Effects of Host-rock Fracturing on Elastic-deformation Source Models of Volcano Deflation

Eoghan P. Holohan,John J. Walsh,Thomas R. Walter,Martin P. J. Schöpfer,Henriette Sudhaus

doi:10.1038/s41598-017-10009-6

Abstract

Volcanoes commonly inflate or deflate during episodes of unrest or eruption. Continuum mechanics models that assume linear elastic deformation of the Earth’s crust are routinely used to invert the observed ground motions. The source(s) of deformation in such models are generally interpreted in terms of magma bodies or pathways, and thus form a basis for hazard assessment and mitigation. Using discontinuum mechanics models, we show how host-rock fracturing (i.e. non-elastic deformation) during drainage of a magma body can progressively change the shape and depth of an elastic-deformation source. We argue that this effect explains the marked spatio-temporal changes in source model attributes inferred for the March-April 2007 eruption of Piton de la Fournaise volcano, La Reunion. We find that pronounced deflation-related host-rock fracturing can: (1) yield inclined source model geometries for a horizontal magma body; (2) cause significant upward migration of an elastic-deformation source, leading to underestimation of the true magma body depth and potentially to a misinterpretation of ascending magma; and (3) at least partly explain underestimation by elastic–deformation sources of changes in sub-surface magma volume.

Highlights

The underlying cause(s) of deformation at volcanoes may be investigated by comparing surface displacement patterns to solutions obtained from analytical or numerical models that contain one or more deformation sources[1,2,3]
While displacements and strains during a volcano inflation episode are generally small, they can become very large during a volcano deflation episode. Deflation can in this case progress to a collapse of the host-rocks[25, 26], which can occur exclusively underground[27] or with formation of a caldera at the surface[28, 29]. This progression from low-strain inflation to high-strain collapse is exemplified by activity at Piton de la Fournaise volcano in 2007
We tested the hypothesis that such changes in the elastic-deformation source inferred for the March-April 2007 activity at Piton de la Fournaise are related to the formation and propagation of collapse-related fracture systems[30]

Summary

Introduction

The underlying cause(s) of deformation at volcanoes may be investigated by comparing surface displacement patterns to solutions obtained from analytical or numerical models that contain one or more deformation sources[1,2,3] The attributes of such sources may include shape, size, and orientation, as well as changes in volume or pressure. While displacements and strains during a volcano inflation episode are generally small, they can become very large during a volcano deflation episode Deflation can in this case progress to a collapse of the host-rocks[25, 26], which can occur exclusively underground[27] or with formation of a caldera at the surface[28, 29]. The 2007 eruption occurred with a complex seaward motion of the volcano flank[41, 42], but the flank displacements are largely distinct from the summit area displacements, which are the focus of this study

Methods

Results

Conclusion