W strząśy sejsmiczne w Polsce (bez Karpat i Sudetów) l ich związek z budową głębszego podłoża

Abstract

Polska znajduje się zasadniczo w strefie asejsmicznej, jednak na ·przeważającej jej części rejestrowano w ostatnim tysiącleciu słabe wstrząsy sejsmiczne, sporadycznie powodujące szkody materialne. Ich występowanie pozostaje w związku z dyslokacjami głębszego podłoża. Na obszarze Wielkopolski impulsy idące z podłoża tłumi gruby płaszcz salinarnego cechsztynu. Na przedgórzu podobną rolę spełnia miocen, osiągający tu znaczne miąższości. Również usztywniony dawno obszai' platformy wschodnioeuropejskiej jest niemal pozbawiony wstrząsów.

EARTH TREMORS IN POLAND (EXCEPT FOR THE CARPATHIANS AND SUDETY MTS) AND THEIR RELATION TO DEEP BASEMENT STRUCTURE

The major part of the area of Poland is assigned to aseismic zone (W.W. Bielousow i in., 1967). The exceptions. are the Sudety. Mts, Upper Silesian Coal Basin, and western part of Polish Carpathians, placed in "transitional peneseismic zone" J. Pagaczewski, 1958). However, quakes were also affecting the area of the Polish Lowlands in a wider sense (i.e. including the zone of South- Polish Uplands), sometimes resulting in destruction of buildings, including monumental buildings like churches and medieval city walls (Brzeg in 1443, Głogów in 1454, Warsaw in 1680, Wieliczka in 1785, etc.). Some towns were repeatedly affected .by quakes in their history.

The enclosed map shows non uniform distribution of towns in which quakes have been recorded. The points often display a tendency to concentration along some lines or zones. It should be .noted that a part of them follow the .course of the proven faults or deep fractures, as it is the case in the Sudety Mts. Therefore, it may be assumed that the remaining lines or zones of higher concentration of places affected by quakes mark the course of still unrecorded faults or deep fractures. This is further supported by consistent course of these lines and zones of marked increase in gradients in gravimetric and magnetic maps.

The map clearly shows almost complete lack of points corresponding to the recorded quakes in areas of the East-European Platform. The exception is here the Bialyst6k region, situated at the contact of two "escarpments" in gravimetric map. It should be noted that the platform margin was relatively often affected by the quakes and the last of large quakes were recorded there in 1932 (E. Janczewski, 1932), affecting several towns in the Podlasie region and right-bank part of Mazowsze. Attention should be paid to the fact that the localities are situated in a zone interpreted by A. Dąbrowski and K. Karaczun (1956) as one of "escarpments" delineating the East-Eurepean Platform in SW. Warsaw, affected by several quakes (for the last time by Roumanian quake in March, 1977), is also situated in that zone.

It is also worth to note concentration of quake records at two sides of a tectonic element known as the Nida or Miechów Basin. The course of the two zones may be also traced in gravimetry and magnetic map so it may be assumed that the unit is delineated by large faults or crustal fractures at two sides. Therefore, the term "basin" may be used here with reference to shallower-seated structural stage only. Concentrations of localities affected by quakes may be also noted along Silesian section of the Odra River. Directions from 135° to 140°, i.e. Hercynian ("Godów" in the Góry Świętokrzyskie Mts) dominate in the course of these zones. Such directions are regarded as related to original fracture pattern in the Earth crust, resulting from rotation (J.J. Czebanenko, 1963). Attention should be also paid to distances between major zones of concentration of places atrected by quakes and the major known faults (induding Sudetic and Góry Świętokrzyskie faults) . are relatively uniform, equal c. 100 - 110 km.

It is also worth to note aseisimic nature of the Wielkopolska region including Kujawy and right-bank part of Mazowsze. This is presumably due to large thickness of salinary Zechstein rocks (Iocaly over 1000 m), having a damper effect on impulses coming from the basement. The area of the Carpathian Foredeep, with fill of Miocene rocks locally over 3000 m in thickness and partly salinary in character, also appears aseismic. The lack of quake records in the Radom-Lublin region appears related to the presence of a gravimetric high, found by T. Olczak (1951) and others. Aseismic nature, of gravimetric highs is also known from· other areas, e.g. from France (S. Coron, 1954, fide H. Termier, G. Termier, 1956).