Lg-wave propagation and crustal structure differences near Denmark and the North Sea

• Published in: Geophysical Journal International
• Main Author: Gregersen, Søren
• Format: Text
• Language: English
• Published: Oxford University Press 1984
• Subjects: Articles; Fennoscandian
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/79/1/217; https://doi.org/10.1111/j.1365-246X.1984.tb02852.x

Recordings of regional earthquakes near Denmark and the North Sea have been collected in an attempt to find the Lg-wave propagation properties in the area. Propagation paths of lengths between 300 and 1100 km are investigated. The amplitudes of the Lg-waves and the other seismic phases in the seismograms are compared. The propagation paths are classified according to whether the Lg-wave propagation compared to P - and S -wave propagation is good, intermediate or poor. It is noted that there is a distinct regional pattern in the Lg-wave attenuation. The Lg-wave propagation is poor across the North Sea. The North Sea is surrounded by areas of good, continental Lg-wave propagation except towards the south-east in the Netherlands and western Germany. An area in Skagerrak is characterized by good, but slow propagation of a surface wave similar to Lg. Since this is exactly the area of a cluster of earthquakes in Skagerrak, it is suggested that the two phenomena are caused by the same structural inhomogeneities in the area, just off the coast of north-western Denmark. The Lg-wave propagation is as good in the northern part of Denmark as it is in the Fennoscandidn shield. The extent to which the propagation is poor across the North Sea is dependent on the frequency of the Lg-waves observed with different seismographs. The Lg-waves are more attenuated, relative to the other seismic phases, the higher the frequency is between 0.25 and 2 Hz. As the structural differences between the Fennoscandian bedrock area and the Norwegian—Danish basin with 8–10 km of sediments has no effect on the Lg-wave propagation, it is suggested that the structural anomalies causing the poor Lg-wave propagation across the North Sea are deep crustal anomalies, possibly significant variations in crustal thickness. It is likely that such deep crustal differences are located along the most proniinent upper crustal differences, in the sedimentary grabens in the middle of the North Sea. The poor propagation across the North Sea is similar to that ...