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.

Stress axes of the Magadi Area

(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.