The Devene fault system, a major strike-slip structure at the boundary between the Balkan Range and the Moesian Platform in NW Bulgaria, remains a subject of debate regarding its Quaternary activity. This study investigates the shallow expression of the fault at two representative sites, Tri Kladentsi and Beli Breg, using high-resolution electrical resistivity profiling to differentiate tectonic deformation from climatically driven landscape evolution. At Tri Kladentsi, resistivity profiles confirm a steeply dipping structural boundary within the Miocene bedrock, juxtaposing limestone against sands. The overlying 25 m thick loess cover, however, remains sub-horizontal and undisturbed. Likewise, at Beli Breg, the complex architecture of stacked channel sequences and tributary deposits at the Ogosta River confluence reveals no identifiable fault displacement. Our results suggest a high degree of morphological mimicry, where asymmetric river valleys produced by selective erosion and differential loess accumulation superficially converge with tectonic signatures. The long-term left-lateral slip rate is estimated at 0.14–0.19 mm/yr based on a 20 km Miocene offset. Nevertheless, the lack of modern surface rupture indicates a deceleration of fault slip rate and a transition to a buried fault top during the Quaternary. These findings necessitate a re-evaluation of regional seismic hazard assessments, because the absence of continuous surface traces physically constrains the maximum earthquake potential.

The Devene fault system, a major strike–slip structure at the boundary between the Balkan Range and the Moesian Platform in NW Bulgaria, remains a subject of debate regarding its Quaternary activity. This study investigates the shallow expression of the fault at two representative sites, Tri Kladentsi and Beli Breg, using high-resolution electrical resistivity profiling to differentiate tectonic deformation from climatically driven landscape evolution. At Tri Kladentsi, resistivity profiles confirm a steeply dipping structural boundary within the Miocene bedrock, juxtaposing limestone against sands. The overlying 25 m thick loess cover, however, remains sub-horizontal and undisturbed. Likewise, at Beli Breg, the complex architecture of stacked channel sequences and tributary deposits at the Ogosta River confluence reveals no identifiable fault displacement. Our results suggest a high degree of morphological mimicry, where asymmetric river valleys produced by selective erosion and differential loess accumulation superficially converge with tectonic signatures. The long-term left-lateral slip rate is estimated at 0.14–0.19 mm/yr based on a 20 km Miocene offset. Nevertheless, the lack of modern surface rupture indicates a deceleration of the fault slip rate and a transition to a buried fault top during the Quaternary. These findings necessitate a re-evaluation of regional seismic hazard assessments, because the absence of continuous surface traces physically constrains the maximum earthquake potential.

This study presents a multidisciplinary investigation of the Svidnya high-potassium alkaline pluton in the Western Stara Planina Mountains, Bulgaria, combining Electrical Resistivity Tomography (ERT), Induced Polarization (IP), and field X-ray fluorescence (pXRF) to assess their joint potential for exploration. The target rocks of the Svidnya association, represented mainly by melasyenites (shonkinites) and alkaline to quartz syenite dikes, are known from previous studies to be enriched in large-ion lithophile and high-field-strength elements (Ba, Sr, Rb, Zr, Th, U, Nb) and light rare earth elements (La, Ce, Nd), with apatite identified as a principal REE-bearing phase. A geophysical profile crossing the contact between the potassic-alkaline intrusion and Ordovician metasediments reveals two main subsurface domains: a low-resistivity zone corresponding to altered rocks, and a higher-resistivity zone associated with relatively fresh syenite varieties. IP data indicate additional heterogeneities related to variations in lithology and alteration intensity. Field pXRF measurements detected trace concentrations of U, Th, Rb, Sr, Ba, and Nb in slightly altered rocks, confirming the presence of CRM-related elements, although only at qualitative to semi-quantitative levels due to methodological limitations. The results demonstrate that the integrated ERT/IP–pXRF approach is effective for environmentally friendly, non-destructive preliminary delineation of lithological variability and altered zones in prospective alkaline intrusions, and that it also has practical applicability for solving problems in engineering geology, such as the characterization of rock mass heterogeneity, degree of alteration, and subsurface conditions relevant to construction and stability assessments.

In loess terrains, identifying near-surface deformation from blind, slow-slipping faults is challenging because eolian blankets can obscure its subtle and distributed expression. In the Danubian Plain (Northern Bulgaria), the Quaternary loess cover masks the Devene fault system, a major lineament separating the Moesian Platform from the Balkan Range, thus leaving its recent activity debatable. The study integrates high-resolution Unmanned Aerial Vehicle morphometry with 2D electrical resistivity profiling near Tlachene to resolve this ambiguity. Resistivity inversions reveal a sub-vertical lithological contact and a 220-m-wide zone of structural uplift within Miocene sediments characteristic of strike-slip kinematics. The bedrock uplift correlates with a 25-cm deep topographic sag. This inverse relationship uncovers a process of structurally controlled geomorphic inversion, where enhanced fault-zone fracturing accelerates localized erosion, concealing a positive structural feature beneath a negative landform. At the surface, swath profiles isolate a subtle vertical offset of merely 5 cm, aligning with the fault zone. Given the Quaternary age of the loess, this topographic step suggests that tectonic adjustment has persisted into recent times. The negligible cumulative vertical fault displacement and absence of lateral offsets across minor landforms indicate a low long-term fault slip rate for the near-surface portion of the studied fault segment.

The Iskrets spring is one of the most significant karst springs in Bulgaria. А specific feature of this spring are irregular and abrupt reductions of its discharge after some earthquakes as well as other reasons related to the recharge. This study aims to present a reasonable hypothesis for the reasons of the discharge interruptions. A main feature of the Iskrets karst system is the fact that it is dominated by unconfined water flow in the unsaturated zone with velocities varying in a wide range. This creates conditions for significant erosion, transport and redeposition of the fluvial cave sediments. As a result, in the zones where there is a sharp decrease in the water velocity, a considerable accumulation of sediments occurs which can even fill the entire section of the passage-way and leads to a temporary blocking of the spring discharge. It is supposed that an appropriate section where this happens is located near to the discharge point of the spring. Most likely the main reason for the cave deposit displacement is the soil liquefaction caused by the seismic impact. It has been found that the duration of the discharge interruption period is longer at low water flow rates.