Backward Propagation of Seismicity During the 2014–2015 Bárðarbunga Diking Events

Dike propagation governs how magma is transported and emplaced within the crust, fundamentally controlling eruption dynamics and the mechanical state of volcanic systems. Understanding its evolution is therefore essential for assessing volcanic hazards and crustal stress redistribution. Seismicity, which occurs as a dike fractures and deforms the surrounding host rock, provides key evidence for tracking the geometry, velocity, and temporal evolution of dike propagation. While the forward (tipward) propagation of dikes, accompanied by migrating seismicity, has been extensively studied, episodes of backward seismic migration—where earthquakes progress opposite to the main propagation direction—remain poorly understood. The physical mechanism responsible for this phenomenon and its relationship to magma pressure evolution and host-rock damage are still uncertain. To address this, we developed a damage-mechanics-based finite element model that couples fluid dynamics and solid mechanics to simulate the interactions between magma pressure, fracture propagation, and inelastic deformation of the surrounding rock. The model reproduces both forward and backward seismic migration patterns by incorporating stress redistribution and fracture reactivation following transient pressure drops during dike propagation. We apply this framework to the 2014–2015 Bárðarbunga diking events in Iceland—one of the most comprehensively monitored lateral intrusions—to identify the controlling processes behind the observed backward propagation of seismicity. Model results suggest that back-propagation arises from the reactivation of previously damaged segments as magma pressure decays and stress is transferred back along the dike. Our findings provide a mechanistic explanation for the dual propagation behavior of seismicity during dike intrusions and establish a physically grounded approach for linking seismic migration to magma dynamics and crustal damage evolution in active volcanic systems.