Seismic site characterization and classification using HVSR and MASW analyses for urbanization suitability assessment in the Azilal urban area, Central Morocco

Abstract

The Azilal urban area, located in the Central High Atlas and approximately 220 km from the 6.8 Mw Al Haouz earthquake epicenter, was investigated for seismic site effects using 24 Multi‑Channel Analysis of Surface Waves (MASW) profiles and 99 Horizontal‑to‑Vertical Spectral Ratio HVSR. The Vs30 values derived from MASW inversion range from 500 to 1300 m/s, allowing the subdivision of the study area into three zones: central (Vs30 ≤ 830 m/s), northern (830 < Vs30 < 1300 m/s) and southern (Vs30 up to 1300 m/s). According to the Moroccan seismic design code (RPS 2000, version 2011), soils are classified into categories S1 (dominant) and S2, with S1 including the northern and southern zones formed by dense limestone and consolidated sandstone, and S2 corresponding to the central zone where karstified limestone and friable red clays dominate, while the NEHRP 2001 classified the study area into B and C classes. The amplification factor ranges from 1.0 to 1.5, indicating slight to moderate amplification. HVSR spectral analysis reveals natural frequencies between 0.5 and 2.3 Hz, with two main clusters: 1.38‑1.69 Hz along the southeastern margin and 1.1‑1.4 Hz in adjacent zones. These resonance zones correspond closely with the geomechanical boundaries, highlighting the critical role of subsurface geology in seismic response.