Active Magma Systems in the East African Rift · Deformation in the East African Rift Juliet Biggs...
Transcript of Active Magma Systems in the East African Rift · Deformation in the East African Rift Juliet Biggs...
Recent Surface Deformation in the East African Rift
Juliet Biggs1; Ian Bastow1, Derek Keir2, Elias Lewi3,Michael Hutichinson1; Michael Chivers1
1University of Bristol; 2University of Leeds; 3University of Addis Ababa
2007: Lake Natron Dyke Intrusione.g. Calais et al, 2009; Biggs et al, 2009;
Baer et al, 2008.
2005: AfarDyke Intrusione.g. Wright et al, 2006; and
many, many more….
2009: Saudi Arabia Dyke IntrusionJonsson et al,. 2010
Baer et al, 2010.
Recent Surface Deformation in East Africa
2009: Karonga Earthquake Sequencee.g. Biggs et al, 2010.
2009: BukavuEarthquakee.g. d’Oreye et al, 2010.
Main Ethiopian Rift13 volcanoes, 350 km
• Current Spreading Rate - 6.5 mm/yr
• Extension on Mid-Miocene border faults began ~18Ma.
• Since 12Ma, strain has progressively localized towards the magmatic segments.
• 80% strain now accommodated in the rift.
Alutu-Gedemsa
Fentale-Dofen
Angelele
Boset
Corbetti
Evidence for melt in the MER
Seismic Refraction: Velocities compatible with a cooled gabbroic body (Maguire et al, 2006)
Gravity: Shallow high density body beneath Boset volcano (Cornwell et al, 2006)
Shear-wave splitting: melt filled pockets in the top 90km of the lithosphere (Kendall et al, 2005)
Crustal Tomography: high velocity mafic intrusions ~7km beneath the magmatic segments (Kerenan et al, 2004)
Seismicity: Ongoing microseismicity beneath Fentale-Dofen MS (Keir et al, 2006)
Petrology: Fractional crystallisation in the upper 5km (Rooney et al, 2007)
Monitoring Level
No eruption since 1820’s.Hazard and Risk Assesment
GFDRR World Bank Report, 2011
Main Ethiopian Rift
13 volcanoes, 350 kma) Haledebi
b) Bora
c) Alutu
d) Corbetti
10 km
Main Ethiopian Rift
13 volcanoes, 350 km
a) Haledebi
Corbetti
13 volcanoes, 350 km
d) Corbetti
10 km
Ayelew et al, 2004
At least 14cm of subsidence between 1997 and 2000.
Rejuvenated uplift in 2010
Main Ethiopian Rift
13 volcanoes, 350 km
c) Alutu
Alutu-Langano Geothermal Field
•Exploration drilling in 1980’s T=350C at 2500m• 7.3MW power station installed in 1999; reactivated in 2009
Cum
ulat
ive
Dis
plac
emen
t (cm
) Time Series
Simple Fit: Exponential decayDecay const: 200-300 daysUplift Period: 6-9 months.
RAPIDUPLIFT(2004)
RAPIDUPLIFT(2008)
Slow Subsidence
Alut
u
Bora
Cor
betti
Hale.
Temporal Pattern Source GeometryStart Displ.
(cm) Duration (days)
Decay (days)
Model Type
Depth (km)
Radius (km)
Dec 0 3 + 1 5 2 6 0 2 3 0 Penny 0.7-2.5 2.8-8.9 Sep 04 -4 .7 1400 3 2 0 Penny 0.5-1.9 2.9-10 Jul 08 +9.9 1 5 0 2 3 0 Penny 0.7-2.4 4.0-8.2 Dec 0 8 <-4.3 >750 3 2 0 Penny Feb 08 +5.3 9 0 0 1600 Penny 0.9-1.3 4.7-8.1 Feb 94-Jan 96* >1.4 - - - - - Sep 97-Sep 00* < 1 4 - - Mogi 5.8-7.8 0 - -3 .3 >2100 1500 - - - Aug 09 +4.2 2 9 0 1700 Mogi 3.3-5.3 0 Jun 07 +4.1 1 7 0 6 3 0 Dipping Sill 2.7-8.8 5.8x8 Dec 0 7 -5 .4 >1000 6800 Dipping Sill 2.7-8.8 5.8x8
Rate:Exponential decay at Alutu;~ Constant rate at Bora, Corbetti, Haledebi.
Shape:Alutu, Bora, Corbetti fit a radially symmetric source; Haledebi is asymmetric/dipping.
Depth:All depths are shallow (<10 km): Alutu + Bora <3 km.
Time Series and Source Modelling
Lake Natron
OldonyoLengai
Gelei
Dyke
2007 Lake Natron Dike Intrusion(Calais, 2008, Baer, 2008, Biggs, 2009)
Lengai EruptionLengai EruptionGelei Swarm Gelei Swarm
Kervyn et al, 2010
Eruptive Activity at Oldonyo Lengai
Uplift at Lengai:071001-071210
Subsidence at Lengai:080218-080915
Subsidence at Gelei:071001-080811
Example Interferograms
Continued Graben Subsidence
Inflation Rapid DeflationSlow Deflation
Gelei Dyke Intrusion
Triggering Mechanism1. Static (Coulomb) Stressa) Increase in compressional stress
squeezes magma upwards.b) Decrease in compressional
stress causes melting, exsolution of gases, unclamping of conduits
2. Dynamic StressPressure oscillations cause bubble growth
3. Magma-drivenBoth dyke intrusion and eruption are driven by a new pulse of magma from deep.
PetrologicalTimescales
Conclusions•ERS/Envisat archive still contains many interesting, but unstudied deformation signals.•In East Africa alone have identified deformation at 8 unmonitored volcanoes previously considered dormant.•Link to other processes e.g. ground cracks, dike intrusions etc.
NGRS
NGRS
GEL1
GEL1
GEL1
Comparison with GPS(thanks to E. Calais and S. Stamps)