Monitoring Dynamic Magma Movement in the Lower Crust During the 2015 Eruption of Axial Seamount

Abstract Observations of lower crustal magma movement during a volcanic eruption are critical to understanding the magma dynamics in the plumbing system and the mechanism that controls the eruption. However, capturing the local and transient magmatic process in the lower crust is seismically challenging, and therefore the dynamic magma movement has been poorly observed. Axial Seamount, an active submarine volcano located at the intersection of the Juan de Fuca Ridge and the Cobb Hotspot and monitored by a permanent cabled array, provides an unprecedented opportunity to study lower crustal magma movement. Here, we use a novel marine geophysical method, Rayleigh‐wave admittance, to reconstruct top‐lower crustal shear‐wave velocity ( Vs ) variation from 3 months before the latest eruption on 24 April 2015, to March 2022. From the variations of daily Vs in the top‐lower crust, we discovered a localized multi‐stage magma movement in the top‐lower crustal channel beneath the southeast caldera. This multi‐stage process reveals a complete pressure evolution in the crustal plumbing system through an eruption cycle of Axial Seamount, which may be applicable to eruptions in ridge‐hotspot interactive volcanoes. In particular, the exponential decrease in Vs initiated 8 weeks before the eruption indicates a pre‐eruption magma accumulation in the top‐lower crustal channel. This accumulation drove the crustal plumbing system to a critical overpressure state and subsequently induced the 2015 eruption. The exponential decrease in Vs caused by lower‐crustal magma accumulation therefore provides a timely warning for eruption forecasting several weeks in advance.