Suspension rheology under oscillatory shear and its geophysical implications

Dynamic strain generated by seismic waves is often invoked to explain coseismic responses to earthquakes, such as liquefaction, dilatancy and the remote triggering of earthquakes and eruptions. In order to expand our understanding of the conditions under which such responses occur, we measure the rheology of non-Brownian particle suspensions under oscillatory shear at frequencies corresponding to the seismic band (0.1-10 Hz). We characterize the changes in rheology as a function of volumetric particle packing fraction (0.2-0.6), particle diameter (9, 39 mu m), and the viscosity of the, suspending fluid (0.97, 12.14 Pas). By varying the stress (corresponding to a strain range 10(-5)< 10(1)) we identify three rheological regimes. In order of increasing strain amplitude these are (I) linear viscoelastic, (II) shear-thinning and (III) shear-thickening. Transitions between regimes are better defined by a critical strain, rather than stress, particularly for large packing fractions. We find that critical strains are independent of the shearing frequency and fluid viscosity, whereas they decrease with particle packing fraction. We also monitor how the rheology recovers after imposing a large amplitude shear and find that the time scale for recovery is shorter for larger particles, suggesting that structural recovery is controlled by permeable fluid flow. Our results are consistent with the different regimes identified in previous shear tests of water-saturated sands which have a smaller fluid viscosity, suggesting that the same strain criteria can be applied to a wide variety of geological situations. (C) 2008 Elsevier B.V. All rights reserved.