THERMAL STATE, RHEOLOGY AND SEISMICITY IN THE PANNONIAN BASIN, HUNGARY

On the basis of data on crustal structure and terrestrial heat flow, a 3-D geothermal model for the lithosphere in the Pannonian basin, Hung ary, has been calculated. This model, together with information on cru stal composition, laboratory data on rock friction, and certain assump tions about fluid conditions and strain-rate levels within the lithosp here, has been used to construct a rheological model of the area. The results obtained show a layered rheological structure where an aseismi c part of the crust is ''sandwiched'' between an upper and a lower sei smogenic crustal layers. According to the proposed rheological model, seismic activity in the upper crust may be expected down to depths of 10-12 km, which is confirmed well by the observed depth distribution o f seismicity. The model also predicts a lower crustal seismogenic laye r down to 20-22 km. Because of infrequent occurrences of deep earthqua kes and/or a generally small number of reliable hypocenter depth deter minations in the study area, this seismogenic zone is less constrained by observations. The depth of the different theologic horizons within the crust is governed mainly by thermal conditions. The lower boundar y of both seismogenic layers appears isothermal. Brittle-ductile trans ition in the upper crust coincides with the similar to 200 degrees C i sotherm, while in the lower crust it coincides with the similar to 375 degrees C isotherm. The lowermost crust and the upper mantle beneath Hungary show ductile behavior, thus the possibility of siesmic activit y at these horizons can be excluded. Copyright (C) 1996 Elsevier Scien ce Ltd