An investigation of the structure beneath Magadi area in southern Kenya rift using gravimetric data

Abstract

Magadi area is located in the southern part of the Kenyan rift, an active
continental rift that is part of the East African Rift system. Local seismic activity monitored previously around Lake Magadi revealed an earthquake cluster caused by swarm activity in the rift centre at shallow depths, which was probably triggered by magma movements. There was need for a follow-up to locate any body at depth with sufficient density contrast that may represent magmatic intrusions. Gravity measurements were carried out in 58 established stations and data from 52 other stations merged from existing coverage of previous measurements. Necessary corrections were applied to the gravity data and a Bouguer contour map prepared. Euler deconvolution technique was used to image depth to the causative bodies along selected profiles on the Bouguer anomaly map. Two  dimensional gravity forward models of the subsurface structure were generated by using Euler depth solutions in the start models. Among others, a unique body of density of 3.20 gcm^-3 was modelled on the northern region near little Magadi at a depth of approximately 0.4 km. The location of the body coincides with the area where earthquake swarm occurs. Such a body of high density contrasts may be caused by mafic intrusions into the crust. Discontinuities in Euler solution cluster along the profiles indicated buried faults in the volcanic rift infill. The high seismicity may hence be associated to magma intrusions.

Key words: Gravity, Bouguer anomaly, Euler Deconvolution, Kenya