Active fault segments as potential earthquake sources: inferences from integrated geophysical mapping of the Magadi fault system, Southern Kenya Rift

Abstract Southern Kenya Rift has been known as a region of high geodynamic activity expressed by recent volcanism, geothermal activity and high rate of seismicity. The active faults that host these activities have not been investigated to determine their subsurface geometry, faulting intensity and constituents (fluids, sediments) for proper characterization of tectonic rift extension. Two different models of extension direction (E–W to ESE–WNW and NW–SE) have been proposed. However, they were based on limited field data and lacked subsurface investigations. In this research, we delineated active fault zones from ASTER image draped on ASTER DEM, together with relocated earthquakes. Subsequently, we combined field geologic mapping, electrical resistivity, ground magnetic traverses and aeromagnetic data to investigate the subsurface character of the active faults. Our results from structural studies identified four fault sets of different age and deformational styles, namely: normal N–S; dextral NW–SE; strike slip ENE–WSW; and sinistral NE–SW. The previous studies did not recognize the existence of the sinistral oblique slip NE–SW trending faults which were created under an E–W extension to counterbalance the NW–SE faults. The E–W extension has also been confirmed from focal mechanism solutions of the swarm earthquakes, which are located where all the four fault sets intersect. Our findings therefore, bridge the existing gap in opinion on neo-tectonic extension of the rift suggested by the earlier authors. Our results from resistivity survey show that the southern faults are in filled with fluid (0.05 and 0.2 Ωm), whereas fault zones to the north contain high resistivity (55–75 Ωm) material. The ground magnetic survey results have revealed faulting activity within active fault zones that do not contain fluids. In addition, the 2D inversion of the four aero-magnetic profiles (209 km long) revealed: major vertical to sub vertical faults (dipping 75–85° east or west); an uplifted, heavily fractured and deformed basin to the north (highly disturbed magnetic signatures) characteristic of on going active rifting; and a refined architecture of the asymmetry graben to the south with an intrarift horst, whose western graben is 4 km deep and eastern graben is much deeper (9 km), with a zone of significant break in magnetic signatures at that depth, interpreted as source of the hot springs south of Lake Magadi (a location confirmed near surface by ground magnetic and resistivity data sets). The magnetic sources to the north are shallow at 15 km depth compared to 22 km to the south. The loss of magnetism to the north is probably due to increased heat as a result of magmatic intrusion supporting active rifting model. Conclusively, the integrated approach employed in this research confirms that fault system delineated to the north is actively deforming under E–W normal extension and is a potential earthquake source probably related to magmatic intrusion, while the presence of fluids within the south fault zone reduce intensity of faulting activity and explains lack of earthquakes in a continental rift setting.

[1]  I. W. Somerton,et al.  Magnetic interpretation in three dimensions using Euler deconvolution , 1990 .

[2]  A. B. Kampunzu,et al.  Magmatism in Extensional Structural Settings , 1991 .

[3]  G. R. Keller,et al.  The KRISP 94 lithospheric investigation of southern Kenya — the experiments and their main results , 1997 .

[4]  Gordon R. J. Cooper,et al.  Obtaining dip and susceptibility information from Euler deconvolution using the Hough transform , 2006, Comput. Geosci..

[5]  B. H. Baker.  Geology of the Area South of Magadi : degree sheet 58, N.W. quarter : with coloured geological map , 1963 .

[6]  M. I. Seht,et al.  Seismicity, seismotectonics and crustal structure of the southern Kenya Rift—new data from the Lake Magadi area , 2001 .

[7]  C. Morley,et al.  Tectonic evolution of the northern Kenyan Rift , 1992, Journal of the Geological Society.

[8]  M. I. Seht,et al.  Earthquake swarms in continental rifts — A comparison of selected cases in America, Africa and Europe , 2008 .

[9]  P. Maguire,et al.  Some remarks on the structure and geodynamics of the Kenya Rift , 1992 .

[10]  P. Molnar,et al.  A microearthquake survey in Kenya , 1971, Bulletin of the Seismological Society of America.

[11]  W. Bosworth.  Geometry of propagating continental rifts , 1985, Nature.

[12]  G. Randy Keller,et al.  Rift localization in suture-thickened crust: Evidence from Bouguer gravity anomalies in northeastern , 1997 .

[13]  M. Strecker,et al.  Rotation of extension direction in the central Kenya Rift , 1990 .

[14]  C. Prodehl,et al.  Crustal structure on the northeastern flank of the Kenya rift , 1994 .

[15]  R. Shackleton.  Precambrian collision tectonics in Africa , 1986, Geological Society, London, Special Publications.

[16]  G. R. Keller,et al.  Upper crustal structure in the vicinity of Lake Magadi in the Kenya Rift Valley region , 1998 .

[18]  C. Swain.  The Kenya rift axial gravity high: a re-interpretation , 1992 .

[19]  B. H. Baker,et al.  Geology of the Eastern Rift System of Africa , 1972 .

[20]  P. Young,et al.  Seismicity distribution from temporary earthquake recording networks in Kenya , 1992 .

[21]  J. Byerlee,et al.  Magnetic and electric fields associated with changes in high pore pressure in fault zones : Application to the Loma Prieta ULF emissions , 1995 .

[22]  G. R. Keller,et al.  The East African rift system in the light of KRISP 90 , 1994 .

[23]  D. Hollnack,et al.  Neotectonics and extension direction of the Southern Kenya Rift, Lake Magadi area , 2003 .

[24]  W. Bosworth,et al.  Stress field changes in the Afro-Arabian rift system during the Miocene to Recent period , 1997 .

[25]  E. Nickel.  Hypersaline Brines and Evaporitic Environments: A. Nissenbaum (Editor). Developments in Sedimentology, 28, Elsevier, Amsterdam, 1980, 278 pp., US $48.75, Dfl. 100.00 , 1981 .

[26]  D. Falebita,et al.  Integrated geophysical mapping of the Ifewara transcurrent fault system, Nigeria , 2008 .

[27]  Martin L. Smith.  Stratigraphic and structural constraints on mechanisms of active rifting in the Gregory Rift, Kenya , 1994 .

[28]  P. Mohr,et al.  Magmatic Evolution and Petrogenesis in the East African Rift System , 1991 .

[29]  Peter Maguire,et al.  The influence of pre-existing structures on the evolution of the southern Kenya Rift Valley - evidence from seismic and gravity studies , 1997 .

[30]  H. Pollack,et al.  A gravity model for the lithosphere in western Kenya and northeastern Tanzania , 1992 .

[31]  Walter R. Roest,et al.  Magnetic interpretation using the 3-D analytic signal , 1992 .

[32]  R. Barker,et al.  Least-squares deconvolution of apparent resistivity pseudosections , 1995 .

[33]  J. Miller,et al.  Sequence and geochronology of the Kenya rift volcanics , 1971 .

[34]  P. Young,et al.  Implications of the distribution of seismicity near Lake Bogoria in the Kenya Rift , 1991 .

[35]  C. Morley,et al.  Geoscience of Rift Systems—Evolution of East Africa , 1999 .

[36]  R. E. Long,et al.  The Structure on the Western Flank of the Gregory Rift (Kenya). Part I. The Crust , 1976 .

[37]  T. C. Davies,et al.  Geology and the environment in Kenya , 1996 .

[38]  B. H. Baker.  Tectonics and volcanism of the southern Kenya Rift Valley and its influence on rift sedimentation , 1986, Geological Society, London, Special Publications.

[39]  R. Searle.  Evidence from Gravity Anomalies for Thinning of the Lithosphere beneath the Rift Valley in Kenya , 1970 .

[40]  W. Green,et al.  A three-dimensional seismic image of the crust and upper mantle beneath the Kenya rift , 1991, Nature.

[41]  F. Masson,et al.  Seismic tomography of continental rifts revisited: from relative to absolute heterogeneities , 2002 .

[42]  G. R. Keller,et al.  A Seismic Investigation of the Kenya Rift Valley , 1987 .

[43]  G. R. Keller,et al.  An integrated geophysical analysis of the upper crust of the southern Kenya rift , 2001 .

[44]  D. Bailey.  The chemical and thermal evolution of rifts , 1983 .

[45]  R. Riddihough.  DIURNAL CORRECTIONS TO MAGNETIC SURVEYS—AN ASSESSMENT OF ERRORS* , 1971 .

[46]  J. Fairhead,et al.  The Seismicity of the East African Rift System , 1972 .

[47]  G. W. Lamplugh,et al.  The Geological Society of London , 1961, Nature.

[48]  C. Lepvrier,et al.  Seaward extension of the East African Rift , 1986, Nature.

[49]  Martin Smith,et al.  Crustal heterogeneity and basement influence on the development of the Kenya Rift, East Africa , 1993 .

[50]  D. H. Griffiths,et al.  Large-scale variation in lithospheric structure along and across the Kenya rift , 1991, Nature.

[51]  D. Hollnack,et al.  The seismicity related to the southern part of the Kenya Rift , 1998 .

[52]  W. C. Smith.  A Classification of some Rhyolites, Trachytes, and Phonolites from part of Kenya Colony, with a Note on some Associated Basaltic Rocks , 1931, Quarterly Journal of the Geological Society of London.

[53]  D. H. Griffiths,et al.  Seismic and gravity surveys in the Lake Baringo–Tugen Hills area, Kenya Rift Valley , 1981, Journal of the Geological Society.