Seismic Microzonation of Urban Site Using the Target Spectral Ratio Method (HVSR): A Case Study of Algiers City.

Abstract

This paper presents a microzonation study of the urban area of Algiers city in Algeria, using 640 microtremor measurements and horizontal over vertical spectral ratios (H/V). The sesmic history of the region changed from moderate to high seismic activity since the destructive Boumerdes earthquake (Mw = 6.8) on May 21st, 2003. 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 of the top 30 m (Vs30). In the absence of Vs30, using H/V measurements provides comparable site information and offers more descriptive value than Vs30 for deep soils, the objective is to prepare a first order microzonation map using a new classification scheme proposed by Laouami et al. (2018) based on H/V target spectral ratio functions defined for each of the 4 RPA99/2003 soil classes (S1, S2, S3 and S4). It serves as a practical tool to empirically determine the site class for 640 sites using microtremor measurements, along with identifying their fundamental frequencies and estimating the average shear wave velocities of the top 30 meters. The comparison of the obtained iso-frequency and site classification maps with the in situ data reveals a high correlation level, emphasizing the new scheme's robustness. The results obtained are vital for urban planning and seismic analysis, improving Algiers' resilience and guiding structural design against seismic loads.