HR: 09:00h
AN: T31F-05    [Abstracts]
TI: Melt distribution in the Ethiopian rift system: Constraints from seismic observations and modelling
AU: * Hammond, J
EM: j.hammond@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, BS8 1RJ, United Kingdom
AU: Kendall, J
EM: gljmk@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, BS8 1RJ, United Kingdom
AU: Angus, D
EM: d.angus@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, BS8 1RJ, United Kingdom
AU: Wookey, J
EM: j.wookey@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, BS8 1RJ, United Kingdom
AU: Keir, D
EM: d.keir@gl.rhul.ac.uk
AF: University of Leeds, School of Earth and Environment, Leeds, LS2 9JT, United Kingdom
AU: Ebinger, C
EM: cebinger@earth.rochester.edu
AF: University of Rochester, Department of Earth and Environmental Sciences, Rochester, 14627, United States
AB: Seismic observations from the EAGLE experiment in the Main Ethiopian Rift have been interpreted in terms of melt-induced anisotropy and support ideas of magma-assisted rifting in continental regions. Following the 2005 Dabbahu rifting event in Afar a further 9 broadband seismometers were installed around the newly active rift segment. These recorded more than one year of continuous data and shear-wave splitting observed in core phases (SKS/SKKS) shows considerable variability across the array. Three stations centred above the Dabbahu rift segment show markedly different splitting characteristics from the other stations. The fast direction is oriented roughly north/south and parallel to the Dabbahu magmatic segment, compared to NNE/SSW orientations at nearby stations. Also the magnitude of splitting is slightly larger at the rift stations compared to those nearby (~1s compared to 0.7-0.9s). These observations supports previous work in the Main Ethiopian Rift (MER), where fast directions change abruptly from being rift parallel on the rift flanks to magmatic-segment parallel in the rift valley. Furthermore, observations of frequency-dependent splitting in the data further suggest that the underlying cause of the anisotropy is related to aligned melt inclusions. Poroelastic modelling support mechanisms for melt-induced anisotropy due to vertically-aligned melt pockets that are on the order of centimetres in length scale. The abrupt change in splitting parameters over small lateral distances (~ 30 ° over ~30~km) suggests that the source of anisotropy is shallow. To further constrain the location of the anisotropy and study the influence of the rift transition on shear-wave splitting results, we model finite-frequency waveforms for a suite of model representations of the rift zone. In each model, the orientation of the anisotropic fabric varies laterally (i.e., the symmetry axis of the HTI symmetry rotates between 0° and 30°), but the strength of anisotropy and depth of transition differs. Waveforms are modeled using a narrow-angle one-way elastic vector wave equation to simulate finite-frequency waveform effects for an incident near-planar S-wave. The modelling is used to examine the influence of changing anisotropic symmetry across the rift as a function of wavefront curvature, and strength and depth of transition as well as lateral width of transition zone. The results show how a simple model with two regimes of anisotropy can explain the variability across the rift, in both delay time and shear-wave polarization, over short length scales.
DE: 3619 Magma genesis and partial melting (1037)
DE: 7218 Lithosphere (1236)
DE: 7290 Computational seismology
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting