[S-CG44] Science of slow-to-fast earthquakes

[SCG44-P29] Depth-dependence of shallow and deep slow earthquakes: ductility comes from friction or plasticity?

*Ryosuke Ando¹, Kohtaro Ujiie², Yasushi Mori³, Naoki Nishiyama² (1.Graduate School of Science, University of Tokyo, 2.Graduate School of Life and Environmental Sciences, University of Tsukuba, 3.Kitakyushu Museum of Natural History and Human History)

Keywords:Rheology, Fault rocks, Physical model

The Discovery of slow earthquakes illuminates the existence of a strange depth dependence of seismogenesis, which apparently contradicts our common understandings of brittle-ductile transitional mechanics in the Earth’s surface layers. The brittle-ductile transition has been strongly believed to describe the change of brittle/seismogenic layer to ductile/aseismic layer as increasing depth and ambient pressure/temperature. However, within the transitional layer of typical plate interfaces, recent observations have clarified slip velocities of slow earthquakes changing from those of slow (more ductile) to fast (more brittle) with increasing depth, as if described by the “seismogenic inversion layer. (SIL)” Here we propose a new mechanical model that can consistently explain the classic brittle-ductile transition and this inversion phenomenon. The unified understanding is possible only by considering the first-order dependence of the rock rheology on the ambient temperature. We find that the important mechanism to present SIL is the reduction of the viscosity of plastic flows as increasing temperature. This requirement contradicts the dependence of the rate-dependent friction, which shows the increasing “a-b”, equivalent with the viscosity, as increasing temperature. Our results also have implications for the depth-dependence of shallow-slow earthquakes; if SIL does not exist, the rate-dependent friction is preferred as the ductility mechanism there.