Zusammenfassung
Local seismic activity has been monitored in the southern part of
the Kenya Rift in the area around Lake Magadi. An earthquake recording
network consisting of 15 station sites was operated for 8 months
from November 1997 to June 1998. During this period, the Magadi area
proved to be seismically active. Approximately 10 events per day
were detected and found to be equally distributed over the rift floor.
The hypocentre depth distribution shows surprisingly large depths
of up to 27 km in the south and much shallower depths in the northern
part of the area. Apart from the background activity, swarm activity
with rates of more than 300 events per day was also recorded. The
epicentres are clustered and trace a linear structure lying SSW-NNE
over a length of 10 km north of Lake Magadi. Hypocentres in the region
of the earthquake cluster are shallow and exhibit a sharp cut-off
at 9 km depth. A surface crack that occurred during the recording
period is connected to the earthquake swarm.Analysis of the focal
mechanisms of selected earthquakes indicates predominantly normal
faulting in response to a WNW-ESE-directed tensional stress field.
This direction corresponds to the general alignment of the southernmost
part of the Kenya Rift and to the surface faulting pattern inside
the rift.The crustal structure of the area has been investigated
using local earthquake tomography and the spatial distribution of
hypocentres. The main results of the tomography are a linear positive
velocity anomaly following the rift axis and a negative anomaly at
shallow depth underneath Lake Magadi. The high velocities can be
explained by mafic material that has intruded into the upper crust.
The negative anomaly is attributed to highly fractured rocks. Maximum
hypocentre depths indicate a body of low shear strength centred at
the rift axis and a general deepening of the brittle-ductile transition
from north to south.
Nutzer