Earthquake sequence in the NE Lut, Iran: observations from multiple space geodetic techniques

M Marchandon,M Vergnolle,H Sudhaus,J Hollingsworth,O Cavalié

doi:10.1093/gji/ggy364

Abstract

An increasing number of observations supports temporal clustering behaviour of earthquakes within fault systems. As earthquake occurrence is mainly controlled by the crustal stresses, it is crucial to determine their spatio-temporal evolution to understand the generation of catastrophic seismic sequences. A possible way to constrain these variations is to measure the surface displacement field induced by seismic sequences. However, the observation time of modern satellite geodesy (InSAR/GPS) is short compared to the duration of an earthquake sequence. Thus, the goal of this paper is to extend the temporal range of observations of a seismic sequence. We focus on the largest earthquakes of the 1936–1997, Northeast Lut, Iran, sequence that is composed of 11 Mw ≧ ∼ 6 earthquakes. Using subpixel correlation of historic (KH9) and recent (Sentinel-2) optical satellite images, we measure for the first time the surface displacement field of the 1979 Mw 7.1 Khuli-Boniabad earthquake, which broke the eastern part of the Dasht-e-Bayaz fault. Using subpixel correlation of optical (SPOT2-4) and SAR (JERS-2) images, we also measure the surface displacement field of the Mw 7.2 Zirkuh earthquake, which ruptured the Abiz fault. We found that both earthquakes have a mean slip of 2.5 m but the Khuli-Boniabad earthquake broke two main segments (total rupture ∼ 60 km), whereas the Zirkuh earthquake broke three main segments (total rupture ∼ 125 km). We suggest that the differences are controlled by the maturity of the faults, the Dasht-e-Bayaz fault being less mature than the Abiz fault. Furthermore, we succeed to measure offsets up to 2.60 m for the 1979 Mw 6.6 Korizan earthquake that broke the northern part of the Abiz fault. It is the first time that the surface displacement field for such a small historic earthquake has been measured using optical correlation. Finally, our study confirms the potential of historical optical imagery for retrieving surface displacements for past earthquakes (pre-modern geodesy era).

Highlights

It has long been thought that large earthquakes (Mw > 6) were separated by a long period of interseismic quiescence in which stresses, relaxed by a preceding earthquake, were restored under the constant tectonic loading
An increasing number of observations indicates that large earthquakes occur in temporal clusters within fault systems (e.g. Stein et al 1997; Pollitz et al 2003; Sieh et al 2008; Schlagenhauf et al 2011; Michele et al 2016) suggesting that additional processes are involved in the build-up and relaxation of crustal stresses
Milliner et al 2015; Vallage et al 2015; Hollingsworth et al 2017), but we think our results demonstrate the capacity of optical image correlation data to revisit pre-modern geodesy earthquakes that may have not been intensively investigated in the field after the event

Summary

Introduction

It has long been thought that large earthquakes (Mw > 6) were separated by a long period of interseismic quiescence in which stresses, relaxed by a preceding earthquake, were restored under the constant tectonic loading. It is crucial to understand the spatio-temporal evolution of crustal stresses to more accurately assess seismic hazard. While measuring the crustal stresses at depth currently remains out of reach, modern spatial geodetic techniques (e.g. GPS, InSAR) enable an estimation of lithospheric strain from its expression at the Earth’s surface. To understand the behaviour of fault systems throughout a seismic cycle (lasting as long as several 10–1000 yr) requires geodetic measurements over similar timescales. It is necessary to extend the temporal range of geodetic observation to document the longest possible period of a seismic sequence in order to obtain a comprehensive overview of the surface displacements induced by the sequence. Optical image correlation technique (Michel & Avouac 2002), which allows

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