SEISMOLOGICAL AND GEOLOGICAL INVESTIGATION FOR EARTHQUAKE HAZARD IN THE GREATER ACCRA METROPOLITAN AREA

ABSTARCT A seismological and geological investigation for earthquake hazard in the Greater Accra Metropolitan Area was undertaken. The research was aimed at employing a mathematical model to estimate the seismic stress for the study area by generating a complete, unified and harmonized earthquake catalogue spanning 1615 to 2012. Seismic events were sourced from Leydecker, G. and P. Amponsah, (1986), Ambraseys and Adams, (1986), Amponsah (2008), Geological Survey Department, Accra, Ghana, Amponsah (2002), National Earthquake Information Service, United States Geological Survey, Denver, Colorado 80225, USA, the International Seismological Centre and the National Data Centre of the Ghana Atomic Energy Commission. Events occurring in the study area were used to create an Epicentral Intensity Map and a seismicity map of the study area after interpolation of missing seismic magnitudes. The least square method and the maximum likelihood estimation method were employed to evaluate b-values of 0.6 and 0.9 respectively for the study area. A thematic map of epicentral intensity superimposed on the geology of the study area was also developed to help understand the relationship between the virtually fractured, jointed and sheared geology and the seismic events. The results obtained are indicative of the fact that the stress level of GAMA has a telling effect on its seismicity and also the events are prevalent at fractured, jointed and sheared zones. University of Ghana http://ugspace.ug.edu.gh 1 CHAPTER ONE INTRODUCTION 1.1 BACKGROUND Globally nobody is immune from disasters or disaster related losses. Therefore, this gives us enough reason to study them to know how best they can be handled in case they strike. Disasters are hazard situations that pose a level of threat to life, property or the environment. There are two major types of disasters. These are natural and man-made disasters. Hazards, on the other hand, are exposure or vulnerability to harm or risk (Microsoft Encarta Premium, 2009). Natural disasters include floods, fires, tropical cyclones, earthquakes, tsunamis and others which are environmentally related. However, hazards that strike in low vulnerability regions never become disasters. Natural and man-made hazards usually come with deadly forces or retribution. Manmade hazards, however, are associated with transport, industry and health. Geological hazards include internal earth processes such as earthquakes, volcanic activities, emissions and related geophysical processes such as mass movements, landslides, rockslides, surface collapses and debris or mud flows are also hazardous. In Ghana, disasters that can be identified include drought, earthquake, epidemics, storm, mass movement, extreme temperature, flood, insect infestation etc. The National Disaster Management Organization (NADMO) of Ghana in collaboration with other Agencies has been in the fore front trying to mitigate these hazard situations. University of Ghana http://ug