The mechanism of partial rupture of a locked megathrust: The role of fault morphology

Qiang Qiu,Emma M Hill,Eric O Lindsey,Paul Tapponnier,Sylvain Barbot,Sergey V Samsonov,Wanpeng Feng,Lujia Feng,Keren Dai,Judith Hubbard

doi:10.1130/g38178.1

Abstract

Assessment of seismic hazard relies on estimates of how large an area of a tectonic fault could potentially rupture in a single earthquake. Vital information for these forecasts includes which areas of a fault are locked and how the fault is segmented. Much research has focused on exploring downdip limits to fault rupture from chemical and thermal boundaries, and along-strike barriers from subducted structural features, yet we regularly see only partial rupture of fully locked fault patches that could have ruptured as a whole in a larger earthquake. Here we draw insight into this conundrum from the 25 April 2015 M w 7.8 Gorkha (Nepal) earthquake. We invert geodetic data with a structural model of the Main Himalayan thrust in the region of Kathmandu, Nepal, showing that this event was generated by rupture of a decollement bounded on all sides by more steeply dipping ramps. The morphological bounds explain why the event ruptured only a small piece of a large fully locked seismic gap. We then use dynamic earthquake cycle modeling on the same fault geometry to reveal that such events are predicted by the physics. Depending on the earthquake history and the details of rupture dynamics, however, great earthquakes that rupture the entire seismogenic zone are also possible. These insights from Nepal should be applicable to understanding bounds on earthquake size on megathrusts worldwide.

Highlights

Earthquake magnitudes are directly related to the spatial extent of the fault patch that ruptures and the amount of slip on that patch
In this study we use the three-dimensional (3-D) fault morphology of Hubbard et al (2016) for inversion of geodetic data to estimate the extent of rupture, showing that it was bounded by ramps on all sides, and for dynamic models of slip evolution that explain the role of this morphology in generating a complex earthquake history with single and multisegment ruptures
The model for fault morphology was determined by studying the surface geology, topography, and seismicity of the area (Fig. 1; Hubbard et al, 2016)

Summary

Introduction

INTRODUCTION Earthquake magnitudes are directly related to the spatial extent of the fault patch that ruptures and the amount of slip on that patch. In this study we use the three-dimensional (3-D) fault morphology of Hubbard et al (2016) for inversion of geodetic data to estimate the extent of rupture, showing that it was bounded by ramps on all sides, and for dynamic models of slip evolution that explain the role of this morphology in generating a complex earthquake history with single and multisegment ruptures.

Results

Conclusion