Recent Seismicity and Rupture Process of Some Earthquakes in the East African Rift System
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Date
2020-11-24
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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 NS 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 NWSE 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 WNWESE
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 NWSE 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 NWSE direction with NESW 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 NWSE direction. The resulted
focal mechanism of the event shows normal faulting with NESW 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.
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Keywords
B-Value, Catalog, East African Rift System, Magnitude, Moment Tensor Inversion, Northern Tanzania Divergence Zone, Okavango Delta Region, Okavango Rift Zone, Seismicity