Controlling factors on the shallow co-seismic slip at the Sumatra subduction zone megathrust
Abstract
Rupture models of the 2004 Mw 9.2 Sumatra-Andaman EQ imply shallow co-seismic slip with suggestions that slip could have even extended close to the trench particularly along the southern portion of the Sumatran margin. Geophysical imaging of the Sumatran accretionary prism suggested that the shallow plate boundary interface propagates through sediments deposited in the Warton Basin near the base of the Bengal-Nicobar fan and even in pre-fan layers. However, imaging was not able to identify the precise location of the frontal megathrust, and three "candidates" for this sediment horizon were proposed in the input section. IODP Exp. 362 sampled these layers to investigate the characteristics of the input material and the factors controlling shallow seismogenic slip potential. We conducted an innovative set of experiments - "creep tests" - where shear stress is increased step-wise until the onset of accelerated fault slip. Unlike the usual creep tests, this novel technique allows us to observe a very broad spectrum of slip behaviors that spontaneously occur upon faulting, from aseismic creep to accelerated slip at seismic slip rates (1 m/s). Creep tests will also avoid forcing dynamic weakening as a consequence of the seismic slip rate - i.e. at 1 m/s all materials tend to have a remarkable slip weakening behavior. These tests reveal that although the dynamic weakening behaviour seems similar for all the "candidate" fault materials, the way in which they accumulate and adjust their shear stress differs one from the other, resulting in two endmember fault stability characteristics, explained using the energy budget for rupture propagation. Specifically, we identified a pre-fan layer that can accumulate high stress and release it all at once through a fast (>1 m/s) event, behaving like a locked fault patch. Most of the other layers, although still able to accumulate stress, release elastic shear stress both by creeping and also by short-lasting fast (> 0.1 m/s) events, behaving like creeping patches which can accelerate up to seismic slip rates. These conclusions hold if elastic strain is not released by adjacent weaker lithological units. Deformation features in the lower plate as well as in the prism, and plate flexural bending can all exert a key role in building up elastic stress favoring fault propagation - as well as fault nucleation - in the candidate decollement layer.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T43B..02V
- Keywords:
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- 8045 Role of fluids;
- STRUCTURAL GEOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS;
- 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS