Seismic Microzonation Study of Calabar Metropolis, Cross River State

This study conducts a seismic microzonation investigation of Calabar metropolis, Cross River State, South South Nigeria, characterising site response variability relevant to building design and urban risk management. Calabar is a rapidly developing coastal city situated on Tertiary and Quaternary sediments overlying Precambrian basement, and the variability of near-surface geology creates heterogeneous site amplification conditions that affect seismic performance of structures. This study measures ambient seismic noise at 45 stations distributed across Calabar Municipality and Calabar South local government areas using three-component broad-band seismometers deployed for minimum 45-minute recordings. HVSR analysis extracts fundamental site frequency and amplification factor at each station following Sesame project guidelines for data quality assessment. Spatial interpolation of HVSR parameters produces site frequency maps, amplification factor maps, and vulnerability index maps. Geotechnical data from available borehole logs in the area are correlated with HVSR results for validation. Findings reveal a systematic spatial pattern of decreasing resonance frequency from 6 to 9 hertz on high ground underlain by shallow basement to 1.5 to 3 hertz in low-lying areas underlain by thick alluvial and estuarine sediments. Amplification factors reach 8 in the southern coastal zone, indicating very high potential amplification. The vulnerability index, combining amplification factor and resonance frequency, identifies the Duke Town and State Housing areas as highest priority for seismic-aware building regulation. The study recommends adoption of site-specific design spectra for critical facilities in Calabar based on the produced microzonation maps.

Keywords: seismic microzonation, HVSR, ambient noise, Calabar, site amplification.