Dynamic restrengthening and fault heterogeneity explain megathrust earthquake complexity

TL;DR

This study shows that the complex rupture behavior of megathrust earthquakes like Tohoku-Oki can be explained by dynamic restrengthening and fault heterogeneity, using 3D simulations to match observed rupture features.

Contribution

It introduces a self-consistent model demonstrating how fault heterogeneity and dynamic frictional processes produce observed earthquake complexities.

Findings

Reproduces depth-dependent rupture speeds

Simulates multiple rupture fronts

Explains large tsunamigenic slip near trench

Abstract

Megathrusts host Earth's largest earthquakes. Understanding the physical conditions controlling their rupture dynamics is critical for assessing seismic and tsunami hazards. These earthquakes often display complex rupture dynamics, exemplified by the 2011 Tohoku-Oki earthquake, which exhibited multiple rupture episodes, depth-dependent seismic radiation, and substantial tsunamigenic slip near the trench. However, how such complexity arises from preexisting physical conditions remains uncertain. Here, we demonstrate that the observed rupture complexity of the Tohoku-Oki earthquake can spontaneously and self-consistently emerge, driven by rapid coseismic frictional restrengthening and data-informed fault heterogeneity. We use an ensemble of 3D dynamic rupture simulations to identify that mixed downdip pulse-like and updip crack-like rupture are driven by dynamic stress redistribution with episodic rupture reactivation. By featuring low fault strength compared to its dynamic stress drop, a preferred model can consistently reproduce the observed complex depth-dependent propagation speeds, multiple rupture fronts as imaged by back-projection, and large tsunamigenic slip at the trench. Our findings demonstrate that preexisting fault heterogeneity conjointly with dynamic frictional weakening and restrengthening drives seemingly unexpected megathrust rupture complexity, highlighting the need to include dynamic effects into physics-based seismic and tsunami hazard assessments of future earthquakes.