Velocity structure of the Earth’s crust and structural control of seismicity in the Krupnik area, Southwestern Bulgaria
The present study investigates the depth-dependent structure and seismotectonic behavior of the fault zone associated with the 1904 doublet earthquakes in southwestern Bulgaria (4th April, 1904, M7.2 and M7.8 in a time domain of 20 min.). The principal contribution of this study is an integrated interpretation combining previously published local earthquake tomography with updated seismicity, active fault geometry, historical earthquake information, and gravity observations. Despite the availability of geological, geodetic, and geophysical data, the geometry and mechanical behavior of the fault system at depth remain insufficiently constrained.
The results reveal depth-dependent variations in crustal structure. In the 10–15 km depth interval, a comparatively coherent upper–middle crust is inferred from neutral-to-positive Vp and Vs anomalies and strong lateral contrasts in Vp/Vs. This interval is broadly compatible with previous macroseismic depth estimates for the 1904 earthquakes, although the historical focal depths remain uncertain. At depths of 20–30 km, reduced S-wave velocities and elevated Vp/Vs values are observed in parts of the model. These patterns are interpreted cautiously as possible indicators of reduced shear rigidity within the middle crust, potentially related to temperature, fluids, mineralogical variations, or lithological contrasts.
The Krupnik fault is expressed in the analyzed datasets primarily as a structural boundary between crustal domains rather than as a simple localized low-velocity zone; this interpretation is supported qualitatively by the spatial correspondence between tomographic contrasts and Bouguer gravity gradients. Contemporary seismicity exhibits clear structural control by the fault-system geometry, with hypocenters preferentially associated with fault segments and lateral velocity gradients rather than with isolated velocity extremes.
The obtained results support a model of a structurally controlled fault system that might extends into the middle crust, where contemporary seismicity reflects distributed deformation within the investigated fault zone and its immediate crustal surroundings.
Keywords: Krupnik Fault; seismotectonics; local seismic tomography (Vp, Vs, Vp/Vs), structural control, seismicity.
