Inchworm-like source evolution through a geometrically complex fault fueled persistent supershear rupture during the 2018 Palu Indonesia earthquake

Ryo Okuwaki,Shiro Hirano,Yuji Yagi,Kousuke Shimizu

doi:10.1016/j.epsl.2020.116449

Ryo Okuwaki, Shiro Hirano + Show 2 more

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https://doi.org/10.1016/j.epsl.2020.116449

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Abstract

How does fault slip follow an earthquake rupture front propagating faster than the local shear-wave velocity (i.e., at supershear speed)? How does a supershear rupture front pass through a geometrically complex fault system? Resolving the evolution of such complex earthquake ruptures is fundamental to our understanding of earthquake-source physics, but these events have not been well captured by conventional waveform inversions of observational data. We applied a new framework of finite-fault inversion to globally observed teleseismic waveforms and resolved both the spatiotemporal evolution of slip and the fault geometry of the 2018 Palu earthquake (moment magnitude 7.6) in Sulawesi, Indonesia. We show that supershear rupture propagation for this event was sustained by transient slip stagnation and advancement as the rupture front passed through the geometrically complex fault system. This peculiar inchworm-like slip evolution was caused by the rupture front encountering fault bends with favorable and unfavorable orientations for rupture propagation. Our analysis also identified the possible existence of a fault junction beneath Palu Bay connecting an unmapped primary fault in northern Sulawesi with the Palu-Koro fault in the south.

Highlights

How earthquake ruptures evolve within geometrically complex fault systems is an intriguing issue in earthquake science
Bouchon et al (2010) reported that supershear rupture is likely promoted along smooth faults, rather than along those that are geometrically complex, and Bao et al (2019) showed that supershear rupture can persist across major bends in a fault system
Given the known geometry of the Palu-Koro fault (Bellier et al, 2001) and the fault geometry we modeled, the northern part of the fault can be considered to represent the optimal plane for maximum mean horizontal stress, which likely explains the higher slip rates we modeled there

Summary

Introduction

How earthquake ruptures evolve within geometrically complex fault systems is an intriguing issue in earthquake science. Seismic-waveform analyses have resolved complex evolution of ruptures associated with geometric barriers and have shown that such barriers can control both rupture direction and speed (Bouchon et al, 2001, Uchide et al, 2013, Okuwaki and Yagi, 2018). There is a need for further investigation of the relationship between the geometric complexity of a fault system and irregular high-speed rupture propagation that exceeds the local S-wave velocity (known as supershear rupture). Analyses of observed waveforms have generated diverse views of the relationship of supershear rupture to the geometric complexity of fault systems. The details of the kinematic evolution of supershear fault rupture across geometrically complex fault systems have not been well resolved from analyses of observational data

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