5/29/2013 - On the relationship between forearc deformation, friction properties, pore pressure, and megathrust


12:00 PM - 1:00 PM
Geology 1707

Presented By:
Nadaya Cubas - Caltech


Determining the spatio-temporal variations of frictional properties is a key issue in seismotectonics, since these properties are thought to determine the seismic potential of a subduction as well as the deformation style of a continental margin. In this presentation, we propose to compare frictional properties of the regions of the 2010 Mw 8.8 Maule earthquake and the 2011 Mw 9.0 Tohoku-Oki earthquake. To retrieve these properties, two kinds of mechanical analyses are conducted. The first one relies on the critical taper theory and yields the effective basal friction on the subduction interface, internal friction, and internal pore fluid pressure. The second is based on the limit analysis approach that allows constraining variations of frictional properties based on the location and style of forearc faulting. For the Maule area, we first show that the rupture area of the earthquake coincides with the mechanically stable part of the wedge. In the surrounding area, the wedge is critical, consistent with various evidence for active deformation. This is in particular true for the Arauco peninsula area, which seems to have stopped the Maule earthquake's rupture to the South. This observation lends support to the view that the seismic rupture is inhibited when propagating beneath a critical area. In the frontal aseismic zone, we found a long-term hydrostatic pore pressure within the wedge and an intermediate effective friction along the megathrust (? ~ 0.3) probably due to the presence of clays. In the rupture area, a low effective dynamic friction (? < 0.14) is found that probably reflects strong dynamic weakening. On the contrary, the frontal wedge of the Tohoku-Oki area is characterized by a long-term high internal pore pressure and a low effective friction along the megathrust (? ~ 0.1). Moreover, the earthquake activated a landward normal fault downdip of the patch of maximum slip. From the modelling of this splay fault with limit analysis, we show that the frontal wedge was submitted to a strong increase of pore pressure during the earthquake. The difference of properties of the frontal wedge of these two regions might actually reflect differences in permeability. A lower permeability would enhance dynamic weakening and allow a frontal propagation of the rupture. This hypothesis is confronted with 2D earthquake sequence model. We then study whether the low dynamic friction in the seismogenic zone is due to an intrinsically lower static friction and/or chronically elevated pore pressure, or due to a dynamic weakening process induced by thermal pressurization.

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