Tomography on PP-P waves and its application for investigation of the upper mantle in central Siberia

Abstract

We develop the method of tomographic inversion, called RR-R scheme, which is based on joint use of teleseismic P or S refracted rays and corresponding PP or SS rays with bounce points located within a study region. This scheme allows imaging the deep seismic structure beneath “blank” areas where there are neither recording stations nor earthquakes. Utilization of differential travel times makes it possible to avoid the difficulty of source and station corrections, which cause problems in teleseismic tomography. The RR-R scheme has been applied to more than 10 000 ray pairs from the ISC database to investigate a large region from the North Arctic Ocean to the northern part of China and Mongolia. Inversion was carried out in several separate windows covering the study area down to the depth of 530 km. Velocity anomalies were computed in grid nodes distributed in the study 3D area according to ray density. The tomography results from different windows were combined into one general map of the lithospheric structure in central Siberia. Principal positive P-wave velocity anomalies (+(1–3)% down to 400 km) are associated with the Precambrian Siberian craton. Negative velocity anomalies are related to the thin lithosphere of the West Siberian plate, the Altai Mountains and the Hangai plateau (−3–1% in the depth range 30 to 430 km). A strong negative anomaly beneath the Precambrian Hangai block is apparently associated with a mantle plume. Positive velocity anomalies in sedimentary basins at Southern Siberia are believed to have the crustal origin.