ASPERITY: Aseismic SliP and Earthquake Ruptures: Interrogating Transitions in rheologY

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Where and why tectonic faults produce earthquakes or slip aseismically is a critical Earth Science question that remains unanswered, despite representing prerequisite knowledge for probabilistic earthquake forecasting. It is known that earthquakes are typically generated by frictional failure, which initiates where stress is high or strength is low relative to bulk fault zone strength. Fault zones therefore have earthquake-generating patches ('asperities') surrounded by areas likely to creep aseismically. This varied behaviour has been ascribed to variation in fault zone properties. Current models for the spectrum of fault slip styles, however, are based on laboratory-scale observations, typically in single rock types, described by sophisticated empirical constitutive laws. These laws lack insights into underlying physical properties and interaction of multiple materials over km-scales and multiple earthquake cycles. This is a critical knowledge gap that prevents development of realistic earthquake models - ASPERITY will bridge this gap. ASPERITY proposes a generalised model for natural faults where 'asperities' are defined as areas where, over an earthquake cycle, the amount of co-seismic slip exceeds the magnitude of aseismic creep. This model raises the specific hypothesis that interaction between asperities and surrounding fault rock determines fault slip style. To test this hypothesis, we will collect quantitative geological evidence from the rock record, link natural and laboratory deformation microstructures, and develop numerical models to bridge the scale to plate boundary faults. This will lead to specific scenarios that forecast where earthquakes, creep, and slow earthquakes occur in terms of variables that can be quantified in nature. This outcome is a step-change from empirical to physical understanding of where and why some tectonic faults move in episodic, potentially damaging earthquakes, while others creep silently and pseudo-continuously.