Site characterization using active and passive surface wave analysis: a case study for some representative sites in Addis Ababa, Ethiopia

Abstract

The subsurface condition of Addis Ababa City is heterogeneous and distinctly varies from place to place, ranging from very soft soil formation to competent rock mass units. This study characterized subsurface variations and estimated site-specific average shear wave velocity changes within the upmost 30 m (Vs30) geological medium at representative sites. We implemented combined active-source Multichannel Analysis of Surface Waves (MASW) and passive-source Microtremor Array Measurements (MAM) surface wave methods to effectively characterize and estimate the shear wave velocity (Vs) of shallow and deep subsurface materials at nine selected sites in the city. The Vs30 of geological profile at the selected sites was attained by joint dispersion inversion analysis of active and passive surface wave data. The result shows that the Vs30 calculated at the selected sites in the city ranges from 256 to 1787 m/s. The study results were used for site classification of near-surface geological materials according to the Vs30 classification guideline proposed by the National Earthquake Hazards Reduction Program (NERHP) and Eurocode-8 (EC-8) classification. Based on Vs30 results, the near-surface materials at the representative sites of the study area are categorized as site types C and D as per NEHRP and as site types B and C as per EC-8. Exceptionally, the rocky Arat Killo site is classified as a site type A in accordance with both NEHRP and EC-8. In addition, we validated acquired shear wave velocity profiles with local geological and engineering geological mapping and geotechnical boreholes proximate to measured sites to control the final inversion quality of Vs values to the corresponding layers. Since this study is the first in Addis Ababa City, the findings will be used for preliminary local site characterization and site specific seismic response analysis.