Deep seismic structure of the Earth's crust along the Baltic Sea profile

Abstract

Abstract In summer of 1989 the Institute of Oceanology of the Russian Academy of Sciences (Moscow) for the first time used a new large volume airgun for deep seismic sounding (DSS) in the Baltic Sea. The experiment was carried out during the 23-rd cruise of R/V Professor Shtockman. The wave-field analysis showed clear P and S wave arrivals, including Moho reflections. The Moho reflections were distinctly observed at distances exceeding 100 km. The P waves can be followed to a maximum distance of 368 km. Two-dimensional raytracing modelling of the data revealed a pronounced topography of the Moho along the BALTIC SEA profile: a depression in the middle of the profile at a depth of 45 km is bounded by 2–3 km steps and uplifts on both sides. Reflections off a sub-Moho boundary at a depth of 60 km have also been recorded. The upper mantle velocities vary between 7.8 km/s under the depression to 8.1 km/s to the side of it. The crustal P waves velocities define three layers: 5.8–6.3, 6.3–6.7 and 6.9–7.1 km/s. All crustal velocity boundaries bend over the Moho depression and reach their deepest points in the central part of the profile. The velocities in the crust also change laterally, reaching their minimum within the central depression. The crustal thickness along the profile varies notably between 38 and 45 km. A comparison with the FENNOLORA and the BABEL B profiles indicates that the Moho depression found along the BALTIC SEA profile correlates with similar depressions on the other profiles.