Seismic Hazard Analysis for the Debrezeit Area
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Date
2015-05-05
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Addis Ababa University
Abstract
In this thesis work we have presented a seismic hazard investigation conducted at Debrezite area,
central Ethiopia. In this work we carried out a probabilistic seismic hazard analysis with an
emphasis on the local site conditions. The analyses were carried out using the ISC-GEM
(International Seismological Center and Global Earthquake Model) earthquake catalogue
available over a radius of 200 km around Debrezeit city considering destructive earthquakes
from 1906 to 2011. The analysis was also carried out by considering known seismogenic sources
that are identified on the basis of seismicity and tectonics. Earthquake data from ISC-GEM
(International Seismological Center and Global Earthquake Model) catalogue were analyzed
statistically and Gutenburg-Richter recurrence relation is used to obtain the b-value of 0.67, 1.03
and 1.11 for the three seismic zones: Main Ethiopian Rift, Afar and Escarpment respectively that
are considered to generate earthquakes that affect the Debrezeit area.
Seismic refraction data was collected at Kuriftu site, Debrezeit using McSEIS-SX24
seismogram. Multi-Channel Analysis of Surface Waves method is used to analyze the surface
waves and determine the shear wave velocity at the depth of 30m. SeisImager/ SW software
were used to produce a 2-D shear-wave velocity section. The shear wave velocity observed at
Kuriftu site has the mean Vs30 value of 360m/s. Therefore, according to the National Earthquake
Hazard Reduction Program classification the site can be classified under soil class C. Based on
the site classification value, we can conclude that the main part of the study area is under
substantial risk of soil amplification.
After the implementation of a seismic hazard analysis by using CRISIS2012 software, the
probabilistic seismic hazard curves: Exceedance probability and Uniform Hazard Spectrum
curves were developed based on the selected attenuation relationship, with a probability of
exceedance of 10% in 50, 100, 250 and 500 years to a return period of 475 years for bed rock
condition and considering local site effects. On the other hand the highest hazard levels were
determined at soil site for Debrezeit, the peak ground acceleration and spectral acceleration
values calculated based on attenuation relationship by (Campbell and Bozorgnia 2008) were
found to be applicable for 10% probability of exceedance in 50, 100, 250 and 500 years, taking
into consideration the fact that a considerable portion of the city is founded over alluviums.
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Keywords
Debrezeit, Earthquake, B-Value, Hazard Curve, Attenuation Relationship, Peak Ground Acceleration