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Seismotectonic Investigations in the Lake Magadi area, Kenya
- Diploma thesis abstract -
Physical
Map of East Africa.
The
diploma thesis deals with the seismotectonic investigation of
the Lake Magadi area in the southern Gregory Rift, Kenya. A net
of seismological stations was installed for a period of 8 months.
From November 1997 to June 1998, more than 5500 earthquakes have
been recorded, 1174 of which are located in the Magadi area. Apart
from a normal activity of approx. 10 events per day, a swarm activity
was observed starting from May 1998. The events of the swarm activity
form a linear NNE-SSW earthquake cluster north of Lake Magadi.
The cluster events occur in depths between 0 km and 9 km, forming
a steeply WNW dipping plane. The hypocenters of the other earthquakes
in the area have focus depths between 7 km and 27 km. The magnitudes
range up to ML=4.1. Magnitudes greater than ML=3.5
exclusively occur in the cluster. Here, a strong earthquake in
May 1998 caused a surface crack that trends in the same direction
as the cluster and is situated on its eastern margin. During May
and June 1998, an epicenter migration from south to north could
be observed for the cluster.
Network
of seismic stations in the Lake Magadi Area.
Well located epicenters from Nov. '97 - June '98.
Fault plane solutions were determined for a subset
of well located earthquakes, using first motion p-polarities.
96 single solutions and 2 composite solutions for the cluster
were made. The composite fault plane solutions contain the data
of 230 earthquakes. The strike directions of the fault planes
mainly range between 0° and 30°, with a maximum frequency at 10-20°.
Subordinate directions are NNW (160-170°), given by precambrian
lineaments, as well as E-W striking planes, trending perpendicularly
to the rift axis (approx. 90°). The direction of extension was
determined from the arrangement of tension(T)-axes to be 100-110°
(WNW-ESE). It is thus oriented perpendicularly to the trend of
most faults in the area. The focal mechanisms predominantly show
normal and vertical faults. The pressure(P)-axes usually dip steeply,
whereas the T-axes lie approximately horizontally. The composite
fault plane solution of the cluster shows a NNE trending normal
fault, dipping 65° towards WNW. The slip vector on the plane points
to the west (270°).
Distribution
of fault plane solutions ("beachballs") in the research
area.
The first motion p-polarities are displayed in black (compression)
and white (dilatation).
Cumulative
fault plane solution of the earthquake cluster north of Lake Magadi.
Black dots mean compression, red circles mean dilatation. The
P- and T-Axes are displayed in green.
(A)
Distribution of axes (Wulf-Net) in the Magadi area, determined
from fault plane solutions.
(B) Resulting direction of extension. (C) Predominant strike directions
of the fault palnes.
(D) Distribution of dislocation vectors (Wulf-Net).
In the southern part
of the area investigated, earthquakes that nucleated at depths
greater than 20 km were detected, whereas in the north of the
area, the maximum focus depth is approx. 17 km. The depth of the
transition between ductile and brittle crust is therefore assumed
to be 20-22 km in the south and 15-17 km in the north. In comparison,
its depth at Lake Bogoria is assumed to be approx. 14 km.
The crust underneath the earthquake cluster (> 9 km) is virtually
free of hypocenters. The results of a simultaneously developed
3D velocity model show a zone of increased vP below
Lake Magadi at depths between 3 km and 10 km. These results indicate
the presence of high density material, the origin of which is
likely to be the upper earth's mantle. Below 10 km this material
must be ductile, giving evidence for the absence of earthquakes.
Above 10 km it must be solid, thus causing the positive anomaly
of vP. The anomaly might be caused by a dyke that follows
the regional tectonic structures and is situated underneath Lake
Magadi. Probably, the earthquake swarms were triggered through
active pressure from below. Fluid circulation in the northern
part of Lake Magadi causes a decrease of vP and vS
within shallow depths (0-3 km) and prevents the increase of stress.
This explains the sharp southern cut-off of the cluster at the
northern margin of Lake Magadi. The focus of stress increase is
assumed to be at the northern edge of Lake Magadi.
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