Blockage of regional seismic waves by the Teisseyre-Tornquist zone

Abstract

SUMMARY During the Group of Scientific Experts Technical Test (GSETT, second experiment, 22 April-2 June 1991), several hundred seismic events were located in Europe. Associating these events with the detecting stations-altogether 28 European stations including seven arrays participated in the GSETT-2 experiment-clearly shows that the Teisseyre-Tornquist Zone (TTZ) influences the propagation of regional seismic phases. Large explosions in the Bay of Gdansk, for example, were observed by the wellestablished Scandinavian arrays'NORSAR (A % 830 km) and ARCESS (A z 1650 km), but not by the Polish station KSP (A~470km) nor by the new highly sensitive GERESS array (A % 750 km), both situated south-west of the TTZ. For events in central Europe with comparable magnitudes, we observe a similar increase of the detection threshold at stations located north-east of the TTZ in Scandinavia. To explain these observations, the wave propagation of Pn and Pg perpendicular to the TTZ was modelled for a profile from the Estonian/Russian border region to GERESS with Gaussian-beam seismograms. Published crustal and uppermost mantle models for Poland and for Europe were used as a starting point for developing a model of the TTZ. The observations cannot be explained only by a graben-like crustal structure with a jump in Moho depth from 30 km to 50 km. To defocus the seismic energy, the TTZ as a structural anomaly between eastern and western Europe must reach down into the upper mantle to a depth of at least about 200 km. The proposed model has such a deep-reaching root of the TTZ.