Evolution of the 2007 earthquake swarm, Tanzania: Envisat and ALOS interferometry, ground observations and elastic modeling

Transcription

1 Evolution of the 2007 earthquake swarm, Tanzania: Envisat and ALOS interferometry, ground observations and elastic modeling Gidon Baer, Yariv Hamiel, Gadi Shamir, Ran Nof Geological Survey of Israel

2 East African Rift North Tanzania Divergence

3 North Tanzania Divergence 35º 36º 37º 38º 2º m 0 50km Kenya Rift Kenya Tanzania * High level of seismicity * Upper mantle thermal anomaly * Active volcanism Lake Natron Oldoinyo Gelai Kilimanjaro 3º Ngorongoro Oldoinyo Lengai Meru Eyasi Rift Manyara Rift Pangani Rift 4º

4 12/7-16/9/ M>4 EQs (NEIC+CMT) 19/7/ April/2008: Eruptions of Oldoinyo Lengai: The only carbonatitic volcano currently active on Earth 17/7 M 5.9 Lake Natron

5 12 Cumulative Seismic Moment (dyncm x ) carbonatites Cumulative seismic moment (2º x 2º rectangle centered at Lengai) ash Oldoinyo Lengai Volcano eruptive and seismic history: Ash eruption Ash eruption ash Carbonatites Ash eruption ash Dawson et al.,

6 Cumulative Seismic Moment (dyncm x ) 1960 carbonatites /7 Cumulative seismic moment (2º x 2º rectangle centered at Lengai) ash Oldoinyo Lengai Volcano eruptive and seismic history: Spatter cones ash 23/8 4/9 Lava flows carbonatites Ash eruptions ash 2 Seismic Cumulative seismic moment Summer 2007 earthquake swarm 0 7/7 22/7 6/8 21/8 5/9 20/9 5/10 Time

7 Sep. 4, 2007 to April 2008 hybrid magma formed by the assimilation of natrocarbonatite by a nephelinitic magma. Seems to be an extreme variant of the 1966 ash Mitchell & Dawson, Mineralogical Magazine, , 1993 silicatecarbonatite mixture Ne Ne Nepheline N Natrocarbonatite P Na,K,Ca-phospatecarbonate Dawson et al., 1992

8 Oldoinyo Lengai observations: Rare episodes of magma mixing and ash eruptions: 1966, 1967, 1993, 2007 All violent events preceded by earthquake swarms Question: Are there any causative relations between the earthquake swarms and the violent ash eruptions?

9 Methodology InSAR measurements of the surface deformation associated with the earthquake swarm Inversion of the InSAR measurements for fault and dike parameters Verification in the field Reconstruction of the complete evolution of faulting, dike intrusion, and eruptions Propose a mechanism for the earthquake-volcano sequences

10 SAR Acquisition times Cumulative Moment (dyncm x ) Envisat I6 ALOS Envisat I2 Envisat I /7 22/7 6/8 21/8 5/9 20/9 5/10 Time Cumulative seismic moment 8 λ=56mm; θ=41º λ=236mm; θ=34.3º λ=56mm; θ=23º λ=56mm; θ=23º Processing: Envisat: ROI-PAC ALOS: Gamma Data supplied by ESA under ALOS-ADEN project AO-3580

11 Descending track ALOS PALSAR G 14 G - Gelai L - Lengai 0-14 L mm (1) (2) (3) Fault Dike 41 (4)

12 Descending track mm 23 (5) 41 (6) (7) Fault Dike (8)

13 Elastic modeling (slip inversion) Purpose Obtain fault and/or dike parameters from observed deformation Procedure Choice of initial location of fault (or dike) trace by discontinuities of fringes In the wrapped interferograms Division of fault (or dike) into 1.5 x 1.5 km patches Iterative comparison between data and Okada elastic solution, allowing for variations in: slip (each patch), strike, dip, rake, fine-location Choice of best-fit solution lowest residual between data and model Output: fault (or dike) models with slip (or opening) distribution

14 Cumulative Moment (dyncm x ) For time series: additional hybrid interferograms Envisat I6 ALOS Envisat I2 Envisat I2 Time series made of interferograms: 1, 9, 10, 5, /7 22/7 6/8 21/8 5/9 20/9 5/10 Time = Interferogram 2 model 1 (reprojected) = Interferogram 3 model 2 (reprojected)

15 Elastic Modeling Interferogram Model Residual Cumulative moment Interval I 4/3/07 17/7/ Cumulative Moment (dyncm x ) I II III IV V Total geodetic moment Fault slip geodetic moment Seismic moment 0 7/7 22/7 6/8 21/8 5/9 20/9 Geodetic moment: M = Σ μau 5/10 Interval II 17/7/07 21/7/07 Interval III 21/7/07 23/7/07-10 cm RMS=0.39 cm RMS=1.9 cm RMS=1.6 cm First week: faulting (possibly induced by deep-seated dikes) Next ~month: dike intrusion+faulting Last month: minor faulting Geodetic moment: ~ 2.5 times seismic moment Fault slip moment ~ seismic moment (no aseismic slip) Interval IV 23/7/07 27/8/07 Interval V 27/8/07 1/10/07 RMS=0.6 cm RMS=0.5 cm

16 Dip slip Opening 23/7/07 27/8/ cm 0 cm 0 D E D E N N

17 Sequence of events Oldoinyo Gelai Oldoinyo Lengai Km

18 View to north Ground Verification

19 Ground Verification View to north M 5.9, July Gradual ground subsidence (above dike), July 17-21

20 Relations between the earthquake swarm in Gelai and the eruptions at Lengai Gelai Lengai

21 Spatial relations Common chamber possible Close vicinity (~15 km) Earthquakes induced by dikes (should come from somewhere) Dike and fault propagation from SW to NE Anatomy of a continental volcano - Hill et al., 2002

22 Temporal relations Lengai (i) (iii) Natrocarbonatites (vi) (v) (vii)? Ash eruptions Until April 2008 Gelai (ii) Dikes Earthquakes (iv) July August September October November Suggested scenario: (i) pressurization of a deep-seated magma chamber below Oldoinyo Lengai (ii) Lateral dike injection, dike-induced faulting, seismicity (iii) Triggered (?) natrocarbonatite effusion (iv) Magma chamber depleted, pressure drop, bubble growth (v) Continued magma supply, ascent of bubble-rich silicate magma in conduit (vi) Reaction between silicate and carbonatitic magma, CO 2 release (vii) Ash eruption

23 Summary 1. InSAR captured the complete evolution of a dike-induced earthquake swarm: Intense faulting during first week (probably induced by deep-seated dikes) ~ 1 month of shallow dike propagation and induced faulting Mainly faulting during next (last) month 2. Provides precise relocation of earthquakes and dikes, verified in the field 3. In contrast with the pure carbonatitic composition of the effusive eruptions, all major ash eruptions in Lengai in the last 50 years were of mixed silicatecarbonatite composition and were preceded by EQ swarms 4. Magma withdrawal by dikes could be the possible cause for vesiculation and upward ascent of silicate magma leading to reaction and mixing with the carbonatites. Dynamic triggering is also a plausible mechanism for this process (not shown here). 5. Ongoing rifting in Africa

24 Thank you!