Investigating Thermal Shear Instability as a Viable Failure Mechanism for Deep Earthquakes in High Strain-rate Regions of Subducting Slabs

Abstract:

Although the majority of intraplate earthquakes observed rupture in subducted lithosphere more than 50 km below Earth's surface, the rupture mechanics of these deep earthquakes are still not fully understood. At the high temperatures and pressures within subducting slabs, intermediate (70 - 300 km) and deep (300-700 km) earthquakes cannot rupture by the result of the same brittle failure mechanism as shallow earthquakes (0-70 km). There are currently three proposed failure mechanisms for intermediate and deep earthquakes: dehydration embrittlement, transformational faulting, and thermal shear instability. It has been recently proposed that in addition to thermal constraints on where they occur, the spatial distribution of deep earthquakes may be controlled by strain-rate variations in slabs. It was found that earthquakes occur in areas of high strain-rate within subducting lithosphere, but this correlation does not distinguish between the proposed earthquake failure mechanisms. Here we test the newly hypothesized strain-rate controls by investigating when high strain-rate conditions lead to failure in the deep slab by the mechanism of thermal shear instability. A 2D dynamic visco-elastic-plastic subduction model will be used to determine the physical state of the slab at depth (e.g. stress, temperature, pressure) and the spatial distribution of regions of high strain-rate in the Tonga, Japan, and South America subduction zones. The conditions found to occur in high strain-rate regions will then be used in a 1D thermal shear instability model to assess whether thermal runaway occurs and therefore can potentially trigger a deep earthquake. Regions of the slabs identified to contain the necessary conditions for thermal runaway to occur will be evaluated through comparison with observed earthquakes (e.g. hypocenter locations, geometry, stress orientations) in those specific subduction zones. This work will help understand the feasibility of thermal shear instability as a potential mechanism for deep earthquake rupture under high strain-rate conditions.