Faulting Modulation of Earthquakes at Axial Seamount, Juan de Fuca Ridge

Abstract:

The occurrence of faults in active volcanic regions is well documented. Seismicity is frequently identified as an eruption precursor and large earthquakes are commonly observed either prior to, or syn-eruption. It has also been postulated that caldera-style eruptions even require faults to trigger their formation. However, numerical models of volcano unrest seldom incorporate these structures, and their impact on triggering eruption is debated. A recent investigation of eruption triggers at the submarine volcano, Axial Seamount, suggests that seismicity on caldera-related faults may relieve some of the host rock stresses induced by the expansion of its magma reservoir. For the Axial magma system, fault slip appears to work as an eruption buffer rather than a catalyst, delaying the onset of Axial’s eruptions until critical overpressurization of the magma reservoir is achieved and eruption commences. We perform a series of numerical experiments to test this hypothesis by assessing the impact of faulting on predictions of volcanic deformation, stress distribution, and triggering mechanisms of eruptions at Axial Seamount. In particular, we utilize COMSOL Multiphysics finite element modeling (FEM) software to develop an Axial-specific FEM with known caldera ring faults simulated by regions of reduced Young’s modulus. Numerical experiments are conducted using a range in Young’s modulus of the simulated ring faults of 0-25% of the surrounding host rock to identify optimal values which reproduce the observed deformation and eruption timing coincident with that of the 2015 Axial eruption. We find that incorporating simulated faults into unrest models impacts both the model predicted surface deformation and the timing of eruption onset. Similarly, we find that fault strength has a significant impact on deformation and the onset of model-predicted eruption. Models with a 15% reduction in fault stiffness and strength perform well, reproducing the observed deformation at Axial Seamount during the 2011 – 2015 unrest period, and hindcasting the timing of its 2015 eruption coincident with the date observed. Results of these numerical experiments lend strong support to the hypothesis that surface deformation and triggering of eruptions at Axial Seamount are modulated by faulting.