Electrokinetic contributions to self-potential signals from magmatic stressing

Pre-eruptive electrical signals at active volcanoes are generally interpreted in terms of electrokinetic processes. Spatio-temporal self-potential (SP) signals can be caused by strain-induced fluid flow in volcanic aquifers, however, previous studies lack the quantitative assessments of these phenomena and the underpinning poroelastic responses. Here we use Finite-Element Analysis to study poroelastic responses induced by subsurface stressing from sill and dike sources by jointly solving for ground displacements, pore pressure and SP signals. We evaluate the influence of pressure source orientation on the poroelastic response in two different volcanic aquifers (pyroclastic and lava flow) to provide insights on emergent geodetic and SP signals and their sensitivity to governing parameters. Strain-induced SP amplitudes deduced from a reference parameter set vary in both aquifer models and are of negative polarity (-0.35 mV and -22.6 mV) for a pressurized dike and of positive polarity (+4 mV and +20 mV) for a pressurized sill. Importantly, we find uniquely different SP and ground displacement patterns from either sill or dike intrusions. Our study shows that SP signals are highly sensitive to the subsurface Young's modulus, streaming potential coupling coefficient and electrical conductivity of the poroelastic domains. For the set of parameters tested, the dike model predicts SP amplitudes of up to -947 mV which are broadly representative of recorded amplitudes from active volcanoes. Our study demonstrates that electrokinetic processes reflect magma-induced stress and strain variations and highlights the potential of joint geodetic and SP studies to gain new insights on causes of volcanic unrest.