Multiple ruptures on the North-Sofia fault at Gorni Bogrov from scarp profile and shallow geophysics

Abstract

The seismic hazard assessment across the densely urbanized region in the Sofia Basin in Bulgaria can be improved through using fault data, in search of which we first aim to attain to a better knowledge on the past earthquake ground ruptures. Currently focused on the North-Sofia fault, we investigated whether, and how, the fluvial system responded to fault displacements at a site located near the village of Gorni Bogrov. At the studied site, a scarp delimits a distal alluvial fan on the upper surface from an alluvial plain on the lower surface. Although the scarp forms a complex structure that resembles a breached relay ramp, its fault origin is questionable, because the basin axial stream could have caused lateral erosion approaching the toe of the fan. The electrical resistivity profiles that we measured imply that recent deposits are associated with fault displacement. A fault located beneath the middle slope juxtaposes Neogene sand layers, covered by an alluvial fan in the footwall, with colluvial deposits laterally merged with deposits from the alluvial plain in the hangingwall. The scarp also contains a non-tectonic component related to lateral erosion affecting mainly the lower slope. Breaks on the scarp profile are interpreted to have originated through erosion repeatedly renewed due to multiple fault displacements in recent times. The interseismic intervals of the North-Sofia fault are inferred to be in order of thousands of years but not longer. The inferred interseismic intervals correspond approximately to those of the adjacent South-Ihtiman and distant Chirpan faults; however, they are noticeably shorter compared to the adjacent Zlatitsa and distant Krupnik faults. It seems likely that faults in a broad region on the Balkans release strain at remarkably different rates, independently from their proximity.