Abstract
The 420-km-long northwest-southeast KRISP (Kenya Rift International
Seismic Project) flank line F, part of the KRISP 94 experiment, extends
from Athi River (30 km southeast of Nairobi) to the Indian Ocean
near Mombasa, Kenya. Line F crosses the Chyulu Hills area, a young
Quaternary volcanic field, surrounded by the basement of the Mozambique
belt. The basement is at the surface almost along the entire profile
with two exceptions, one in the Chyulu Hills area due to the presence
of volcanic pyroclastics, the other next to the Indian Ocean where
sediments reach a thickness of about 8 km. Below the basement the
crust can be divided into three layers. The upper layer extends to
about 10 km depth with P-wave velocities of roughly 6.25-6.4 km/s.
The mid-crustal layer reaches a depth of about 20 km where P-wave
velocities range from 6.55 km/s to 6.7 km/s. The lower crust in the
area of the Chyulu Hills is unexpectedly thick (>20 km) and generates
strong refracted phases with velocities of about 7.0 km/s. Crustal
thickness is about 40 km but thickens in the area of the Chyulu Hills
and thins towards the Indian Ocean to 22 km. Pn-phases, refracted
waves travelling through the uppermost mantle, are identified on
four record sections and give good control over the upper mantle
velocities which are slightly reduced from 8.1-8.2 km/s to 7.9 km/s
underneath the Chyulu Hills area. The upper and lower crust in the
region of the Chyulu Hills are significantly heterogeneous, producing
strong signal-induced noise which masks secondary arrivals in the
corresponding distance range. Underneath the Chyulu Hills PMP-reflections
are hard to identify indicating that the crust-mantle boundary is
a transition zone, rather than a first-order discontinuity.
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