The Characteristics of Mb 4.9 Turkana Rift Mainshock-aftershocks Sequence in East Africa

Abstract

The volcanically and seismically active Main Ethiopian and East African (Kenya) rifts are linked in a complex zone within the Turkana Depression bounded by the Ethiopian plateau to the north and East African plateau to the south. Subsurface imaging beneath Lake Turkana and surrounding sub-parallel basins reveal multiple half-graben basins bounded by N-S striking normal faults. Lake Turkana includes at least 3 Quaternary volcanoes within the central rift basins. The objectives of the study are to use the mainshock-aftershock spatial distribution and focal mechanisms of the mainshock and large aftershocks to determine the main rupture fault plane and surrounding minor rupture planes, develop an improved statistical model for assessing earthquake hazards in Kerio Basin and do a comparative study between the Mb 4.9 Turkana and Mw 5.2 Karonga mainshock-aftershock sequences. Data from 21 out of 32-station of the TRAILS project and a permanent GEOFON station, LODK, recorded May 3, 2020, Mb4.9 event and its aftershocks. The origin times, locations, magnitudes, and fault plane solutions of the earthquakes were determined using SEISAN software. The absolute locations of the events were improved using HypoDD double difference algorithm. Three maximum likelihood estimates (MLEs) parameter values (c, p & K0) describing Omori’s law of decay rate were determined by AFTPOI program while four parameter values (c, p, K0 & α) describing the statistical model MLEs were determined by ETAS program, both programs were written in Fortran language. A magnitude of completeness, Mc, of 1.62 and b-value = 1.33 ±0.24 for the Mb 4.9 Turkana mainshock-aftershocks sequence were determined. The mainshock was located near the borderline of Lake Turkana while most of the aftershocks occurred onshore triggered on an NNW-SSE oriented main rupture plane. There were no notable foreshocks, hence no background seismicity was used in the analysis. The main rupture plane was a normal fault with oblique strike-slip component. Two fault plane solutions for the aftershocks were determined; one corroborated with the main rupture plane geometry while the other one was an oblique strike-slip solution. The ETAS model parameter values were c=0.0007, p=0.7071, K (0) =1.5361 & α=1.217. The spatial distribution of the Mw 5.2 Karonga sequence exhibited intersection of faults caused by both seismic and aseismic slip deformation. Further investigations such as fault measurement around Kerio source area and waveform modelling/inversion should be done to improve the fault plane solution of the main rupture plane. Additionally, fault plane solutions for other large aftershocks can be determined