Near-surface shear-wave velocity heterogeneity and site effects along the intraplate Hockai Fault Zone (Eastern Belgium) from ambient noise measurements

The Hockai Fault Zone (HFZ) is a ~42 km-long intraplate fault system in eastern Belgium, located within the Rhenohercynian Zone of the Variscan orogenic belt. It has produced the strongest historical earthquake in the region (Mw 6.3, 1692 Verviers). It is also associated with clusters of slow-moving and reactivated landslides. Despite this relevance, the geometry of the HFZ, its terminations and its influence on near-surface mechanical properties remain insufficiently constrained.

In this study, we present a dense ambient noise dataset and ongoing quantitative modeling aimed at resolving site-specific seismic response and shallow subsurface structure. Multiple field campaigns were conducted along and across the HFZ using broadband (Guralp 6TD) and short-period (Lennartz LE-3D/5s) sensors. Single-station horizontal-to-vertical spectral ratio (HVSR) analysis was performed following established SESAME-type criteria, including time-window selection, stability tests, and frequency-dependent uncertainty assessment. The resulting HVSR curves display well-defined and spatially variable fundamental resonance frequencies, indicating strong lateral heterogeneity in near-surface conditions.

To quantitatively interpret these observations, HVSR curve inversion was initiated to derive 1D Vs models, constrained by local geological information. Preliminary results reveal pronounced impedance contrasts within the upper tens of meters, interpreted as the combined effect of weathered bedrock, sedimentary pockets, and fault-related damage zones. These velocity contrasts are expected to exert a first-order control on seismic amplification along the HFZ.

Ongoing HVSR inversions, constrained by local geology, reveal strong shear-wave velocity contrasts within the upper tens of meters, attributed to weathered bedrock, sedimentary pockets, and fault-related damage. This work demonstrates the effectiveness of passive seismic methods for site-response characterization in low-seismicity intraplate regions and provides new constraints relevant for seismic hazard and landslide assessment along the HFZ.