HR: 1330h
AN: U53A-02    [Abstracts]
TI: Normal-Mode Excitation by Sumatran Earthquake and Short-Timescale THERMO-MECHANICS AND RHEOLOGY OF THE EARTH'S LITHOSPHERE
AU: Regenauer-Lieb, K
EM: klaus.regenauer-lieb@csiro.au
AF: CSIRO, PO Box 1130, Bentley, WA 6102 Australia
AU: * YUen, D A
EM: davey@krissy.geo.umn.edu
AF: Dept. of Geology and Geophysics and MInnesota Supercomputing Institute, Univ. of Minnesota, Minneapolis, MN 55455-0219 United States
AB: Solutions of the free-oscillation amplitudes,excited by the recent Sumatran wallop, by Okal and Stein ( 2005 ) have revealed a linearly growing trend in the semi-log plot between amplitude and period from 300 seconds to around an hour. This tantalizing plot (http://www.earth.northwestern.edu/people/seth/research/sumatra.html ) is very much reminiscent of the Rayleigh-Jeans portion of the Planck function in radiation physics, which was called the ultra-violet catastrophe. This distinct signature at long periods shows that some other physics must intervene to neutralize this singular tendency at a longer timescale. Thus in earthquake thermo-mechanics the size of an earthquake or moment is analogous to temperature in statistical physics. In this vein we have studied the thermal-mechanical shear interaction within the framework of a two-dimensional time-dependent model wherein a realistic visco-elastic-plastic rheology is implemented, and the governing equations include the momentum equation without inertia, the rheological and energy equations. We have retained all mechanical heating terms and heating terms involving volumetric expansion in the energy eq uation. In our simulations wherein we have modeled a bending situation, we encou nter two basically different bifurcation phenomena at the brittle-ductile transition-zone in the lithosphere, which can be attributed to two different families of eigenmodes of the system. One in which the shear zone nucleates on thermal perturbations in the ductile field, and the second which is fully associated with elasto-plastic (brittle, pressure-dependent) displacements. A quartz slab has all two modes operating simultaneously at three different depth levels. The bottom of the crust is controlled by the elasto-visco-plastic mode while the top is controlled by the elasto-plastic mode. The exchange of the two modes appears to communicate on a sub-horizontal layer in a flip-flop fashion, which may yield a fractal-like signature in time. The timescales of instabilities are found to decrease strongly with a decrease in the activation energy of the thermally activated processes. For olivine the timescales are around 10**5 years, whereas for quartz it goes down to around one month to a few years. Therefore, we have demonstrated that with a properly formulated thermal-mechanical and rheological model, we can generate timescales now very close to those of earthquakes and of the same order as slow earthquakes, which are the hallmarks of very large earthquakes, like Sumatra. These long-period seismological results together with future data acquisition from GPS data near subduction slabs, such as the Cascades, would help to link the two regimes of timescales and enable us to develop a uniformly valid rheological law for the lithosphere, which provides a link between the two regimes of elastic behavior and nonlinear dissipative dynamics.
DE: 3902 Creep and deformation
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Union [U]
MN: 2005 Joint Assembly