HR: 11:40h
AN: T52A-06    [Abstracts]
TI: Forearc Rigidity and Rupture During Great Earthquakes
AU: * Hackney, R
EM: rhackney@geophysik.uni-kiel.de
AF: Institut für Geowissenschaften Universität Kiel, Otto-Hahn-Platz 1, Kiel, 24118, Germany
AU: Tassara, A
EM: andres@dgf.uchile.cl
AF: Departmento de Geofisica Universidad de Chile, Av. Blanco Encalada 2002, Santiago, 00000, Chile
AB: Mapping along-strike variations in subduction zone rigidity is useful for understanding great megathrust earthquakes. This is because: 1) the integrated long-term strength of the coupled slab-forearc system, which is reflected in rigidity, should be an important control on earthquake frequency and magnitude distribution; and 2) forearc rigidity is likely to play a role in the generation of great earthquakes through its control on the degree to which the overriding plate can absorb strain accumulated during convergence. Motivated by these factors, we have calculated lithospheric rigidity along circum-Pacific subduction zones using a wavelet-based Bouguer coherence technique. Whilst our results show a general correlation between oceanic plate-age and rigidity -- as expected -- intriguing along-strike rigidity variations are evident. The most interesting along-strike rigidity variations exist in the subduction zones subjected to the four largest recorded earthquakes: the Valdivia (1960, M9.5), Sumatra--Andaman (2004, Mw 9.3), Alaska (1964, 9.2) and Kamchatka (1952, 9.0) earthquakes. For all but the 2004 Sumatra-Andaman earthquake, the extensive rupture associated with these earthquakes tends to initiate in a region of higher rigidity and propagate into regions of lower rigidity. This suggests that rupture dominantly occurs in weakened parts of the plate interface that slip when triggered by rupture in an adjacent area where substantial interseismic strain has accumulated. At least for the Chilean subduction zone, this weakness appears to be linked to the thick sediments that fill the trench. In contrast to the 1960 Valdivia event, maximum slip during the 2004 Sumatra--Andaman earthquake correlates with the most rigid parts of the forearc. In this case, slip is highest where the most inter-seismic strain has accumulated, perhaps where the plate interface is not lubricated by subducting sediments. These results highlight the potential influence of trench sediments on plate coupling in subduction zones, but the influence of other factors (e.g. wedge geometry, forearc elastic properties, thermally induced oceanic plate rigidity variations) also need to be considered.
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8104 Continental margins: convergent
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 9355 Pacific Ocean
SC: Tectonophysics [T]
MN: 2007 Fall Meeting