Active fault segmentation in southwest Bulgaria and Coulomb stress triggering of the 1904 earthquake sequence

We combined field mapping and structural analysis of Landsat imagery in order to identify active faults in the broader area of the Simitli graben and to the east towards the cities of Razlog and Bansko, in southwest Bulgaria. We mapped five large active fault segments with normal-slip kinematics and down-to-north displacement and three smaller, antithetic faults near Razlog. Our work suggests that: (a) present-day deformation in SW Bulgaria is extensional and is accommodated by seismic slip along E-W, NE-SW and WNW-ESE normal faults; (b) inversion of fault slip data shows a σ3 axis oriented 336-356°; (c) the Krupnik fault comprises one earthquake segment with a general NE-SW strike and dip to the N-NW; its length is about 20 km so its earthquake potential is of the order of Mw = 6.7 ± 0.3; (d) as the 4 April, 1904 earthquake comprised two events, a static stress triggering hypothesis may apply, which is also compatible with the fault segmentation, geomorphology and the macroseismic reports. Source faults for the first event (10:02 a.m.) may have been either the 12 km long Gradevo fault or the 11 km long Elovitsa fault. We estimate a moment magnitude of 6.3 for this event. The first event triggered the second one (10:28 a.m.) on the Krupnik fault.