| dc.description.abstract | A single-station three component method of measuring the
amplitude and noting the direction of first motion in all the
three short period components was used. A minimum magnitude
beyond which the whole data set is homogeneous and therefore,
representative of the seismic nature of the region was
determined. The minimum magnitude was found by plotting log N =
a - bML and fitting the best line through the points. The
lowest point beyond which the line passes through most of the
points corresponds to the minimum magnitude which in this case
was ML= 4.0. Therefore, all earthquakes read from the Nairobi
station seismograms and those from other publications containing
local magnitude (ML) = 4.0 which occurred in Kenya were used in
the study. Subsequently, a total of 510 earthquakes were
studied for the period between 1964 - 1984 inclusive .
.
The distribution of magnitudes is such that they range between
4.0 and 5.6 with over half of the data falling between 4.0 and
4.2. Beyond ML = 5.4 the number of earthquakes falls off
remarkably. The local magnitudes were determined from the
maximum ampli t.ude recorded on the short or long period
components and th~ amplitudes were then converted to that of a
hypothetical Wood-Anderson seismograph using the formula;
Awa
2
=
VeT)
AE are largest amplitudes obtained from the north-south, east-
~
west components respectively; Vwa (T) - magnification of the
Wood-Anderson Seismograph at period Ti VeT) - magnification of
the recording instrument; A - amplitude measured in mm from the
Z-component of the recording station; Awa - amplitude of the
phase in Wood-Anderson instrument in micrometers.
Using the overlapping square method of varying overlapping
degrees to delineate areas of high seismicity, it was found that
the overlapping degree of moving the square longitudinally by
0.250 and latitudinally by 0.50 revealed more details and
seismicity corresponding to surface geological features unlike
the overlapping of 0.5 by 0.5 degree. It was found that the
earthquakes in the region are controlled by both tectonic and
volcanic processes. Most of the tectonically induced
earthquakes are found along the Rift Valley especially towards
the south. There is a marked decrease of seismicity towards the
north as compared to the south due to the younging southwards of
the rifting activity.. Although most of the earthquakes are
found along the Kenya Rift, there are a number of volcanic areas
which have th~ highest concentration of earthquakes for example,
Chyulu, Nyambene an~~arsabit Ranges~ The lowest seismic areas
are found to be associated with the Lamu embayment and the north
eastern corner of the country.
The East Africa Rift System forms a. triple junction with with
the Gulf of Aden and .Red Sea mid-ocean ridges. It has been
found that b-values are dependent on ridge characteristics;
whether it is a continental rift or a developing or developed
oceanic ridge.
Whep the b-value was investigated for the Kenya, it was found
that it is within the range of those for the mid-ocean ridges. '
iv
The b-value and seismicity of the region lead to the conclusion
that the Kenya rift is part of a developing ridge along eastern
branch of the East African Rift System. The energy release
pattern was difficult to use to delineate any periodicity of
seismic activity though a weak pattern of about 4 years emerges. | en |
