TOMOGRAPHIC ESTIMATES OF SUB-MOHO SEISMIC VELOCITIES IN FENNOSCANDIA AND STRUCTURAL IMPLICATIONS

Pn and Sn velocity estimates are derived for the sub-Moho structure beneath Fennoscandia using a tomographic conjugate gradient scheme. Observational data stem from local earthquake recordings of crustal phases Pg, Sg (Lg) and the sub-crustal phases Pn and Sn by the Fennoscandian seismograph network. Unknowns are Pn and Sn velocities and event hypocenter parameters. Crustal thicknesses are assumed as known and are not estimated, whereas velocities are allowed to vary. The most prominent P and S velocity anomalies are found in the northwest (inland Lofoten) and southwest (More) of Norway and, for shear waves, also in central Finland. There is a good correlation between the velocity anomalies in Norway and observed negative Bouguer anomalies, indicating the presence of low-density sub-Moho material in these areas. Geologically the Pn and Sn velocity anomalies appear to be associated with geodynamic processes tied to the opening of the Norwegian-Greenland Sea.