Discovering the Čirá–Kopanina Fault: Bridging the Gap between Surface Evidence and Seismic Activity

Abstract

The West Bohemia region in Czechia is well known for its frequent seismic activity, particularly earthquake swarms with magnitudes up to 3-4. Seismicity has been documented since the sixteenth century, with pronounced activity since 1985. Earthquakes concentrate in the Nový Kostel area where hypocenters align along a N-S trending fault zone over 15 km long and at depths of 6-13 km. Despite this activity, a clear surface trace of the responsible fault has remained elusive. This study reports the discovery and investigation of a surface manifestation of the Čirá-Kopanina fault (ČKF), which aligns with the Nový Kostel focal zone (NKFZ). A multidisciplinary approach, integrating geomorphological, geophysical, and seismological analyses, was used to explore the ČKF trace and its near-surface structure. Aligned geomorphological features, including a NNW-trending scarp and linear springs and wet areas, suggest a fault trace extending southward from Čirá village. A combined geophysical survey, including electrical resistivity tomography (ERT), shallow seismic refraction, gravity survey, and electromagnetic induction, confirmed the presence and characteristics of the fault. ERT data revealed a continuous zone, up to 200 m wide, of low resistivity interpreted as fractured, weathered, water-saturated rocks, indicating multiple parallel fault strands. Measurements across the prominent scarp at Čirá village consistently confirmed the fault zone's presence with all applied methods. Relocation of earthquake hypocenters (1991-2022) showed a narrow, steeply dipping fault zone striking 169° and descending from 6 to 13 km depth. Subparallel and oblique segments dipping west support the interpretation that the ČKF trace represents the surface expression of the NKFZ. The fault scarp implies past surface-rupturing earthquakes, and both historical ruptures and present-day swarms with magnitudes up to ML 5 likely occur within the same fault system. These findings emphasize the ČKF's recent tectonic activity and its role in regional seismic hazard assessment.