Global Images of the Upper 200 km in the Earth by Short-Period Surface Waves

Abstract

A global shallow Earth structure is retrieved from short-period surface wave data (40-200 s) using a data set assembled from global digital networks. Phase velocity maps for both Love and Rayleigh waves are retrieved by spherical harmonic inversion. A comparison with results by block inversion is also presented, and the similarity of the results by the two different methods supports the belief that the data are capable of resolving most details in final velocity maps. Strong effects of crustal structure are evident in the patterns of short-period Love wave phase velocity variations, although depth resolution kernels show that the complete resolution of crustal structure is not possible from this period range of data. Two models of S-wave velocity maps are presented, one with an assumed crustal structure and topography and the other without such an assumption. Major differences between the two models occur just below the moho; the one with an assumed crustal structure shows the beginning of tectosphere-like structure immediately below the moho under cratons, while the model without it shows the beginning depths of tectosphere-like structure between 50 and 80 km. It is obvious that the short-period cut-off in this study (40 s) is not sufficiently short to resolve shallow structures in detail. A few interesting tectonic features in the models include: 1) S-wave velocity under ridge axes is slow in the upper 200 km and is very slow in the upper 100-120 km, due possibly to partial melting at shallow depths; and 2) depth slices under the Himalaya-Tibet region show fast velocity anomaly extending from India to the underside of Tibet. While it is difficult to distinguish between a subducted oceanic lithosphere and a thickened continental lid, the shape of the contour lines seems to favor the interpretation that this fast velocity anomaly is the underthrusting Indian ocean lithosphere.