Spatial variations of P residuals and deep structure of the European lithosphere

Abstract

Summary. Relative travel‐time residuals of 224 earthquakes and nuclear explosions at distances from 20° to 100° were calculated for 100 European seismic stations using the Jeffreys‐Bullen tables. The P residuals were corrected for the Earth's ellipticity, altitude of the stations, as well as the thickness of sediments and crust, taking into consideration the velocities and Moho relief determined by DSS measurements. Other effects than those arising from deep‐seated structures in the uppermost mantle beneath the stations were minimized by subtracting the average delays calculated for each event at 15 basic stations, uniformly covering the investigated territory. Spatial variations of these normalized residuals, as large as 2 s, were derived from events in different focal regions. An average residual at each station was calculated from 23 selected groups, compiled from 181 events evenly distributed as to their azimuths and distances. The 3‐D inversion of the residuals yielded very much the same picture of the relatively high‐ and low‐velocity provinces for the upper layer of the mantle as compared with the map of average residuals. The map of average residuals mainly reflects variations in the thickness of the lithosphere and its thermal state. Regions of low velocities include the Pannonian Basin and Central Carpathians, the Northern Apennines and the Po Plain, as well as the Rhine Graben, the Rhenish Massif and the North German‐Polish Platform. High velocities are observed in the south‐western margin of the East European Platform and in three well‐defined zones situated in the inner part of the Western Alps, in the eastern part of the Austrian Alps and in the Eastern Carpathians. Besides the effects of inhomogeneities, also anisotropic effects are anticipated. From the spatial diagrams of the residuals, which show a similar pattern within large tectonic provinces, systematic orientations of relatively slow and high‐velocity directions are recognized. This suggests the existence of large‐scale anisotropic structures in the uppermost mantle which are consistent as to their dips and orientations over several hundred kilometres. They provide new structural information about the geodynamic development of deep continental tectonics. Copyright © 1984, Wiley Blackwell. All rights reserved