Elastic vs. plastic: Inversion of analogue magma-induced surface displacements in granular materials in laboratory experiments

When magma ascends through the shallow parts of terrestrial planetary crust, it deforms the surrounding host rocks. The deformation patterns observed at the surface offer indirect means to characterize the position, geometry and volume of subsurface magmatic intrusions. To enable real-time eruption forecasting during volcano unrest, most volcano geodetic models assume that magma intrusion induces linearly elastic deformation of homogeneous shallow planetary crust. Other indirect geophysical volcano monitoring data (e.g., seismology, gravimetry) however offer only limited opportunity for validating geodetic model results. Moreover, recent geological observations at exhumed volcano plumbing systems and geophysical observations of recent intrusion events have shown that plastic behaviour can dominate in heavily fractured and heterogeneous volcanic edifices and tectonically active areas. The question remains how large the effect of unaccounted plastic deformation could be on estimated intrusion characteristics.

Scaled laboratory experiments can be an innovative tool to assess by how much modelled magma intrusion characteristics – volume, geometry, position – deviate from reality in circumstances where plastic deformation processes are important. We used a tensile rectangular dislocation in a homogeneous, linearly elastic half-space to invert the three components of near-surface displacements extracted from X-ray Computed Tomography imagery of laboratory experiments of analogue dyke injection in cohesive mixtures of quartz sand and gypsum powder. The model results favored by the inversions are then compared to the three-dimensional characteristics of the analogue magma intrusions observed in the X-ray CT imagery. To further investigate the effect of more complex model geometry, we also used a tensile distributed-opening dislocation geometry. Preliminary results show that inversion results can be improved by fixing values of parameters that control the position of the modelled dislocation, but significant discrepancies remain between the modelled and observed intrusion geometry, orientation and volume. This test study helps gaining insight on the limitations of commonly used volcano geodetic modelling and inversion methods, and provides a novel basis for interpreting geological, geodetic and geophysical data related to volcanic deformation. The experimental results pave the way for developing complex forward models of magma-induced deformation in the heterogeneous shallow crust of terrestrial planets.