Magma Accommodation and Ground Movement Analysis: testing the reliability of analytical volcano deformation models 

Abstract

Space for magma intrusion in the subsurface is often created by uplifting and bending of overlying rock, producing a forced fold. This intrusion-induced forced folding can deform the free surface, driving ground movement at active volcanoes. Monitoring such ground movement using satellite- and ground-based instrumentation plays a key role in volcanic eruption forecasting. Specifically, modelling ground movement allows us to assess eruption threats, by estimating intrusion properties (e.g., geometry, depth, volume, and pressure), and understand of volcanic system evolution and behaviour. Yet building accurate ground movement models to reliably estimate intrusion properties requires knowledge of a volcano’s subsurface composition and structure; information which is often limited or unavailable. Many ground movement models therefore tend to use analytical or numerical approaches that embed simple intrusion geometries within a homogeneous, isotropic, and linearly elastic medium (i.e., a material of uniform composition and structure). To assess the reliability and validity of such ground movement models, it would thus be beneficial to identify scenarios where known and modelled intrusion properties can be confidently compared. Seismic reflection data image Earths subsurface in 3D at metre- to decametre-scale resolutions and can thus capture the detailed geometry of ancient intrusions and overlying forced folds. Where such intrusion-fold pairs have been seismically imaged, these data show: (1) intrusion geometries are typically complex; (2) host stratigraphic sequences comprise multiple lithologies; and (3) forced fold amplitudes are often less than intrusion thicknesses, implying space for magma was generated by both uplift and internal fold deformation (e.g., compaction). Importantly, seismic reflection data uniquely allow us to measure and model syn-emplacement ground movement driven by forced folding, whilst independently determining the geometry, size, and depth of underlying intrusions. Here, we examine an Early Cretaceous laccolith and forced fold pair imaged in 3D seismic reflection data from the Exmouth Plateau, offshore NW Australia. We consider how post-emplacement, burial-related compaction has reduced the fold amplitude by using local borehole data, which describe the lithology and seismic velocity of the folded strata, to decompact the succession and recover estimates of the original fold geometry. From these estimates of the original fold geometry, we calculate possible vertical and horizontal displacement components of deformation; this displacement data can be considered akin to that acquired during monitoring of ground movement at active volcanoes. By applying standard analytical methods to estimate intrusion properties from this pseudo-ground movement data, we compare model outputs to the true location, geometry, and size of the seismically imaged intrusion.