积层,P波速度为5.78km/s,但是该沉积层在西部缺失.和PREM模型给出的全球平均地壳速度值相比,两个分区的地壳速度值整体偏低.

We applied three-component full-waveform inversion combining niching genetic algorithm and reflectivity method to the southeastern Gansu and obtained average crustal structure for two regions.The western and eastern regions are located at the northeastern margin of Qinghai-Tibet plateau and its transition zone to the Ordos block.The waveform data was recorded by 24 broadband permanent seismic stations implemented in 2007 from an aftershock of Wenchuan earthquake occurred on August 1,2008.The final average models show that the western and eastern region has similar P-wave velocity in the upper and lower crust,which are composed of felsic and intermediate materials.The P-wave velocity is 6.96 and 6.41 km/s,and the thickness is 19.8 and 9.3 km at the lower crust for the western and eastern region,respectively.The total crustal thickness is 54.6 and 47.9 km for the western and eastern region.These results imply that the lower crust is composed of mafic in the western region and intermediate material in the eastern region,and support that the crust thickening in the Qinghai-Tibet block in the study region mainly occurred in the lower crust.The P-wave velocity for the upper mantle is 7.73 km/s at the western and 8.05 km/s at the eastern region,which corresponds to the young active tectonic region and old stable block,respectively.A sedimentary layer of 6.5 km thick is found at the eastern region with P-wave velocity of 5.78 km/s,but it is absent at the western region.Comparing with the global average crustal P-wave velocities given by PREM model,the crust in this region shows lower P-wave velocity values.