Crustal Structure Evolution In Magadi Area, Kenya, Using 3D Seismic Tomography

Abstract

In this study, the seismicity, hypocenter distribution, Magadi crustal structure evolution and short term crustal structure changes were explored in order to examine its crustal structure changes using refraction seismic method. Short term crustal structure changes in Magadi have not been studied earlier on. Also, update on seismicity of Magadi area in order to incorporate new events has not been done. The new events were located and relocated using sub sets of seismic data from crafti project for period January 2013 up to December 2014, supplemented by the existing seismic data sub set recorded by Ibs-von Seht et al. for period November 1997 up to June 1998. The combined events were visualized in form of epicenters and hypocenters using GIS and tomo3d softwares respectively. Travel times data was extracted from the two data sub sets. These travel times were used to develop 1D inversion model for Magadi area using velest routine, a seismic technique, in seisan. The 1D inversion model was used as input into 3D inversion process using seismic tomography technique (tomo3d) software package in order to aid in understanding the Magadi crustal structure evolution and short term crustal structure changes. The surface visualization of the combined events revealed clustering of events on the north eastern part of Lake Magadi unlike the northern, north western, western, south western, southern, south eastern and eastern parts of this lake where no clustering of events was evident. These events had a modal maximum depth of 27 kilometers with uneven distribution of hypocenters. The underground components of the examined structures exposed an increment of velocity with constant velocity in between the increasing velocities, except at about 3 kilometers to about 7 kilometers of the final 1D inversion model for this work. Here, there was a decline instead of increment of velocity believed to be a small magma chamber or zone of partial melt due to small magma injection. The final 3D inversion results revealed an oval shaped high velocity zone (6.5 to about 9.0 kilometers per second) representing mafic intrusive body. This high velocity intrusive body is located at 35.60 E to 36.20 E and 2.10 S to 1.70 S and at depths of about 3 kilometers to about 15 kilometers. It later broke, changed or deformed into three smaller oval shaped high velocity v bodies ( 6.5 to about 8.0 kilometers per second) with a low velocity zone ( believed to be small magma chamber, or zone of partial melt due to the intruding magma) in between them with velocity value of about 3.0 kilometers per second. These oval bodies are located at 360 E to 36.60 E and depths of about 3 kilometers to about 16 kilometers. The low velocity zones have values of about 3 kilometers per second up to a maximum of 4 kilometers per second unlike the smaller high velocity oval bodies whose velocity values range from about 6.5 kilometers per second to 8 kilometers per second. The oval shapes change to globular or sausage-like shapes as one moves north to south. These changes stand for varying geometry of the intruding magma conduits. Therefore, in summary, the study concludes that there exist high velocity zones ( 6.5 to 9.0 kilometers per second) within the Magadi area through which small pockets of magma has intruded forming low velocity zones ( about 3 kilometers per second). These low velocity zones are fed by deep seated magma repository chamber. This repository chamber is located at the bottom of Magadi crust (about 47 kilometers deep). It extends to the top of the mantle layer. This supports the fact that Magadi is undergoing continuous crustal structure changes through small scale magmatic and tectonic movements. The faulting process occurring in this area has played a key role in forming conduits through which magma moves upwards through the Magadi crust. This confirms continuous short term small crustal structure changes of the Magadi basin through small magmatic intrusion and tectonism within the Magadi crust. . Keywords: L. Magadi, Seismicity, Epicenters, Hypocenters, Inversion and Seismic Tomography