Global Common Conversion Point Stacking and its Applications

Abstract

Imaging of globally varying subsurface structures is traditionally done by using seismic tomography to map large volumetric anomalies. Methods that image sharp discontinuities in seismic impedance have been restricted to regional or local studies. A global, three-dimensional method is developed that stacks converted earthquake waves originating in such discontinuities, as a function of their conversion points. The method relies on the database GLImER, which contains a global dataset of earthquake recordings that were processed into records of seismic impedance contrasts, namely receiver functions. The implementation of the method follows four steps that are (1) tracing the theoretical conversion points of the receiver functions, (2) creating a global bin grid, (3) binning the receiver functions as a function of their theoretical conversion points, and (4) traveltime-correcting and stacking the binned receiver functions. The stacks are computed with phase-weights to efficiently attenuate incoherencies in the signal. Furthermore, our method takes advantage of the crustal multiples to further enhance the stacks. We test our developed method on both synthetic data created with RAYSUM and real data of the Cascadia subduction zone with the conclusion that dipping discontinuities of up to 20° can be smoothly imaged. Furthermore, the subducting slab was shown to have a 4° southwards dip orthogonal to the convergence of the Juan de Fuca and the North American plate. Following the tests, the method is applied to the East African continent to create two almost continuous cross sections from the Afar Region to Cape Town. The cross sections are used to survey both the topography of the Mohorovičić (Moho) discontinuity and the mantle transition zone (MTZ) discontinuities at ∼410 and ∼660 km. Our images show that (1) the Moho is predominantly affected by Africa’s tectonic history, showing jumps at nearly every tectonic boundary with shallowing from northern Kenya towards the Afar region, and that (2) the MVZ is generally thinning from South Africa to the Afar region with an abrupt thickening towards the Arabian Peninsula. Both the shallowing of the Moho and the thinning of the MTZ are attributed to the mantle upwelling previously imaged by seismic tomography. The abrupt thickening of the MTZ in the north-east of the Afar region is attributed to a change in mantle flow direction which manifests in a steep, negative temperature gradient towards the Arabian peninsula in consistency with geodynamically modelled mantle flow.