Geoscience Reference
In-Depth Information
Figure 10.54.
Epicentres
and focal mechanisms for
African earthquakes.
Dates are in the format
year month day. (From
Foster and Jackson 1998.)
material in the upper mantle. The region is approximately in isostatic equilibrium,
with evidence for up to 15% dynamic support of the topography. Earthquake and
seismic-refraction data show that the crust on either side of the rift valley has a
simple, typical shield structure: upper and lower crustal layers with velocities of
6.0-6.2 and 6.5-7.0 km s
−
1
and a total thickness of 35-45 km overlying a nor-
mal upper mantle with velocity 8.0-8.3 km s
−
1
.Within the rift valley in northern
Kenya the crust is 20 km thick. A positive Bouguer gravity anomaly lies imme-
diately over the eastern rift in Kenya, which is interpreted as being due to a zone
of denser, molten material.
The largest body-wave magnitudes for earthquakes occurring along the rift
system are about 7.2-7.5, but such events are very rare. Earthquakes along the
rift system are normal-faulting events (Fig. 10.54), which are generally consistent
with the expected relative motions. The focal depths extend down to 35 km, with
seismicity taking place throughout the upper and lower crust. Generally, in areas
