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Evolution of the 2007 magma-driven earthquake swarm in the northern Tanzania sector of the East African Rift: Envisat and ALOS interferometry, ground observations and elastic modeling

Gidon Baer(1), Yariv Hamiel(1), Gadi Shamir(1) and Ran Nof(2)

(1) Geological Survey of Israel, 30, Malkhe Yisrael St., 95501, Jerusalem, Israel
(2) Ben Gurion University of the Negev, Department of Geological and Environmental Science, 84105, Beer Sheva, Israel

Abstract

An earthquake swarm struck the North Tanzania Divergence, East African Rift over a 2 month period between July and September 2007. It produced approximately 70 M>4 earthquakes (peak magnitude Mw 5.9), and extensive surface deformation, concurrent with eruptions at the nearby Oldoinyo Lengai volcano. The spatial and temporal evolution of the entire deformation event was resolved by Interferometric Synthetic Aperture Radar (InSAR) observations, owing to a particularly favorable acquisition programming of the Envisat and ALOS satellites, and was verified by detailed ground observations. Elastic modeling based on the InSAR measurements clearly distinguishes between normal faulting, which dominated during the first week of the event, and intermittent episodes of dike propagation, oblique dike opening and dike-induced faulting during the following month. A gradual decline in the intensity of deformation occurred over the final weeks. Our observations and modeling suggest that the sequence of events was initiated by pressurization of a deep-seated magma chamber below Oldoinyo Lengai volcano which opened the way to lateral dike injection, and dike-induced faulting and seismicity. As dike intrusion terminated, silicate magma ascended the volcano conduit, reacted with the carbonatitic magma, and set off a major episode of explosive ash eruptions producing mixed silicate-carbonatitic ejecta. The rise of the silicate magma within the volcano conduit is attributed to bubble growth and buoyancy increase in the magma chamber either due to a temporary pressure drop after the termination of the diking event, or due to the dynamic effects of seismic wave passage from the earthquake swarm. Similar temporal associations between earthquake swarms and major explosive ash eruptions were observed at Oldoinyo Lengai over the past half century.

 

Symposium presentation

 

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