Was the February 2008 Bukavu seismic sequence associated with magma intrusion?

Transcription

1 Was the February 2008 Bukavu seismic sequence associated with magma intrusion? N. d Oreye, C. Lucas National Museum of Natural History, Luxembourg nicolas.doreye@ecgs.lu P. Gonzalez, J. Fernández Univ. Madrid, Spain A. Shuler, G. Ekström Lamont Doherty Earth Observatory, Columbia University, USA A. Oth, C. Lucas European Center for Geodyanmics and Seismology, Luxembourg F. Kervyn, C. Wauthier Royal Museum for Central Africa, Belgium L. Bagalwa, D. Kavotha, F. Lukaya, E. Osodundu Goma Volc. Observatory, DR Congo

2 What happened? February 3, 2008 : Main shock at 07h34 UTC : Mw 5.9 near Bukavu (DRC) and Cyangugu (Rw) Followed by series of a dozen of felt aftershocks recorded by international seismic networks (USGS NEIC catalog) More than 700 aftershocks recorded by local seismic network over the next 3 weeks Aftermath: at least 43 people killed, 1090 people injured, more than 5000 houses destroyed, landslides.

3 Context Tectonic context: 100km long segments linked by accommodation zones (coincide with transfer fault and volcanic provinces) Historical seismicity: Mw > 5 are not common 2 nd largest event since 1973 after the event Goma Nyamulagira Lake Kivu Nyiragongo Gisenyi Birava Surprising shallow depth Associated to an intense seismic (and also volcanic) activity in that portion of the rift Bukavu Cyangugu Mw >5 EQ (USGS ) and geological faults after [Villeneuve, 1980]

4 Seismicity and volcanic activity Year # EQ Mw > 5 Activity of the Nyiragongo First historical eruption of Nyiragongo; drainage of the lava lake Re apparition of lava lake in the crater in 1982? Highest level of lava in the crater since 1977; intense activity Second historical eruption of Nyiragongo Persistent intense activity of the lava lake No direct link but

5 Comparison Lake Natron, 2007 Lake Kivu, 2008 events Lake Natron 2007 (Tanzania) Start with Mw 5,9 event + Seismic swarm Normal faulting, E EAR Shallow depth Lake Kivu 2008 (DR Congo) Start with Mw 5,9 event + Seismic swarm Normal faulting, W EAR (parameters???) Shallow depth [Calais et al., 2008] Volcanic region Extended ground deformation Upper Cenozoic volcanism??? Large aseismic component??? Magma involved: dyke intrusion??? Available data for Bukavu : No local seismic records at the time of main shock Local data 5 days after EQ Global Seismographic Network. + Afar Consortium seismic arrays in Ethiopia SAR (ENVISAT systematic monitoring + ALOS) Field investigations 2 days after event

6 Seismic studies: Global Seismographic Network CMT solutions using LP body and surface waves observed seismograms < > synthetic waveforms (Σ Normal Modes) (Dziewonski et al. 1981; Arvidsson and Ekström 1998) : Main shock : nearly pure double couple, normal faulting earthquake, shallow depth (12 km) Two main aftershocks : normal faulting, shallow focal depths 2008/02/14 Mw /02/3 Mw /02/03 Mw 5.0 Joint analysis of BB P and SH waveforms and CMT data (Ekström 1989) : Main shock : focal mechanism very similar to CMT solution depth < 8 km

7 Seismic studies: local seismicity Analog records: picked up manually at GVO Digital records: picked up manually in Luxembourg Only 68 earthquakes over the 700 recorded could be (re)located using Nonlinloc software. Large uncertainties due to the unfavorable distribution of seismic stations

8 InSAR: Ground deformation [in cm in LOS] Observed Best fit model Residuals Ascending ALOS PALSAR L band (23,6 cm) 2007/12/ /03/30 Bp 111m, Look angle 34 deg. Descending ENVISAT ASAR C band (5,6 cm) 2008/01/ /02/14 Bp 125m, Look angle 22 deg.

9 Modeling and inversion Fault plane modeling: Okada Inversion algorithm : unconstrained direct search nonlinear optimization (Nelder& Mead, 1965) Not a global optimization method => Perturb randomly the starting parameter to generate 100 uniformly distributed random samples Randomly generated starting points for the non linear inversions (light grey) and the histograms for the bestfitting model parameters (dark grey) Matrix plot of the 100 best fitting inverted parameters. Linear trends shows correlation between parameters.

10 Results Localization Shallow depth confirmed Accurate source parameters USGS/NEIC Broadband and CMT InSAR Envisat, ALOS and Joint inversion compared to Broadband and CMT ENVISAT data alone ALOS data alone Joint ENVI + ALOS Broadband + CMT Depth [km] Dip [Deg] Strike [Deg] Longitude [Deg] Latitude [Deg] Moment [10 18 Nm] 5.3 ±0.6 47± ± ± ±0.3 63± ± ± ±0.3 59± ± ± ± Mw Brittle rupture : no magma involved at least at shallow depth.

11 Conclusions Independent SAR data set : similar results Independent inversions of seismic and geodetic data: similar results InSAR provided accurate localization and depth Unusual shallow depth confirmed Source parameters consistent with tectonic context (Strike, Dip ) Under the lake (explains absence of surface traces) Observed ground deformation is almost entirely coseismic => related to a brittle rupture. No magma at shallow depth Demonstrates that source parameters can be accurately estimated from InSAR, even in the case of a moderate sized earthquake in a vegetated area. SAR data are provided in the frame of ESA Cat 1 project nr 3224 and ESA JAXA Cat 1 project nr Precise orbits are provided by the Delft Institute of Earth Observation and Space Systems (DEOS) and ESA. Interferograms are computed with DORIS (TU Delft) and ROI PAC (Caltech/JPL). This work is supported by the Belgian Science Policy under projects SAMAAV and GORISK (SR/00/113), the National Research Fund of Luxembourg under the project FNR/STEREOII/06/01, and a NSF Graduate Research Fellowship.

12 Thank you for your attention Photo by J. Durieux