THE EUROPEAN PLATE BOUNDARY ZONE: HOW RELIABLE ARE THE EUROPEAN SEISMIC HAZARD MAPS?

Plate boundaries in continents almost always appear as wide plate boundary zones. The concept of a plate boundary zone was first defined by Burke, Grippi and Şengör in 1980 and later became popular as our knowledge of the seismicity and current motions all over the world increased. In Europe, extensive deformation southwest of the Tornquist-Teisseyre Lineament (edge of the Russian Craton) following the Hercynian orogeny lasted until now and created a vast area of faulting and folding. In this area repeated deformations took place on older structures creating fields of reactivation. The reactivations were either resurrected structures, repeating an older deformation in the same sense; replacement structures that reactivated an older structure in a rôle different from that it played earlier; and finally revolutionary structures that cut across older structures without reactivating any. In Europe, many of the structures younger than the Hercynian orogeny belong either to the resurrected or to the replacement class. Almost all European earthquakes north and northwest of the Alps, with the exception of those related to ice removal in Scandinavia and surrounding regions, have taken place on these reactivated structures. These structures seem to have prevented the European lithosphere from recovering after the Hercynian orogeny and today the greatest structure concentrations are seen where the lithosphere seems the weakest. The GPS observations indicate that many points in Europe north of the Alps move with velocities more than 1 mm/a. This gives us a displacement of 1 m in 1ka. Some of the larger earthquakes in the same areas have repeat times of some 1 ka. From Simmons' (1982) equations we deduce that everly thousand years somewhere in extra-alpine Europe a magnitue 6.5 earthquake is inevitable. But the current seismic risk maps do not adequately reflect this, because they are not accompanied by seismic histories of individual regions.