Abstract Title: Near Surface Characterization Using Active and Passive Surface Wave Analysis at Selected Sites in Addis Ababa, Ethiopia

Abstract Submitted to: NEAR-SURFACE GEOPHYSICS

Abstract Text:

The subsurface condition of Addis Ababa city is heterogeneous and distinctly varies from place to place, ranging from very soft soil formations to competent rock mass units. However, the subsurface conditions of the city have not been investigated and classified so far in terms of in-situ shear wave velocity distribution. The main aim of this study was to estimate site-specific average shear wave velocity changes within the topmost 30m geological medium at representative sites in Addis Ababa city. We implemented both active Multichannel Analysis of Surface Waves (MASW) and passive Microtremor Array Measurements (MAM) surface wave techniques to effectively characterize and estimate shear wave velocity of shallow and deep subsurface materials at nine selected sites in the city. The shear wave velocity profile in the upper 30 m of the selected sites was attained by joint dispersion inversion analysis of active and passive surface wave measurements. The result shows that the Vs30 calculated at the selected sites in the city varies from 256 m/s 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 (EC8) classification. Based on Vs30 results, the subsurface materials for selected sites in the city are classified as site classes C and D as per NEHRP and as site classes B and C as per EC8. Exceptionally, the rocky Arat Killo site is classified as site class A. In addition, we correlated attained shear wave velocity profiles with geological and engineering geological mapping and geotechnical boreholes proximate to measured sites to control the final inversion quality of shear wave velocity values to the corresponding layers. The correlation testifies that the Vs obtained from surface wave measurements agree with the borehole geological profiles and prior information. Since this study is the first in Addis Ababa city, the findings will be used for preliminary local site classification and site-specific ground response analysis.