Il 10 Giugno2014 Gideon Baer sarà RISSCLab per il seminario "Current rifting in East Africa and the Red Sea margins"
Dipartimento di Fisica - Aula 1G09 - ore 12.00
The spatial and temporal evolution of three earthquake swarms that occurred in East Africa and the Red Sea margins are analysed by Interferometric Synthetic Aperture Radar (InSAR) measurements, field observations, elastic modeling and seismic moment calculations. The first swarm struck the Lake Natron area, Tanzania, over a 2 month period between July and September 2007. It produced approximately 70 M>4 earthquakes, concurrent with eruptions at the nearby Oldoinyo Lengai volcano. The second episode occurred at the Harrat Lunayyir volcanic field, western Arabia, between April and July 2009. It was accompanied by a swarm of more than 4000 M>2 earthquakes, the largest ever documented in that region, with a peak Mw 5.7 shock. The third swarm struck the Karonga region (northern Malawi) on November 5, 2009 and produced 29 M≥4 earthquakes in a period of about 2 months. Our analysis shows that the sequence of events in Lake Natron 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. For the Saudi event, the far-field deformation is dominated by dike opening, whereas the near-field displacements are mostly associated with movements along the faults. The Karonga event involves dip-slip displacement only and shallow aseismic slip, and shows no direct effect of any intrusive magma body. The cumulative seismic moment released during the Saudi swarm period accounts for about 14% of the total geodetic moment, compared to about 55% at the Natron swarm, about 95% in Karonga and about 8% at the 2005 Dabbahu (Afar) events, analyzed separately by Tim Wright and others. We propose that the differences in moment partitioning ratios are due to the different crustal and seismogenic layer thicknesses in the four regions.