An in depth discussion of the researche and analysis of the CRAFTI project. Localities of earthquake epicenters from dike intrusions were used to better understand the Natron-Manyara Rift Basin and general early stage continental break up.

1. S11A- Rift initiation in cratonic lithosphere: Seismicity patterns in the Manyara-Natron-Magadi basins and Oldoinyo Lengai volcano
C. Lambert1, C. Ebinger1, A. Rodzianko2, S. Roecker2, M. Msabi3, J. D. Muirhead6, N. Rasendra5, C. Tiberi5, R. Ferdinand-Wambura3, K. Mtelela3, A. Muzuka4, G. Mulibo4, E. Witkin1 1. University of Rochester,
2. Rensselaer Polytechnic Institute, 3. University of Dar es Salaam, 4. The Nelson Mandela African Institute of Science and Technology, 5. Université de Montpellier II, 6. University of Idaho
Conclusions
• Our results show that seismicity in Natron and Magadi basins peaks at a depth of
11 km, deeper than but comparable to the 2007 fault-dike sequence, indicating
that shallow earthquakes are typical of background seismicity in these basins.
• There is a strong north-south difference in hypocentral depths between Manyara
basin and Natron/Magadi.
• The site of the 2007 fault-dike sequence remains active.
• In Natron basin, there are some
deeper earthquakes which occur
near Lengai at 20-25 km depth. The
question remains whether these
earthquakes are an expression of the
border fault, and what the relationship
is between the border fault and
magmatism at Lengai.
Support from NSF, IRIS/PASSCAL
gratefully acknowledged. Many
thanks to Fortes Car Hire. Fig 6. Summit of Oldoinyo Lengai
Fig 2.
Cross sections show situations analogous
to Manyara basin (A) and Natron/Magadi
basins (B). Deep seismicity in Manyara has
been constrained by Albaric et al., 2009,
whose studies of Manyara and Natron
indicate north-south differences in
seismogenic layer thickness. Seismicity in
Natron has previously been determined
only from the 2007 intrusion-eruption
sequence.
Main questions to be considered:
•Are shallow earthquakes typical of
background activity in the northern
basins?
•Is there deep seismicity in the Natron and
Magadi basins?
•What is the relation between volcanism at
Oldoinyo Lengai and the border fault?
Depth Distribution of Earthquakes
Fig 5.
Depth distribution results for: a. 2007
Gelai fault-dike eruption sequence,
Albaric et al., 2009. b. Manyara basin,
Albaric et al., 2009, and c. CRAFTI results
in Natron and Magadi. CRAFTI
hypocentral depths peak at 11 km,
deeper than during the Gelai sequence,
but with a strong contrast to activity in
the south, which peaks at 28 km depth.
Introduction
The multi-disciplinary CRAFTI project aims
to quantify the partitioning of strain between
faulting and magmatism during the early stages
of continental rifting in Archaean and
Proterozoic lithosphere. The goals of the project
include the determination of the relative
contributions of diking and faulting along and
across the rift, the spatial and temporal
relationships between magmatism and faulting,
and the geometry and kinematics of active fault
systems, dike intrusions, and active volcanoes.
We present preliminary results of seismicity
analyses from 38 broadband seismometers
deployed in January 2013. We present time-space
relations of seismicity for the first 3
weeks of data, and focal mechanisms for the
largest events during that time period.
Comparisons are made with earlier results from
the 2007 fault-dike intrusion-volcanic eruption
sequence, and results from the Manyara basin
to the south.
Fig 1. The <7 My Eastern Rift System in northern Tanzania
and southern Kenya comprises several rift segments at
different stages of the rifting cycle.
Fig 3.
Epicentral earthquake locations and
focal mechanisms. Orange circles are
from the 2008 temporary array; CRAFTI
locations are in red and represent the
first three weeks of the array. Focal
mechanisms were determined using
FOCMEC (Snoke, 2003), and allowed for
no more than two errors. Some errors
were retained due to errors in takeoff
angles caused by shallow earthquake
depths. Focal mechanisms from CRAFTI,
in black, trend both north-south, parallel
to young faults, and north-east, parallel
to the craton’s Archaean basement
fabric. Focal mechanisms from the 2007
teleseisms, in green, are north-east
trending.
In Manyara basin, only 150 km south of
Natron, a swarm of seismic activity has
been long lasting and unlinked to present
active faults (Mulibo and Nyblade, 2009,
Albaric et al., 2010). We examine
differences in hypocentral depths
between this southern seismicity and
activity in Natron and Magadi basins.
Manyara
Natron
Magadi
a.
Fig 4.
Enlarged view of seismicity in
the vicinity of Natron basin and
Gelai volcano. Green circles are
the double-difference epicentral
locations from the 2008
temporary array. Red are
epicentral locations from CRAFTI
(stations indicated in orange),
and were located with
hypoinverse using the 1-D
velocity model of Albaric et al.
2010. The site of the 2007 dike
intrusion and faulting sequence
near Gelai volcano remains
seismically active.
b.
c.