Active faulting in SW Bulgaria: possible surface rupture of the 1904 Struma earthquakes

Two destructive earthquakes, a main shock preceded by a large foreshock, occurred in 1904 in the Balkan region. These events have long been recognized as the largest earthquakes ever recorded in Europe, and the mainshock was assigned a magnitude up to 7.8. The region affected by the shaking straddles the present border between Bulgaria and the Republic of Macedonia, with the immediate epicentral area close to Krupnik in the Struma valley. Neither the surface break nor the source faults of the 1904 events have yet been identified. Using satellite imagery and field observations, we mapped active faults close to, or within, the epicentral area. The most prominent ones are three 20–35 km long normal faults: the Kocani fault in the eastern part of the Republic of Macedonia, and the two neighbouring Krupnik and Bansko faults in south‐west Bulgaria. The latter two are the closest to the immediate epicentral area of the 1904 events. Geologic and morphologic observations allow us to estimate the age of initiation (∼13 Ma) and the long‐term slip‐rate (∼0.15 mm year  − 1 ) of the Krupnik fault. The onset and rate of faulting suggest that the region experienced a limited amount of stretching (< 10 per cent) during the Tertiary, in conflict with the values (> 100 per cent) deduced from regional models involving extension on low‐angle detachments faults. Along the Krupnik fault we found a recent 2 m‐high scarp that may have formed in 1904. A possible rupture of the Krupnik fault compatible with our observations would account for a moment M o  = 2.8 10 19 Nm and a magnitude M s  = 6.9, significantly smaller than the previous estimates.