Contributions of poroelastic rebound and a weak volcanic arc to the postseismic deformation of the 2011 Tohoku earthquake

Yan Hu,Jeffrey T Freymueller,Roland Bürgmann,Paramesh Banerjee,Kelin Wang

doi:10.1186/1880-5981-66-106

Abstract

A better understanding of fluid-related processes such as poroelastic rebound of the upper crust and weakening of the lower crust beneath the volcanic arc helps better understand and correctly interpret the heterogeneity of postseismic deformation following great subduction zone earthquakes. The postseismic deformation following the 2011 M w 9.0 Tohoku earthquake, recorded with unprecedented high resolution in space and time, provides a unique opportunity to study these ‘second-order’ subduction zone processes. We use a three-dimensional viscoelastic finite element model to study the effects of fluid-related processes on the postseismic deformation. A poroelastic rebound (PE) model alone with fluid flow in response to coseismic pressure changes down to 6 and 16 km in the continental and oceanic crusts, respectively, predicts 0 to 6 cm uplift on land, up to approximately 20 cm uplift above the peak rupture area, and up to approximately 15 cm subsidence elsewhere offshore. PE produces up to approximately 30 cm of horizontal motions in the rupture area but less than 2 cm horizontal displacements on land. Effects of a weak zone beneath the arc depend on its plan-view width and vertical viscosity profile. Our preferred model of the weak sub-arc zone indicates that in the first 2 years after the 2011 earthquake, the weak zone contributes to the surface deformation on land on the order of up to 20 cm in both horizontal and vertical directions. The weak-zone model helps eliminate the remaining systematic misfit of the viscoelastic model of upper mantle relaxation and afterslip of the megathrust.

Highlights

Geodetic observations of deformation before, during, and after M ~ 9 megathrust earthquakes illuminate the mechanics and rheology of the subduction zone system. Wang et al (2012) summarized three primary subduction processes that dominate earthquake cycle deformation following a great megathrust earthquake: aseismic afterslip on the subduction thrust, viscoelastic relaxation of the upper mantle, and re-locking of the fault
We present a three-dimensional (3D) viscoelastic finite element model to illuminate the effects of the poroelastic rebound in the crust and the rheology heterogeneity below the arc
We have constructed finite element models to study the effects of poroelastic rebound on the postseismic deformation following the 2011 Tohoku earthquake

Summary

Introduction

Geodetic observations of deformation before, during, and after M ~ 9 megathrust earthquakes illuminate the mechanics and rheology of the subduction zone system. Wang et al (2012) summarized three primary subduction processes that dominate earthquake cycle deformation following a great megathrust earthquake: aseismic afterslip on the subduction thrust, viscoelastic relaxation of the upper mantle, and re-locking of the fault. Geodetic observations of deformation before, during, and after M ~ 9 megathrust earthquakes illuminate the mechanics and rheology of the subduction zone system. Wang et al (2012) summarized three primary subduction processes that dominate earthquake cycle deformation following a great megathrust earthquake: aseismic afterslip on the subduction thrust, viscoelastic relaxation of the upper mantle, and re-locking of the fault. Later in the earthquake cycle (e.g., McCaffrey et al, 2013), the earthquake-induced stresses in the mantle are mostly relaxed, and the effects of the re-locking of the fault dominate leading to a landward displacement gradient consistent with elastic deformation about the subduction thrust coupled in the upper approximately 50 km of the lithosphere (Savage, 1983). The recent devastating M ~ 9 megathrust earthquakes in Sumatra, Chile, and Japan provide unique opportunities to improve our understanding of the subduction earthquake cycle through observations of the deformation with modern space-geodetic techniques

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