Recent Seismicity and Rupture Process of Some Earthquakes in the East African Rift System

No Thumbnail Available

Date

2020-11-24

Journal Title

Journal ISSN

Volume Title

Publisher

Addis Ababa University

Abstract

This work principally deals with the recent seismicity for the period 2013 to 2016 and rupture process of some selected earthquakes across the di_erent tectonics regimes of the East African Rift System (EARS). We analyzed phase and waveform data from temporary and permanent seismic networks which are installed along EARS and hosted by di_erent agencies. In this investigation three di_erent works have been conducted. In the _rst part, spatial and temporal distribution of earthquakes in the East African Rift System has been investigated for the years 2013 to 2016. The spatial earthquakes distribution in the time window considered in this study has revealed that the Main Ethiopian Rift (MER) is structurally connected to the eastern branch of the East African Rift System (EARS). A relatively high level of seismicity has been obtained in this study compared to the ISC (International Seismological Centre) catalogue database for the same time period which implies the region is seismically more active than we thought. In addition to the rift oor, the rift margins and the surroundings plateaus are found to be seismically active. The western and eastern branches of the EARS are found to be more active than the others rift segments for the time period considered. The frequency-magnitude distribution of earthquakes during this study period provided an average b- and a -values of _1.01 and _6.5, respectively. The b-value result is implying relatively high stress with tectonic origin of earthquakes occurrence dominating the region. On the other hand, threshold magnitude completeness of Mc 3.0 is obtained which is the minimum value that has never been observed previously for such large area. The seismic energy map of the region has been systematically investigated and peaks energy-release is observed at Afar depression, eastern and western branches of the EARS during this study period. The energy mapping shows that the MER is structurally connected to the eastern branch of the EARS which is consistent with seismicity distribution. The overall depth of occurrence of earthquakes in the eastern and the western branches of the EARS are relatively deeper than the one found for Afar and MER. This implies that the seismogenic thickness increases in EARS when we go from north to south which is in agreement with rift age, magmatism and inuence of the African Super plume on the crust and upper mantle deformation in EARS. In the second part, complex tectonic deformation of the circum-Tanzania Craton has been investigated. The reliable earthquakes source parameters and the current deformation in relationship with the dynamic change of Victoria plate rotation are generally poorly understood owing to the lack of the proper techniques used in the region. Here, moment tensor inversion is made from broadband seismic data for _ve 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 signi_cant 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. We therefore _nd that the observed normal, strike-slip and oblique slip deformation seems to be inuenced by the extensional stress regime of the EARS 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 EARS 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 e_ective elastic thickness compared to the northern part of the EARS in the Afar region. In the third part, a deep rupture process of the April 3, 2017 Mw 6.5 Botswana main shock has been investigated. The event has occurred in Botswana in a region where there was no recent tectonic activity and where present-day deformation is believed to be negligible. The event was followed by several aftershocks distributed along NW􀀀SE direction with NE􀀀SW extension direction. We focused on the determination of reliable source parameters for the Mw 6.5 main shock using a moment tensor inversion, both in time and frequency domains from regional, broadband waveform data. We retrieve the source depth at 38.4 km, deeper than other studies in the region. The estimated hypocentral depth of this earthquake is roughly about the Moho depth beneath the region, reecting a deep source that is relatively rare in stable continental regions. The result may suggest that the seismogenic depth is as deep as the average global Moho thickness indicating the upper mantle lower crust region is actively deforming due to a reactivation of the pre-existing fault that may oriented in the NW􀀀SE direction. The resulted focal mechanism of the event shows normal faulting with NE􀀀SW extension. In a summary, the _ndings of this work will provide useful information for geodynamic modeling of earthquakes occurred at plate boundaries and along the rift oor of the EARS. In Addition, the result of this study is intriguing and exciting which will provide useful information about the future seismicity in a stable continental region where rare large magnitude and a relatively deep earthquake occurred in the past.

Description

Keywords

B-Value, Catalog, East African Rift System, Magnitude, Moment Tensor Inversion, Northern Tanzania Divergence Zone, Okavango Delta Region, Okavango Rift Zone, Seismicity

Citation

Collections