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

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.