FY 2022

Geodetic monitoring at Axial Seamount since its 2015 eruption reveals a waning magma supply and tightly linked rates of deformation and seismicity

Chadwick, Jr., W.W., W.S.D. Wilcock, S.L. Nooner, J.W. Beeson, A.M. Sawyer, and T.-K. Lau

Geochem. Geophys. Geosyst., 23(1), e2021GC010153, doi: 10.1029/2021GC010153, View online (open access) (2022)

Axial Seamount is a basaltic hot spot volcano with a summit caldera at a depth of ∼1,500 m below sea level, superimposed on the Juan de Fuca spreading ridge, giving it a robust and continuous magma supply. Axial erupted in 1998, 2011, and 2015, and is monitored by a cabled network of instruments including bottom pressure recorders and seismometers. Since its last eruption, Axial has re-inflated to 85%–90% of its pre-eruption level. During that time, we have identified eight discrete, short-term deflation events of 1–4 cm over 1–3 weeks that occurred quasi-periodically, about every 4–6 months between August 2016 and May 2019. During each short-term deflation event, the rate of earthquakes dropped abruptly to low levels, and then did not return to higher levels until reinflation had resumed and returned near its previous high. The long-term geodetic monitoring record suggests that the rate of magma supply has varied by an order of magnitude over decadal time scales. There was a surge in magma supply between 2011 and 2015, causing those two eruptions to be closely spaced in time and the supply rate has been waning since then. This waning supply has implications for eruption forecasting and the next eruption at Axial still appears to be 4–9 years away. We also show that the number of earthquakes per unit of uplift has increased exponentially with total uplift since the 2015 eruption, a pattern consistent with a mechanical model of cumulative rock damage leading to bulk failure during magma accumulation between eruptions.

Plain Language Summary. Axial Seamount is an underwater volcano located offshore Oregon, USA, that is frequently active and an ideal site for studying volcanic eruptions, hydrothermal vents, and deep-sea ecosystems. Axial is monitored by a network of seafloor instruments connected to shore by a fiber-optic cable, which is part of the Ocean Observatories Initiative, supported by the National Science Foundation. Monitoring of vertical movements of the seafloor at Axial have shown that it has a repeatable pattern of inflation and deflation that can be used for eruption forecasting. Since its last eruption in 2015, Axial has re-inflated almost to the level of its previous high, but we believe the next eruption is still some years away because the rate of inflation is currently quite low. The monitoring data also show that the rates of earthquakes and uplift are evolving in a predictable way with time, because they are both related to the on-going magma accumulation, which causes the uplift, stresses the crust, and generates earthquakes. Eventually that increasing stress will open a pathway for magma, which will lead to an eruption. This work seeks to understand these processes so that we can better predict the behavior of Axial Seamount and other active volcanoes.