Inversion of 3-D crustal p-wave velocity structure in Ningxia and its neighborhood by using direct, reflected and refracted waves

In this paper, simultaneous inversions for hypocentres and three-dimensional (3-D) crustral P-wave velocitystructure of Ningxia and its neighborhood are performed, by using the data of direct P wave, reflected Pm andrefracted Pn phases from the Moho discontinuity. The number of arrival-time applied to the inversion is 11 816total, which are recorded by the Seismic Station Network of Ningxia from 1 107 local natural earthquakes happened since 1970. The LSQR algorithm with damping that has been developed in recent years to solve the largescale sparse matrix is adopted. At different steps of iterative computing process, weighting is made on the actionsof each earthquake at first and on each ray further, to aim at the inhomogeneous features of spatial distribution ofearthquake and ray. Numerical analogue and computational results of actual multi-group data and different iterative control processes, show that under a certain residual standard, there is obvious trade-off on the assignment oftravel-time residuals between medium parameters and earthquakes parameters (especially the origin time anddepth of earthquake), so the obtained solution is related to the selected initial values of medium parameters tosome extent, but there are the basically same variation features on different results in the almost same area. That isthe solution of surface layer has a close relation to the strata and landform. In Yinchuan basin there is obvious lowvelocity feature. and it coincides with the results of a man-made earthquake section crossing the region, tomographic patterns show that there are many low velocity regions in middle-lower crust, and it seems to have someconnections between the positions of historical strong earthquakes and the low velocity region or the anomalousvariations of velocity gradient.