Seismic microzonation of urban site using the target spectral ratio method (THVSR): a case study of Algiers city

Abstract

This paper presents a seismic microzonation study of the urban area of Algiers city in Algeria, using 640 microtremor measurements and site classification based on target horizontal over vertical spectral ratios (THVSR) approach. Northern Algeria lies along the Africa-Eurasia plate boundary, and the Algiers city is located in one of the most seismically active areas in the western Mediterranean. The area's geological context suggests the prevalence of alluvial soils, especially in its central part, which undergoes significant site amplification. The seismic design codes use the concept of soil class to categorize common-soil conditions into broad classes bounded by typical average shear wave velocities in the top 30 m (V_s30). Geophysical methods involving boreholes are required for the measurement of V_s30, however, their prohibitive cost makes them unappealing. In order to prepare a first order seismic microzonation map for the urban area of Algiers city, an alternative new classification scheme based on THVSR is performed over 640 microtremor measurements. The results of the classification of sites based on THVSR consist of providing the site class according to the recent updated version of Algerian Seismic Code (RPA, 2024), the main frequency of the site, and an average estimate of V_s30. To assess the reliability of the THVSR approach, microtremor measurements were compared with shear-wave velocity profiles at 29 sites where V_s30 data were available. In the absence of extensive geophysical surveys, the proposed technique provides a cost-effective and efficient tool for seismic microzonation, with direct applications to urban planning.