Site-Specific Soil Classification Of Seismic Site Effects For The Greater Accra Metropolitan Area Of Ghana Using Multichannel Analysis Of Surface Waves

Abstract

The study presents site classification for Greater Accra Metropolitan Area (GAMA) of Ghana. Multichannel analysis of surface waves (MASW) is used to calculate shear wave velocity to various depths within the study area. Calculated shear wave velocity up to a depth of 30 m has been used to assess seismic site effects and soil conditions for a major part of the GAMA. Seismic risk has become overwhelmingly apparent from many studies carried out over this area. The area is overpopulated, poorly planned, with the largest convergence of major and minor faults, as well as several historical and recent events. Despite the exposed seismic risk, a regional soils conditions map is yet to be completed for the GAMA. Dispersion data from 42 test sites was inverted and classified according to suggested seismic site classification scheme by National Earthquake Hazard Risk Program (NEHRP). From results of calculated shear wave velocity up to 30 m of soil, two main site classes have become apparent (D and C). Considering the geology of the underlying subsoils, two subdivisions for site class D is suggested (D1 and D2). The residual soils to the southwest and northwest of the study area are derived predominantly from granitoids, and their response is characteristically low. These values are generally within a range of 180 -360 m/s and thus fall into suggested site class D1, the most vulnerable within the test site. Similar to site class D1, D2 is also characterized by low shear wave velocity values (180 -360 m/s), however the underlying subsoils derived from phyllites within the study area. D2 occupies area to the southeast, lower and upper middle of the study area. Several vulnerabilities were identified for soils of site class D within the study area.