Seismotectonics of the Ethiopian Rift from Some Selected Earthquakes in 2001
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Date
2007-07
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Addis Ababa Universty
Abstract
Earthquake data mainly between May and July 2001 are used in this study of the seismicity
of the Main Ethiopian Rift system. The major source of data is the IRIS/PASSCAL
(Incorporated Research Institutes of Seismology/Program for the Array Seismic Studies of
the Continental Lithosphere) Broadband Seismic Experiment recently conducted in Ethiopia
(Nyblade and Langston, 2002). This experiment was operational between 2000 and 2002.
Hypocentral parameters for 144 earthquakes are determined using the location program
Hypoinverse-2000. The locations stability is tested by relocating the earthquakes using the
double-difference algorithm. The distribution of epicenters in this study shows high seismic
activity around 90N and 400E, and 9.50N and 39.50E during the study period. These
epicenters are close to the N-S trending Ankober border fault and Dofen volcano. Local
magnitudes are estimated for the reported events. Fault plane solutions are determined for
five selected earthquakes using the FOCMEC program and show pure normal faulting
mechanisms. Four of these mechanisms, which were located near the Ankober Border Fault,
show N-S trending fault orientation while the fifth solution for an earthquake from the
western plateau has nodal planes trending NW-SE. On the other hand, three separate bvalues
are estimated using both the least-squares method and the maximum-likelihood
method for the whole rift system and the high seismicity (Ankober-Dofen) area. For the
whole rift under study, b-values of 1.059±0.13 were obtained using the maximum-likelihood
method and 1.22 using the least-squares method. On the other hand, b-values of 1.17±0.16
and 1.37 using the maximum-likelihood and least-squares methods, respectively, are also
determined for the highly seismic Ankober-Dofen region during the study period. By
including magnitude data from another source for the year 2001 a separate b-value is also
estimated for the region which experienced higher seismic activity - a maximum-likelihood estimate of 0.81±0.05 and a least-squares estimate of 0.91 is obtained. The b-values
determined based on the data only from this study are indicative of a low stress region. The
results could be taken as an indication that seismic energy was released in the region during
the study period mainly in the form of small magnitude earthquakes. The relatively low bvalue
estimated by including data from another source shows a relatively high stress for the
region. The low b-value estimated by including data from another source may be due to the
wide time period considered indicative of the temporal variation of seismicity rate in a given
region.
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Keywords
Seismotectonics