Modeling the effect of layered volcanic material on magma reservoir failure and associated deformation, with application to Long Valley caldera, California

Elastic, gravitationally loaded finite element models of an ellipsoidal magma reservoir in mechanically layered host rock yield new insight into the effect of layer stiffness, thickness, and spatial configuration on the resulting surface displacement curve and tensile rupture of the reservoir wall. Decreasing layer stiffness enhances surface and subsurface deformation, focusing pressure across a narrow area, and generally decreases the likelihood of vertical magma intrusion and surface eruption. In contrast, increasing layer stiffness impedes uplift and distributes strain across a broad area, enhancing the likelihood of vertical magma intrusion. Application of the model to deformation that occurred between 1985 and 1999 at the Long Valley caldera, California shows that the predicted reservoir radius and depth change by 16% and 12% respectively when mechanical layers inferred to be present are included in the model, highlighting the importance of explicitly incorporating mechanical layering. Analysis of rupture conditions for the reservoirs modeled in the present paper indicates that vertical intrusions are likely. In addition, analysis of rupture conditions for Long Valley magma reservoirs recently inferred from surface displacement inversions indicates that they will fail before achieving sufficient pressure to reproduce measured surface displacement.