Shear instability in a viscoelastic material as the cause of deep focus earthquakes

Abstract

The mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs. We assume subducting slabs deform such that the spatially averaged strain rate becomes time independent. Furthermore, we assume that rheology depends on temperature in the manner of an activation process. Using this model, we investigate quantitatively the conditions under which shear instability takes place, i.e., the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs. First, we study instability in a material whose rheological properties are spatially homogeneous. Shear instability takes place in a homogeneous material when exceeds a certain critical level ε˙c , which depends strongly on the stress in the slabs but does not depend on the detail of the creep law (the value of the activation energy). Next, we study instability in a material that contains shear zones similar to those often observed by field geologists, i.e., a thin layer made of material with lower vicosity than the surrounding region. We find that shear zones trigger shear instability even for the case of provided that G, the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones, exceeds 1, and that f, the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones, becomes lower than 1. Applying this condition to subducting slabs with shear zones, we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude. From these results, and from an estimate of in subducting slabs, we conclude that shear instability is a mechanism likely to induce deep focus earthquakes.

Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

“…A number of processes have been suggested for causing local weakening. These include shear instabilities (Ogawa, 1987;Kelemen and Hirth, 2007), thermal runaways (John et al, 2010), and local weakening and embrittlement of the crust due to dehydration (e.g., Raleigh and Paterson, 1965;Kirby et al, 1996;Seno and Yamanaka, 1996;Hacker et al, 2003;Yamasaki and Seno, 2003). Possible consequences of dehydration include stress changes to the change in volume of the oceanic crust , pore pressure changes (Wong et al, 1997) or just the release and transport of water (e.g., Jung et al, 2004;John and Schenk, 2006).…”

Source and aftershock properties of the 1996 Flores Sea Deep Earthquake

“…Several proposed mechanisms for deep earthquakes, including plastic shear instabilities [Ogawa, 1987] and frictional melting [Kanarnori et al, 1997], would be expected to show a strong temperature dependence. The form of this temperature dependence may provide one of the few observational constraints on the mechanism of deep earthquakes.…”

Rapid conversion of elastic energy into plastic shear heating during incipient necking of the lithosphere

“…In this paper we wish to address the role played in the plate-breaking problem by lithospheric elasticity coupled to a temperature-dependent visco-plastic rheology. Previous work based on a one-dimensional time-dependent model [Ogawa, 1987] has demonstrated that visco-elasticity could be a very efficient mechanism for transferring rather rapidly potential energy from elasticity into shear heating, which might cause deep-focus earthquakes. The conversion of elastic strain energy to shear heating can potentially be an attractive mechanism for facilitating localized deformation in the lithosphere.…”