Mapping the soil thickness in the Main Ethiopian Rift using passive seismic data and HVSR, case for Northern water divided between the Ziway-Shala lakes basin and the Awash basin

Abstract

A passive seismic survey wasconducted in the northern water dividebetween the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift (CMER), and Awash basin. Previous geophysical studies in the same areahad mainly focused on mapping intermediate to deeper Earth geological structures, and little attention given to shallowest layer.In contrast, the study aimed to map these shallow geologic structures of soil layer undulations at this specific location of CMER. A rapid and non-invasive technique was employed, which involve recording three component measurements of ambient seismic noise data. The collected seismic noise data was processed and analyzed using the average of Horizontal to Vertical Spectral Ratio (HVSR) which helpsto determine resonance frequencies of the survey stations. After frequency estimatesweremadefor each station, corresponding depth estimates were obtained using the method of Nakamura.The point depth to the shallowest layer was determined for each siteand these depth valueswere gridded and mapped to show subsurface topographic undulations or thickness of topsoil cover.The result showed that the shallowest bedrock layer wasthickening northwards, toward the Awash basin.