Complex tectonic deformation in Circum-Tanzania Craton: East African Rift System

Abstract

The reliable source parameters, the current deformation associated to the earthquakes and the current deformation in relationship with dynamic change of Victoria plate rotation are generally poorly understood owing to the lack of the proper techniques used in the Circum-Tanzania Craton. Therefore, the reliable estimated source parameters and the detail interpretation are very crucial. Here, moment tensor inversion is made from broadband seismic data for five earthquakes that occurred in the region in the years 2014 and 2016 with magnitude ranging from Mw 4.1 to 5.7. The 2014 sequence comprises four earthquakes where two of them occurred north of Kondoa with purely normal faulting mechanisms following the N–S oriented rift structure. The other two earthquakes from the 2014 group occurred north of Dodoma and are found to be normal faulting with significant strike-slip component with dextral sense of motion trending in the NW-SE direction which seems to agree with fault structure and aftershock distribution. On the other hand, focal mechanism of the 2016 earthquake in Kagera region with magnitude Mw 5.7 shows dominantly normal faulting trending WNW-ESE with strike-slip component and dextral sense of motion. The interpreted fault orientation for the Kagera earthquake seems to agree with macroseismic information and aftershock distribution which were recorded by seismic stations deployed immediately after the main shock (Mulibo, 2019). We therefore find that the observed normal, strike-slip and oblique slip deformation seems to be influenced by the extensional stress regime of the East African Rift System and the anti-clockwise rotation of the Victorian plate. On the other hand, it is not uncommon to observe deactivation of the NW-SE oriented weak zones in the East African Rift System which seems to agree with our fault mechanism results. In addition, the waveform inverted depths of the studied earthquakes vary from 12 to 26 km in the Circum-Tanzania Craton is implying relatively larger effective elastic thickness compared to the northern part of the East African Rift System in the Afar region.