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)