Improving the vertical and horizontal resolution of seismic anisotropy and heterogeneity using surface waves

Brown University, Providence RI

Investigators

Abstract

Earthquakes and ocean waves generate seismic waves that travel along the surface of the earth, but with vibrations that penetrate tens to hundreds of kilometers into the solid interior. The velocities of these waves are one of our primary means of measuring the structure of the tectonic plates (lithosphere) and the underlying convecting mantle (asthenosphere). One of the best ways to measure the velocity is with arrays of seismic stations, either on land or on the sea floor, but the measurements are made more difficult due to multiple propagation paths from the earthquakes to the stations and by uneven distribution of ocean wave sources. In this project, we will develop new techniques to overcome these difficulties to improve our measurements of seismic surface wave velocity and therefore our knowledge of the structure of the lithosphere and asthenosphere. One primary goal of the new measurements is to test whether deformation of the oceanic lithosphere is fundamentally different than the asthenosphere, a controversial hypothesis that a number of recent studies have supported. The analysis of Love and Rayleigh wave dispersion within ocean-bottom-seismometer arrays in the Pacific will test the hypothesis that radial anisotropy within the oceanic lithosphere is distinctly different than within the asthenosphere. Several groups of investigators have reported from global tomographic studies that radial anisotropy in the upper lithosphere is either small or characterized by SV>SH in contrast to the strong anisotropy in the asthenosphere with SH>SV. This contrast has been interpreted as indicating either the depth extent of the lithosphere or the depth extent of a depleted layer, but is in disagreement with earlier studies that showed SH>SV decreasing gradually with increasing depth. To improve the depth resolution and resolve the ambiguity about the extent of lithospheric radial anisotropy and strength of asthenospheric anisotropy, we will: (1) extend the measurement of fundamental mode Love wave dispersion to shorter periods, (2) add measurements of higher mode phase velocity for Love and Rayleigh waves, and (3) extend higher mode analysis to short periods using ambient noise correlation to fill in the gap in sensitivity to SV in the shallow lithosphere.