The effect of earthquake rupture on the brittle-viscous flow of olivine

Earthquakes are triggered by the sudden release of strain energy accumulated in the Earth’s crust and mantle. Around 55 earthquakes are located every day around the world, and 16 large earthquakes of magnitude greater than 7 are expected in any given year. These events are responsible for many deaths and for major natural disasters. The periodicity of rupture events is controlled by complex variables such as fault surface roughness, fault geometry, fluid-rock interactions, fluid pressure oscillations and the mechanics of the fault rock. Seismology, rock mechanics experiments and modelling have provided vital insights into the behaviour and frictional properties of faults, but the brittle-viscous response of rock to earthquake rupture and the passage of shock waves is currently not understood.

Natural olivine exposed at the Earth surface, but derived from Earth’s upper mantle, display microstructures where brittle, frictional and viscous deformation coexist. Here we study the microstructures and textures locked in the geological record of the Premosello peridotite to understand the transient brittle-viscous deformation mechanisms triggered during large earthquakes, and their contribution to energy dissipation and build-up during the earthquake cycle.

In this study we use electron backscatter diffraction (EBSD) in the SEM, as well as TEM analyses, to detail the micron to nanoscale structures of olivine in samples from the shallow upper mantle, collected near thick pseudotachylytes in proximity of the Mohorovičić discontinuity, which juxtaposes these peridotites with the lower crustal granulites of the Ivrea-Verbano Zone.