Short-lived tectonic switch mechanism for long-term pulses of volcanic activity after mega-thrust earthquakes

M Lupi,S A Miller

doi:10.5194/se-5-13-2014

Abstract

Abstract. Eruptive rates in volcanic arcs increase significantly after subduction mega-thrust earthquakes. Over short to intermediate time periods the link between mega-thrust earthquakes and arc response can be attributed to dynamic triggering processes or static stress changes, but a fundamental mechanism that controls long-term pulses of volcanic activity after mega-thrust earthquakes has not been proposed yet. Using geomechanical, geological, and geophysical arguments, we propose that increased eruption rates over longer timescales are due to the relaxation of the compressional regime that accompanies mega-thrust subduction zone earthquakes. More specifically, the reduction of the horizontal stress σh promotes the occurrence of short-lived strike-slip kinematics rather than reverse faulting in the volcanic arc. The relaxation of the pre-earthquake compressional regime facilitates magma mobilisation by providing a short-circuit pathway to shallow depths by significantly increasing the hydraulic properties of the system. The timescale for the onset of strike-slip faulting depends on the degree of shear stress accumulated in the arc during inter-seismic periods, which in turn is connected to the degree of strain-partitioning at convergent margins. We performed Coulomb stress transfer analysis to determine the order of magnitude of the stress perturbations in present-day volcanic arcs in response to five recent mega-thrust earthquakes; the 2005 M8.6, 2007 M8.5, and 2007 M7.9 Sumatra earthquakes; the 2010 M8.8 Maule, Chile earthquake; and the 2011 M9.0 Tohoku, Japan earthquake. We find that all but one the shallow earthquakes that occurred in the arcs of Sumatra, Chile and Japan show a marked lateral component. We suggests that the long-term response of volcanic arcs to subduction zone mega-thrust earthquakes will be manifested as predominantly strike-slip seismic events, and that these future earthquakes may be followed closely by indications of rising magma to shallower depths, e.g. surface inflation and seismic swarms.

Highlights

One of the fundamental uncertainties in earth science is the mechanism controlling volcanism in compressional environments
Volcanic belts in convergent margins undergo different tectonic regimes that either restrict or promote vertical migration of deep fluids and magmas by the cyclic loading and unloading of the effective normal stress caused by the recurrence of mega-thrust slips at the subduction interface
We propose a mechanism that operates at timescales greater than the immediate effects of dynamic-stress triggering and long after static stress changes from mega-thrust earthquakes have perturbed the system

Summary

Introduction

One of the fundamental uncertainties in earth science is the mechanism controlling volcanism in compressional environments. We explore the mechanical consequences of a decadal change in the kinematics associated with the post-seismic relaxation and suggest that the tectonic switch from pre-earthquake compression to post-earthquake relaxation results in a dominantly strike-slip stress state emplaced during the “less compressive” time window that follows mega-thrust slips. In this context, increases of crustal permeability via normal stress reduction and strike-slip faulting become the most efficient mechanism for vertical migration of melts and deep slab-derived trapped fluids

Short-lived tectonic switch: proposed mechanism

Supporting arguments

Sumatra