Recent Seismicity And Rupture Process Of Some Earthquakes In The East African Rift System

This work principally deals with the recent seismicity for the period 2013 to 2016 and rupturernprocess of some selected earthquakes across the di_erent tectonics regimes of the East AfricanrnRift System (EARS). We analyzed phase and waveform data from temporary and permanentrnseismic networks which are installed along EARS and hosted by di_erent agencies. In this investigationrnthree di_erent works have been conducted.rnIn the _rst part, spatial and temporal distribution of earthquakes in the East African Rift Systemrnhas been investigated for the years 2013 to 2016. The spatial earthquakes distribution inrnthe time window considered in this study has revealed that the Main Ethiopian Rift (MER) isrnstructurally connected to the eastern branch of the East African Rift System (EARS). A relativelyrnhigh level of seismicity has been obtained in this study compared to the ISC (InternationalrnSeismological Centre) catalogue database for the same time period which implies the region isrnseismically more active than we thought. In addition to the rift oor, the rift margins and thernsurroundings plateaus are found to be seismically active. The western and eastern branches ofrnthe EARS are found to be more active than the others rift segments for the time period considered.rnThe frequency-magnitude distribution of earthquakes during this study period providedrnan average b- and a -values of _1.01 and _6.5, respectively. The b-value result is implyingrnrelatively high stress with tectonic origin of earthquakes occurrence dominating the region. Onrnthe other hand, threshold magnitude completeness of Mc 3.0 is obtained which is the minimumrnvalue that has never been observed previously for such large area. The seismic energy map ofrnthe region has been systematically investigated and peaks energy-release is observed at Afarrndepression, eastern and western branches of the EARS during this study period. The energyrnmapping shows that the MER is structurally connected to the eastern branch of the EARSrnwhich is consistent with seismicity distribution. The overall depth of occurrence of earthquakesrnin the eastern and the western branches of the EARS are relatively deeper than the one foundrnfor Afar and MER. This implies that the seismogenic thickness increases in EARS when we gornfrom north to south which is in agreement with rift age, magmatism and inuence of the AfricanrnSuper plume on the crust and upper mantle deformation in EARS.rnIn the second part, complex tectonic deformation of the circum-Tanzania Craton has been investigated.rnThe reliable earthquakes source parameters and the current deformation in relationshiprnwith the dynamic change of Victoria plate rotation are generally poorly understood owing to thernlack of the proper techniques used in the region. Here, moment tensor inversion is made fromrnbroadband seismic data for _ve earthquakes that occurred in the region in the years 2014 andrn2016 with magnitude ranging from Mw 4.1 to 5.7. The 2014 sequence comprises four earthquakesrnwhere two of them occurred north of Kondoa with purely normal faulting mechanisms followingrnthe NÀ€€S oriented rift structure. The other two earthquakes from the 2014 group occurred northrnof Dodoma and are found to be normal faulting with signi_cant strike-slip component with dextralrnsense of motion trending in the NWð€€€SE direction which seems to agree with fault structurernand aftershock distribution. On the other hand, focal mechanism of the 2016 earthquake inrnKagera region with magnitude Mw 5.7 shows dominantly normal faulting trending WNWð€€€ESErnwith strike-slip component and dextral sense of motion. We therefore _nd that the observedrnnormal, strike-slip and oblique slip deformation seems to be inuenced by the extensional stressrnregime of the EARS and the anti-clockwise rotation of the Victorian plate. On the other hand,rnit is not uncommon to observe deactivation of the NWð€€€SE oriented weak zones in the EARSrnwhich seems to agree with our fault mechanism results. In addition, the waveform invertedrndepths of the studied earthquakes vary from 12 to 26 km in the circum-Tanzania Craton isrnimplying relatively larger e_ective elastic thickness compared to the northern part of the EARSrnin the Afar region.rnIn the third part, a deep rupture process of the April 3, 2017 Mw 6.5 Botswana main shockrnhas been investigated. The event has occurred in Botswana in a region where there was nornrecent tectonic activity and where present-day deformation is believed to be negligible. Thernevent was followed by several aftershocks distributed along NWð€€€SE direction with NEð€€€SW extensionrndirection. We focused on the determination of reliable source parameters for the Mw 6.5rnmain shock using a moment tensor inversion, both in time and frequency domains from regional,rnbroadband waveform data. We retrieve the source depth at 38.4 km, deeper than other studiesrnin the region. The estimated hypocentral depth of this earthquake is roughly about the Mohorndepth beneath the region, reecting a deep source that is relatively rare in stable continentalrnregions. The result may suggest that the seismogenic depth is as deep as the average globalrnMoho thickness indicating the upper mantle lower crust region is actively deforming due to arnreactivation of the pre-existing fault that may oriented in the NWð€€€SE direction. The resultedrnfocal mechanism of the event shows normal faulting with NEð€€€SW extension.rnIn a summary, the _ndings of this work will provide useful information for geodynamic modelingrnof earthquakes occurred at plate boundaries and along the rift oor of the EARS. In Addition,rnrnthe result of this study is intriguing and exciting which will provide useful information aboutrnthe future seismicity in a stable continental region where rare large magnitude and a relativelyrndeep earthquake occurred in the past.

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