Assessing the Impact on Ground Motion of Intermediate-depth Earthquakes in the Vrancea Zone, Romania, using a 3D Grid-based Approach

The Vrancea region, located in the south-eastern Carpathians, Romania, is a unique site of intracontinental intermediate-depth seismicity. Renowned for its frequent large earthquakes exceeding magnitude 6.5, this narrow seismogenic volume significantly impacts Central and Eastern Europe. The seismicity of the Vrancea Zone is concentrated within a vertical NW-SE structure extending from 70 to 180 km depth (Ismail-Zadeh et al., 2012). This depth range is critical for understanding the ground motion effects on the surface. Previous models of the area treated this depth range as a single, undifferentiated source, overlooking the depth-dependent characteristics of earthquake generation and consequent ground motion.

In this study, we conduct an in-depth seismic hazard analysis for Vrancea intermediate-depth earthquakes, emphasizing the role of depth variability in shaping surface ground motion. Using the novel 3D adaptive smoothed seismicity approach by Pandolfi et al. (2023, 2024), we forecast earthquake rates based on precise spatial distributions of seismicity within a 3D grid. This method smooths earthquake locations using a depth-sensitive kernel, with adaptive smoothing distances that account for both high- and low-seismicity areas.

Our analysis utilizes the ROMPLUS catalog (Oncescu et al., 1999), spanning over a millennium (1000–2023) and focusing exclusively on depths between 70 and 180 km to isolate intermediate-depth events. We determined the magnitude of completeness (Mc), computed the b-value, and declustered the catalog using a procedure which considers the earthquake’s location in depth. We also applied a 3D log-likelihood optimization to calibrate the neighboring number (NN) for the adaptive smoothing process. Finally, seismic hazard was assessed using the Vrancea-specific ground motion prediction equation developed by Manea et al. (2021).

This study quantifies the contribution of earthquakes at different depths to ground motion, enhancing our understanding of depth-dependent seismic hazard in the region and providing refined and innovative tools for seismic hazard assessments.