Fault size–dependent fracture energy explains multiscale seismicity and cascading earthquakes

Abstract

Earthquakes vary in size over many orders of magnitude, often rupturing in complex multifault and multievent sequences. Despite the large number of observed earthquakes, the scaling of the earthquake energy budget remains enigmatic. We propose that fundamentally different fracture processes govern small and large earthquakes. We combined seismological observations with physics-based earthquake models, finding that both dynamic weakening and restrengthening effects are non-negligible in the energy budget of small earthquakes. We established a linear scaling relationship between fracture energy and fault size and a break in scaling with slip. We applied this scaling using supercomputing and unveiled large dynamic rupture earthquake cascades involving >700 multiscale fractures within a fault damage zone. We provide a simple explanation for seismicity across all scales with implications for comprehending earthquake genesis and multifault rupture cascades.

Editor’s summary

Earthquakes vary in size, with the largest and most complex also being the most dangerous. The challenge is trying to figure out whether the information gleaned from smaller events can simply be scaled up to understand larger events. Gabriel et al . combined seismic observations with physics-based earthquake models to determine the differences. Their results outline conditions for when multiple-fault, complex ruptures occur and show that fundamentally different fracture processes govern both large and small earthquakes. —Brent Grocholski

Abstract

INTRODUCTION

The catastrophic consequences of large earthquakes governed by complex, multifault rupture dynamics, such as the 6 February 2023 Kahramanmaraş, Türkiye, earthquake doublet. The intricate mechanics of earthquakes, however, remain poorly understood. Natural fault zones are structurally complex systems, comprise fractures and faults of millimeters to hundreds of kilometers in length, and may generate earthquakes over many orders of magnitude. But the details of the earthquake energy budget, its scaling properties, and how multiscale fractures and faults interact dynamically remain enigmatic.

RATIONALE

Traditionally, estimates of fracture energy, the average energy dissipated during an earthquake, are derived from seismological observations by using idealized earthquake models. Utilizing advanced mechanical models of earthquake rupture propagation, we introduced physics-based corrections for seismologically observed fracture energy and developed analytical descriptions of three-dimensional (3D) cracklike circular dynamic ruptures with flash-heating friction and coseismic restrengthening as well as bilaterally expanding kinematic pulselike ruptures with coseismic stress recovery. We synthesized global seismological observations earthquakes with physics-based corrections to estimate the total earthquake fracture energy across a range of rupture sizes. We added fracture energy computed from 12 3D rupture simulations of past small repeating and large earthquakes spanning magnitudes of 1.9 to 9.2. We found that the dynamic weakening and typically neglected restrengthening effects are important for the energy budget of small earthquakes.

RESULTS

Our analysis reveals a linear scaling relationship between a minimum fracture energy and ruptured fault size that is independent of rupture propagation details. We propose that fundamentally different fracture processes govern small and large earthquakes. This explains the linear scaling of the minimum “small-slip” fracture energy with ruptured fault size and implies a fundamental break in earthquake scaling with slip. The minimum fracture energy reflects a local fault property, which can be explained by a well-localized near-front process zone. By contrast, a possibly fault-invariant part of fracture energy increases continuously with earthquake slip and dominates at large slip.

We used supercomputing simulations to demonstrate how fault size–dependent fracture energy facilitates the complex mechanisms driving cascading earthquake nucleation, propagation, and arrest with implications for multifault and multiscale earthquake sequences. We simulated 3D dynamic earthquake rupture and interaction across more than 700 partially intersecting fractures in the damage zone of a planar strike-slip fault. We unveiled large dynamic rupture earthquake cascades involving multiscale fractures within the fault damage zone, which can host ruptures spanning four orders of moment magnitude. These models represent a paradigm shift beyond typical physics-based earthquake models, which often idealize fault zones as infinitesimally thin interfaces with separated on- versus off-fault rheologies. The resulting dynamic rupture cascades can generate large earthquakes consisting only of distributed, multiscale slip across the fault zone fractures. These cascades may or may not dynamically trigger main-fault rupture.

CONCLUSION

We offer a simple explanation for seismicity across scales and provide insight into earthquake genesis and multifault rupture cascades. Our proposed scaling of fracture energy aligns with cascading earthquake observations and the physical mechanisms of localization of brittle deformation before and during earthquakes, implying a fundamental change in the mechanics of earthquake rupture with slip.