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Rheology Studies

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Previous Studies

Our group seeks to understand the rheology (viscoelastic strength) of the lower crust and upper mantle in various tectonic settings. Viscoelastic strength controls the rate of plate motions and significantly influences the evolution of crustal stresses through the earthquake cycle. Our main approach is to use large earthquakes as in situ rock squeezing experiments, whereby earthquakes cause an increase in shear stress that causes the hot rocks in the lower crust and upper mantle to flow. This process is called postseismic viscoelastic relaxation and it causes coseismic s tresses to transfer back up to upper crust, which causes postseismic surface deformations. This deformation can be observed by very precise GPS or InSAR measurements and used to constrain a finite element model of the relaxation process thereby determining the viscosity structure of the lower crust and upper mantle. Our group was the first to show that laboratory derived temperature- and stress-dependent power-law flow associated with dislocation creep following an earthquake was a viable mechanism to explain postseismic surface displacements (Freed and Bürgmann, Nature, 2004 ; Freed et al., EPSL, 2006). We have shown that viscoelastic relaxation within the mantle could influence surface displacements more than 4 rupture lengths from an earthquake (Freed et al., JGR, 2006; Freed et al., GRL, 2007), but that a significant postseismic response will only occur for earthquakes larger then M6.5 (Freed, GRL, 2007; Mahsas et al., GJI, 2007). Most recently, we have shown that power-law flow alone could not account for initial, very rapid surface displacement rate changes invariably observed immediately after an earthquake (Freed et al., EPSL, 2010).

Current Studies

Our current work is on the development of a new temperature- and stress-dependent transient flowlaw that can explain the entire span of observed postseismic time-series displacements based on the tectonic environment of the region. This allows us to use short-term postseismic surface displacements to infer the long-term stress-rates and absolute stress levels in the upper mantle, quantities that are impossible to measure directly. We are also working on inferring the rheology of the lower crust and upper mantle beneath Haiti based on observed postseismic surface displacements following the 2010 earthquake. We are currently collecting GPS data, with finite element modeling to follow.

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Department of Earth & Atmospheric Sciences, Purdue University
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