Omori‐like decay of postseismic velocities following continental earthquakes

T Ingleby,T J Wright

doi:10.1002/2017gl072865

Abstract

AbstractVarious mechanisms have been proposed to explain the transient, enhanced surface deformation rates following earthquakes. Unfortunately, these different mechanisms can produce very similar surface deformation patterns leading to difficulty in distinguishing between them. Here we return to the observations themselves and compile near‐field postseismic velocity measurements following moderate to large continental earthquakes. We find that these velocities have a remarkably consistent pattern, with velocity inversely proportional to time since the earthquake. This suggests that postseismic velocities show an Omori‐like decay and that postseismic displacements increase logarithmically over time. These observations are inconsistent with simple, linear Maxwell or Burgers body viscoelastic relaxation mechanisms but are consistent with rate‐and‐state frictional afterslip models and power law shear zone models. The results imply that near‐field postseismic surface deformation measurements are primarily the result of fault zone processes and, therefore, that the inference of lower crustal viscosities from near‐field postseismic deformation requires care.

Highlights

The rheology of the continental lithosphere remains poorly understood, with a number of different disciplines contributing sometimes conflicting observations
We return to the observations themselves and compile near-field postseismic velocity measurements following moderate to large continental earthquakes
Glacial Isostatic Adjustment (GIA) studies in continental cratons suggest that the lithosphere has very high viscosities (>1022 Pa s) or behaves elastically [e.g., Peltier and Drummond, 2008; Zhao et al, 2012] and generally resolve deep, mantle relaxation with viscosities on the order of 1020 –1021 Pa s

Summary

Introduction

The rheology of the continental lithosphere remains poorly understood, with a number of different disciplines contributing sometimes conflicting observations. Apparent viscosities are often seen to increase with time since the earthquake leading to inferences of various transient rheologies [e.g., Pollitz, 2003; Ryder et al, 2007; Freed et al, 2010] Whether these inferred viscosities apply to the lower crust, upper mantle or the fault zone itself is not always clear [Bürgmann and Dresen, 2008; Wright et al, 2013]. The observed surface deformation can be explained by ongoing fault slip (afterslip) and it is often challenging to distinguish between these competing hypotheses [e.g., Savage, 1990; Perfettini and Avouac, 2004; Ryder et al, 2007; Hao et al, 2012; Wright et al, 2013].

Data Collection

Data Analysis

Consistency With Omori’s Law

Modeling

Linear Maxwell Models

Discussion