蒙古中南部地区噪声层析成像

Ambient noise tomography in central-south Mongolia

依托中蒙国际合作项目＂远东地区地磁场、重力场及深部构造观测与模型研究＂,我们首次获取了蒙古中南部地区密集的地震台阵观测资料.本研究收集了69套宽频带台站2011年8月至2013年7月间的垂直向连续记录,利用噪声互相关方法计算了台站间的经验格林函数,并采用基于连续小波变换的频时分析方法,提取了1478条周期6~30s的瑞雷波的相速度频散曲线.利用Ditmar＆Yanovskaya方法,我们构建了研究区6~30s瑞雷波的高分辨率（0.5°×0.5°）相速度分布图.结果表明,蒙古中南部地区的地壳上地幔速度结构存在横向非均匀性,但并不显著（相对变化~±2%）.短周期（如6s）的相速度分布与地表地质构造具有明显的相关性,具体来说,北部山盆显示为高速,南部盆地、戈壁滩均显示为低速;随着周期的增大（如15s,20s）,地形的控制作用相对减弱.较长周期（30s）的相速度分布图上,南部的高速区进一步扩大,与南部地壳厚度较薄有关.蒙古主构造线南北两侧相速度分布有明显差异,暗示它不仅是地表地形和构造的分界线,而且还是地壳结构的分界线.从6~30s中戈壁一直显示为低速,可能与该区新生代火山活动有关;研究区北部杭爱山—肯特盆地一直显示为相对的高速,与该区具有较老、稳定的地层有关.

Currently,the detailed structure beneath central-south Mongolia（103.5°E—111.5°E,43°N—49°N）is poorly known from previous studies.In order to investigate the detailed structure of the study region,69 portable broadband stations were deployed in central-south Mongolia from August 2011 to July 2013.This cooperation project in Mongolia provided dense seismic array datafor the area for the first time.Ambient noise tomography is increasingly used in studying the structure of the crust and upper mantle.We calculated the inter-station empirical green functions（EGFs）from crosscorrelation using the vertical component of the continuous data recorded by these 69 broadband seismic stations from August 2011 to July 2013 in south-central Mongolia.In addition,a timefrequency analysis based on a continuous wavelet transform was used to extract the Rayleigh wave phase velocity dispersion curves.Through quality control and manual screening,we finally obtained a total number of 1478 phase velocity dispersion curves at periods ranging from 6sto30 s.The Ditmar Yanovskaya method was utilized to obtain phase velocity maps of the Rayleigh waves at periods of 6~30sin the study area.Checkerboard tests showed that the tomographic results had a high resolution of 0.5°×0.5°.The results revealed that the phase velocity maps of the Rayleigh waves had a perturbation of about±2%.A phase velocity map with a short period（e.g.,6s）was imaged,with high-speed anomalies corresponding to the mountain ranges in the north and low-speed anomalies coinciding with the sedimentary basin and Gobi Desert in the central-south region.As the period（15s,20s）increased,the imaging still showed a high-velocity zone（HVZ）in the north and low-velocity zone（LVZ）in the middle.The phase velocity maps with a long period（e.g.,30s）showed an HVZ in the north that expanded further to the south than those with shorter periods（e.g.,15 s and 20s）,which is associated with the thinner crust in the south compared to that in the north.On those maps with long periods（e.g.,20 s,30s）,there were significant differences between the northern and southern sides of the main Mongolian lineament（MML）.On maps with periods ranging from 6sto 30 s,the middle Gobi area was imaged with an obvious low speed,while the Hangay-Hentey basin was always imaged with an obvious high velocity in the north. We compared the tomographic result at 15 swith that from the classic two-station method using earthquake data,and a phase velocity difference of only about 1% was found.The S-wave velocity structure of the crust and upper mantle showed weekly lateral heterogeneity（aperturbation of about ±2%）in central-south Mongolia.The phase velocity distribution at a short period（e.g.,6s）was effectively related to the geology tectonic units on the surface.However,the effect of the phase velocity distribution controlled by the surface geological structure was significantly weaker as the periods increased（e.g.,15 s,20s）.In the phase velocity maps with a long period（e.g.,30s）,the phase velocity distribution was mainly associated with the crustal thickness.For the MML,this was not only a boundary for the topography and tectonics,but also for the crustal structure.The middle Gobi area always showed an LVZ,which could have been related to Cenozoic volcanism,while the Hangay-Hentey basin was always imaged with an HVZ,which could have been associated with the old,stable layers in the north.