Receiver Function Imaging of Dipping Structures - Technique and Applications

CCP Stacking is probably the most common technique in receiver function imaging. In this technique, the conversion points are calculated and binned by assuming horizontal interfaces. However, for dipping structures such as tilted Moho or subducting slab, the horizontal interface assumption breaks down so that the image quality is low and the interfaces are misplaced. In these cases, the CCP stacking is not effective because the true conversion points do not necessarily lie in the vertical planes defined by sources and receivers and the delay time of conversion signals are varying with back azimuth. Although pre-stack time migration is an ideal technique to make correct images, the amount of qualified data it requires is often not available. An alternative way is to gather receiver functions based on conversion points and conversion times calculated from 3D velocity models with dipping structures. A dipping interface is defined by depth (d0) at a fixed geographic location, strike (Φ) and dipping angle (α). For an assumed (d0, Φ, α), we computed the conversion locations and conversion times by 3D ray tracing. We varied the depth (d0) and the dipping geometry (Φ, α) in certain range and determined the optimum value that gives the best coherent stacking signals. Note based on different tectonic setting, one or both of the dipping parameters can be fixed in the grid searching. We applied this technique to the imaging of two different geologic structures. One is a subducting slab case in western Mexico where the Rivera plate is subducting under the North American plate. Receiver functions recorded by fifty broadband seismic stations deployed under the MARS project were used to image the subducting Rivera slab. The second case is a tilted Moho lying underneath the GSN station SDV located at the southern flank of the central Venezuelan Andes. In both cases, we found substantial improvements in the receiver function images using our 3D ray tracing technique.