Seismic Site Response and Microzonation of Awka Capital Territory Using HVSR and MASW Methods

Awka, the capital city of Anambra State, is experiencing unprecedented construction activity, and knowledge of site-specific seismic response is essential for earthquake-resistant design of new structures. This study conducts seismic microzonation of Awka Capital Territory using horizontal-to-vertical spectral ratio and multichannel analysis of surface waves techniques. Ambient noise measurements were collected at forty-five sites across Awka using three-component seismometers, and HVSR analysis following SESAME protocol provided resonance frequency and amplification factor at each site. MASW surveys were performed at twenty selected sites using a 24-channel system with 4.5 Hz geophones at 1.5 m spacing, generating dispersion curves inverted for shear wave velocity profiles. Fundamental resonance frequencies in Awka range from 1.1 Hz to 8.3 Hz, reflecting the variable nature of the underlying geology that transitions from Benin Formation sands in the south to the Imo Shale and Nanka Sands in the north. Amplification factors between 2.1 and 7.2 are computed, with the highest values corresponding to sites overlying thick unconsolidated alluvial deposits. Vs30 values range from 162 to 485 m/s, resulting in site classifications ranging from class C to class E under NEHRP. Microzonation maps for resonance frequency, amplification factor, and Vs30 are produced and overlaid with building density data to identify the most hazardous combinations of site response and built environment vulnerability. The vulnerability index map highlights Awka lowlands and flood-prone residential zones as highest priority for structural risk mitigation.

Keywords: HVSR, MASW, seismic microzonation, site amplification, Awka