Integrating Structural, Geomechanical, and Passive Seismic Data to Investigate Site Effects along an Active Normal Fault Zone

Understanding earthquake site effects in fault-controlled geological settings remains a key challenge for seismic hazard assessment, particularly in intramontane basins affected by active normal faulting. In these settings, fault-zone site effects are expected due to the abrupt contact between thick soft and/or granular sedimentary basin-fill and the carbonate bedrock, which is characterized by highly variable fracture intensity and orientation.

In this study, we present the results of a multidisciplinary investigation aimed at characterizing fault-related site effects along the Monte Morrone fault system, a major Quaternary normal fault bounding the eastern margin of the Sulmona intramontane basin (Central Apennines, Italy) and recognized as a key seismogenic structure in the region.

A passive seismic survey was conducted at three sites located along the fault zone: Eremo di Sant’Onofrio, Roccacasale North, and Roccacasale South. The two Roccacasale sites are structurally located within the fault core and damage zone of the Monte Morrone fault system, characterized by intense deformation and pervasive fracturing. Ambient noise data were acquired and processed using the Horizontal-to-Vertical Spectral Ratio (H/V) technique to investigate resonance frequencies and potential directional amplification effects. Where suitable reference conditions were identified, the data were further analyzed using the Standard Spectral Ratio (SSR) technique to provide a more robust estimate of relative amplification.

Geophysical observations were integrated with detailed structural and geomechanical field measurements. These include fault architecture mapping, fracture density and fracture orientation analysis, and in-situ rock mass characterization through Schmidt hammer rebound measurements. The combined dataset highlights significant lateral variations in seismic response between the investigated sites, which can be directly related to the features of the fault-zone structures, damage intensity, and rock mass stiffness. Directional amplification patterns observed in H/V are consistent with the dominant orientation of fault-related discontinuities, suggesting a strong structural control on local seismic response.

Our results are consistent with previous studies documenting fault-controlled site effects and directional amplification within active fault zones in the central Apennines (e.g., Pischiutta et al., 2013, Vignaroli et al., 2019), and further emphasize the role of fault-core properties and damage-zone architecture in modulating seismic ground motion. These findings support the growing evidence that structural heterogeneities within regional fault zones play a key role in controlling seismic wave propagation and site effects, even at rock sites traditionally considered mechanically homogeneous. Our results suggest that fault cores and associated damage zones should be treated as mechanically distinct domains, characterized by stiffness contrasts and velocity anisotropies capable of modifying the amplitude, frequency content, and directionality of seismic ground motion.

From an application perspective, the multidisciplinary dataset presented here provides further evidence of the importance of correctly representing fault zones in two-dimensional subsurface models for numerical simulations of local seismic response. Explicitly considering the internal architecture of faulted rock masses, rather than assuming uniform "bedrock" conditions, can significantly improve ground motion modeling and help reduce uncertainties in seismic microzonation studies in tectonically active regions.

References

Vignaroli et al., 2019 Domains of seismic noise... BEGE, doi.org/10.1007/s10064-018-1276

Pischiutta et al., 2017. Structural control on the directional.. EPSL, doi:10. 1016/j.epsl.2017.04.017