Abstract - Continuum-based modeling of surface displacements measured from SAR interferograms sometimes lead to estimates of sub-surface deformation source geometries that have uncertain geological meaning. For example, recent studies of subsidence during a 2009 eruption at Fernandina volcano and during a 1997-2001 quiescence period at Alcedo volcano have linked the observed surface displacements, at least in part, to contraction of shallowly-inclined (4-25°) prolate ellipsoidal sources (i.e. cigar-like ‘Yang sources’). Directly relating such sources to the shape and orientation of subterranean magma bodies is not readily reconcilable with field data typical of sub-volcanic igneous intrusions, however. A hypothesis proposed here is that such inclined source geometries may represent a deflating magma body that causes discontinuous (i.e. fracture-related) deformation in the surrounding host rock. To test this hypothesis, we extracted surface displacement data from Distinct Element Method (DEM) models of magma body deflation that simulate a transition from continuous to discontinuous deformation with fracture localization. We then performed continuum-based modeling of the DEM surface data to retrieve the apparent sub-surface deformation source. Our results show that discontinuous deformation at depth induces asymmetries in the model’s free surface displacement profile even if no discontinuities reach the surface. This is because the accumulation of discontinuous deformation tends to be path-dependent and ultimately asymmetric. The deformation-induced asymmetry of the surface displacement profile can, in principle, give rise to inclined or prolate deformation source geometries, even if the original magma body itself was not inclined. Consequently, continuum-discontinuum models can provide a basis for more geologically-plausible interpretations of the observed InSAR data than in the case of purely elastic models.
Abstract of poster presented at:
Fringe 2011: 8th International Workshop on 'Applications in the Science and Applications of SAR Interferometry', Frascati, Italy, September 2011.