Geophysical investigation of faults and fractures in Kiamunyi area south west of Menengai volcano in the Kenya Rift

Abstract

The rapidly growing Nakuru town needs expansion to accommodate the increasing population. This vast expansion can only be done at the outskirts of the town. However, no single research has been done with the aim of checking the geological stability of the regions around Nakuru town; either by mapping new fault zones or by confirming using geophysical methods the presence of faults shown on geological maps. Kiamunyi estate being in the outskirts of the town was therefore chosen for this research; to evaluate the stability of the area for development. The resistivity and potential methods were employed in this study. Five apparent resistivity curves were plotted on a log-log graph using the sounding data. From the curves, the study area was found to have three layers to the basement rock. Using the horizontal electrical resistivity data, a resistivity contour map was generated. Two resistivity profiles were also plotted. From these, a low resistivity region that was sandwiched between two highly resistive regions was observed. Gravity and magnetic data were contoured and interpreted. From the gravity contour map, a gravity low was observed between two gravity highs. A low magnetic intensity zone that occurred between two high magnetic intensity regions was also evident. An interpretation was made that the low resistivity region observed between high resistivity regions, low gravity region observed between high gravity regions and low magnetic intensity region observed between high magnetic intensity regions observed on the western side indicate the presence of fractured rocks. These anomalies are observed at the same region where there is a trench like depression on the surface about 30 m wide. The location of these anomalies from the resistivity, gravity and magnetic contour maps coincides. It is therefore concluded that the observed anomalies indicate the presence of a fault zone that strikes Northwest-Southeast. Construction around the faulted zone should be carefully planned and where possible reinforcement done. More research can also be done to map other faults cutting across the whole of Kiamunyi estate and its surrounding. Seismic stations can also be set to monitor the activity of the fault zones. If the faults are observed to be active then construction around the zones should be discouraged. To curb the water scarcity in Kiamunyi, ground water exploration can be done along the mapped fault zone since faults form paths for underground water flow.