Oceanic Anisotropy from PS Splitting

Abstract:

Observations of seismic anisotropy (e.g. shear wave splitting) can provide direct insights into the relationship between mantle convection in the Earth’s interior and plate motions at the Earth’s surface. Receiver based seismic observations over oceanic plates are however currently restricted due to the concentration of seismic receivers on land. This project suggests a method to overcome this limitation by utilizing the distribution of surface reflected seismic phases, such as PS, which bounce once between the source and the receiver. From these information is extracted about the seismic anisotropy beneath oceanic bounce-points, which is used to construct an observational dataset of hundreds of manually analyzed shear-wave splitting results with direct implications for mantle convective patterns beneath oceanic plates. We observe bi-modal fast directions, which may indicate known systematic errors when the back-azimuth is near the fast/slow axis of anisotropy. Delay times are comparable with previous shear-wave splitting studies (≈2 s). Results suggest potential deviations of the anisotropic fabric from absolute plate motions. We suggest that more careful investigation of anisotropy below old seafloor is necessary in order to determine the presence of small-scale convective patterns and layered anisotropy.