Seismic Activity Around Addis Ababa

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Addis Ababa University


All data recorded from January 2016 to August 2017 using the three permanently installed stations around the city namely AAE, FURI and ANKE together with the newly deployed (on December 2016) permanent seismic station WLRA (Wolmera) are used in this study. During this period81 natural earthquakes and 123 quarry explosions were recorded around Addis Ababa. The local magnitude of natural earthquakes registered ranges from 1.3 to 5.1 ML and the local magnitude from quarry explosions ranges from 1.2 to 3.0 ML. The seismic events those from earthquakes clustered around the city of Addis Ababa. However, it is not in random fashion; most of the events from natural earthquakes around the city are concentrated following the trend of the MER which is NE -SW. Most of the epicenters concentrate SW and NE of the city. Few earthquake events are located on north of the city and around its eastern part. No events are located on the western part within the period of our observation. The other seismic events registered are explosions from quarry sites in the neighborhood of Addis Ababa within 10-30 Km radius from the city center with the exception to the southwestern part. Epicenters from quarry explosion trend along the location of quarry sites. Most of the quarry events cluster east of the city since most of active quarry sites are located in that area; to be specific east of the Bole International Airport. One of the discrimination methods of earthquakes from quarry blasts is by using origin time of events. Almost all quarry blasts are conducted in the daytime from 8:00AM to 5:00 PM whereas the natural earthquakes can originate anytime of the day. Locations of event sites are also used as a discriminator. Quarry sites, are clustered in close proximity to the city while natural earthquakes are not limited to specific sites and origin time unlike explosions. The other method used for isolation of the two types of events is by comparing the spectra of the seismogram from natural earthquakes and from the quarry explosion. The spectrogram of events from natural earthquakes and quarry explosions is analyzed using the seismograms from Z component AAE and FURI for this study. The result shows a difference in frequency and energy radiation pattern for the two groups of events. The seismograms from quarry blasts have weaker body waves with high frequency content while earthquakes have stronger body wave arrival with low frequency content relative to the explosion events. In addition I tried to discrimination events from quarry blasts and earthquakes by using ratio of amplitude of body waves to surfaces waves Ap/As. This method is not 100% efficient. However, I used it together with the above mentioned methods to discriminate quarry blasts from natural earthquakes.



Seismic Activity Around Addis Ababa, Methodology, Background of the Study