Crustal motion and strain accumulation in western Bulgaria

Global Positioning System (GPS) data acquired between 1996 and 2004 and fault plane solutions for four seismic zones are analyzed to obtain the velocity and strain rate fields for western Bulgaria. The GPS derived velocities suggest that southwestern Bulgaria moves to the S to SSE at a rate of ∼ 1 mm/year with respect to northern Bulgaria and southern Romania, defining an approximately ESE-trending extensional boundary that marks the northernmost extent of the Aegean extensional domain. The boundary includes the E-W trending Sub-Balkan graben system of central Bulgaria and its westward continuation into the Sofia graben. Active faults within the boundary region trend ENE to WNW, and they have normal or oblique normal and strike-slip displacements consistent with the velocity field. Within the western part of the boundary region, extension is transferred to the north of the Sofia graben across the westernmost part of the Stara Planina Mountains and the ridges of the western Sredna Gora. The geodetically derived N-S extension is in agreement with the seismic data that show a variable pattern, but with projections of the T axes mostly lying within the N-S quadrant. The predominant type of faulting is caused by sub-horizontal extensional stresses in an approximate N-S direction. This is consistent with extension and oblique strike-slip interpreted from geological studies that show numerous WNW, ENE to E-W-trending active normal faults in western Bulgaria.