Linear Relationship Between Aftershock Productivity and Seismic Coupling in the Northern Chile Subduction Zone

S Hainzl,B Schurr,C Sippl

doi:10.1029/2019jb017764

Abstract

AbstractThe aftershock productivity is known to strongly vary for different mainshocks of the same magnitude, which cannot be simply explained by random fluctuations. In addition to variable source mechanisms, different rheological properties might be responsible for the observed variations. Here we show, for the subduction zone of northern Chile, that the aftershock productivity is linearly related to the degree of mechanical coupling along the subduction interface. Using the earthquake catalog of Sippl et al. (2018, https://doi.org/10.1002/2017JB015384), which consists of more than 100,000 events between 2007 and 2014, and three different coupling maps inferred from interseismic geodetic deformation data, we show that the observed aftershock numbers are significantly lower than expected from the Båth's law. Furthermore, the productivity decays systematically with depth in the uppermost 80 km, while the b value increases. We show that this lack of aftershocks and the observed depth dependence can be simply explained by a linear relationship between the productivity and the coupling coefficient, leading to Båth law only in the case of full coupling. Our results indicate that coupling maps might be useful to forecast aftershock productivity and vice versa.

Highlights

IntroductionThe reason might be the variability of the mainshock source mechanisms and rheological properties
An important empirical feature of aftershock sequences is the so-called Båth's (1965) law, which states that the magnitude difference Δm between a mainshock and its largest aftershock does not depend on the mainshock magnitude M and is on average close to Δm = 1.2
Our results show that the average aftershock productivity is in agreement with the aftershock activity associated to the Båth's law with Δm = 1.2 only in the limit of fully coupled fault zones

Summary

Introduction

The reason might be the variability of the mainshock source mechanisms and rheological properties It has been shown by Bouchon and Karabulut (2008) that supershear ruptures lead to depleted on-fault aftershocks, which likely affects the total number of triggered aftershocks. Another source effect is proposed by Marsan and Helmstetter (2017), who analyzed the aftershock productivity in southern California and suggested a relation of the observed variability with the stress drop variability of the mainshocks. The latter explanation is in accordance with the observation that oceanic transform faults trigger relatively weak aftershock sequences (Boettcher & Jordan, 2004; McGuire et al, 2005), indicating that seismic coupling may be an important factor to explain productivity changes

Results

Discussion

Conclusion