Seismicity Patterns around Oldoinyo Lengai and Gelai Volcanoes, Africa
Alexander Weinstein1*, Sarah J. Oliva1, Cynthia Ebinger1, Steven Roecker2, Christel Tiberi3
1U.
Time-Space Relations in ManyaraNatron AZ
Introduction
The East African rift in northern Tanzania is
one of the most seismically active regions
(Fig 1). Here the zone of magmatism and
faulting transects Archaean lithosphere.
Complementary seismic imaging and gas
geochemistry studies (Lee et al., in press)
reveal that significant volumes of magma
have been added to the crust within this < 5
My-old rift sector that includes Oldoinyo
Lengai carbonatite volcano and Gelai
shield volcano. Damaging earthquakes
(Mw 5.9), a dike intrusion, and an eruption
at Lengai occurred in 2007. We present
time-space relations of seismicity recorded
on a 38-station array spanning the KenyaTanzania border: 3332 earthquakes and
>100 focal mechanisms. Results shed new
light on plate deformation processes.
Are nearby volcanoes hydraulically
connected?
Do large offset border
faults intersect magma chambers?
X
We focus on time-space patterns of
earthquakes in the accommodation zone
between the Manyara and Natron basins
(Fig. 2). Focal mechanisms and CMTs to
N and S of the accommodation zone
show ~E-W extension, consistent with
sparse GPS data. Focal mechanisms of
earthquakes > 15 km are normal (NNE
and NE) whereas focal mechanisms of
shallower earthquakes are variable (Fig.
3). Given the close proximity of
volcanoes, variable topography, and
transfer fault zones, shallow earthquakes
may indicate local stress field perturbed
by fluid movement of gas or magma.
X’
Area
shown in
Fig. 3
Bin 1
Y
Bin 2
Y’
Interconnected
Bodies
Melt
Temporal
relations
between Bin 1 (swarm
interpreted as sill under
Gelai)
and Bin 2
(persistent
swarm
between
volcanoes).
Data show a time
correlation
between
swarms. Two of the
spikes in earthquake
activity are migrating
swarms, suggesting they
are dike intrusions.
Jan 2014
Jul 2014
Fig. 6. Histogram of earthquakes from
Bin 1 (Gelai) uncolored and Bin 2 (intervolcanic)in blue. Bin width is 3.65 days.
To what extent is this correlation due
to intrabasinal fluid transfer versus
large scale stress changes?
Fig. 7. Cartoon of
Fig 1. The <7 My Eastern Rift System in Tanzania
& Kenya comprises rift segments at different
stages of the rifting cycle. Red : Holocene eruptive
centers (Smithsonian catalogue). Global CMTs.
X’
X
Rochester, 2Rensselaer Polytechnic Inst., 3U. Montpellier II
Specific Objectives of Natron
Basin Study
•What are geometry and kinematics
of faults associated with the Mw 5.9
2007 EQ?
• New and 2007 dike intrusion
earthquakes are highly oblique to EW extension direction – Is this a
local stress rotation?
•Is there magmatic and/or fault
connectivity between Lengai and
Gelai volcanoes?
•Do large offset border faults
intersect crustal magma bodies?
Fig. 2 Cartoon profile of Natron Basin (top) and map-view (bottom).
Regional stress in E-W direction is accommodated by discrete slipping
events, folding at monocline, and magma intrusions.
possible feeder
body and intrusion
orientation. Intervolcanic conduits
remain to be tested
and modelled.
Fig 3. CRAFTI catalogue earthquake locations first located using best-fitting 1D velocity model (Albaric et
al.,2010) and relocated using double-difference method (hypoDD). Results provide new insight into EQ
and dike intrusion in 2007 (below). Focal mechanisms (colors indicate depths) using FOCMEC with Pwave polarities on 10 or more stations, allowing ≤ 1 error. Blue dashed box encloses bin 1 and red
dashed box encloses bin 2. Focal mechanisms indicate NE striking normal faults below 12 km depth
.
Conclusions
Fig. 4. Cross section Y-Y’ (Fig. 3) at
2.72oS. Earthquakes re-located with
hypoDD and with depth errors ≤500 m.
Patterns indicate distinct melt and fault
systems. Focal mechanisms indicate
slip along planes highly oblique to the
N-S orientation of the Natron border
fault, and regional opening direction
~E-W in this area of East Africa. See
Fig. 3.
Fig. 5. Fault geometry
inferred for 2007 slow-slip
earthquake (Calais et al.,
2008). Rupture initiates
at site of 2013 Mw 4.5,
4.6 earthquakes above
sill/magma body.
• Time-space seismicity patterns suggest strain and fluid
transfer from the Manyara border fault to Gelai shield
volcano (faulting, diking) via Oldoinyo Lengai volcano and
newly discovered sill? complex between the two volcanoes.
• New focal mechanisms and Global CMTs from an intense
fault-dike episode (2007) show a local, temporally stable,
rotation from ~E-W extension to NE-SW extension in this
linkage zone.
• Our work suggests that the ‘slow-slip’ faulting episode in
2007 was a sill emplacement event that triggered fault slip
that detached on the sill.
• Future: Compare high spatial resolution patterns of ground
deformation from InSAR with seismicity patterns, and
merge with joint ambient noise and body-wave tomography
results.
Support from NSF, ANR, IRIS/PASSCAL gratefully acknowledged.