HR: 0800h
AN: T41E-1260    [Abstracts]
TI: Seismological Constraints on the Magmato-tectonic Behavior of the Asal-Ghoubbet Rift (Afar Depression, Republic of Djibouti) Since the Last 1978-Rifting Episode
AU: * Doubre, C
EM: cdoubre@ess.ucla.edu
AF: UCLA - Department of Earth & Space Sciences, 595 Charles E. Young Drive East, Los Angeles, CA 90095 United States
AU: Manighetti, I
EM: manig@usc.edu
AF: Laboratoire de G‚ophysique Interne et Tectonophysique - Universit‚ Joseph Fourier, BP 53X, Grenoble, 38041 France
AU: Bertil, D
AF: BRGM -Guadeloupe, Morne Houelmont Route de l'Observatoire, Goubeyre, 97113 France
AU: Dorbath, C
EM: catherine.dorbath@eost.u-strasbg.fr
AF: EOST - IPGS, rue Ren‚ Descartes, Strasbourg, 67084 France
AU: Dorbath, L
EM: louis.dorbath@eost.u-strasbg.fr
AF: EOST - IPGS, rue Ren‚ Descartes, Strasbourg, 67084 France
AU: Jacques, E
EM: jacques@ipgp.jussieu.fr
AF: EOST - IPGS, rue Ren‚ Descartes, Strasbourg, 67084 France
AB: The Asal-Ghoubbet rift was the locus of a seismic and volcanic crisis in 1978 followed by 8 years of rapid opening (60 mm/yr) before returning to its long-term opening rate of 16 mm/yr. We analyze the space-time evolution of the seismicity that occurred in the rift between 1979 and 2001. The data recorded by the Djibouti Observatory provide only hypocentral locations before 1995 and P and S-wave arrival times since 1996. Additional data acquired during a five months experiment in 2000-2001 allowed us to determine a 3D-velocity model of the rift, used to precisely relocate post 1996 events. The 2545 small-magnitude earthquakes (Md $\leq 3.2$) recorded in the rift since the 1978 crisis provide a negligible contribution to the total extension across the rift, which occurs essentially aseismically. The temporal evolution of the seismicity reveals two distinct phases consistent with those observed in the geodetic data. The post-crisis period (1979-1986) is characterized by large-magnitude earthquakes exclusively located below the northern rift shoulder. These events are associated with the contraction of the side of the rift resulting from the fast opening of the central dyke system. The subsequent period (1987-2001) corresponding to normal opening rate across the rift is characterized by a micro-seismicity essentially located below the major rift caldera (Fieale). Most recorded events during this period concentrate within the rift inner floor at the top of an aseismic, low velocity zone located below the Fiale caldera, which we interpret as hot material above the magma chamber. Outside from post-crisis periods, the seismicity tends to cluster in time in response to stress changes in the brittle layer induced by episodic magmatic movements.
DE: 7230 Seismicity and seismotectonics
DE: 7280 Volcano seismology (8419)
DE: 8109 Continental tectonics--extensional (0905)
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting