A document by William W. Chadwick. Click on the document to view its contents.

Axial Seamount is a submarine volcano on the Juan de Fuca Ridge with enhanced magma supply from the Cobb Hotspot. Here we compare several deformation model configurations to explore how the spatial component of Axial’s deformation time series relates to magma reservoir geometry imaged by multi-channel seismic (MCS) surveys. To constrain the models, we use vertical displacements from pressure sensors at seafloor benchmarks and repeat autonomous underwater vehicle (AUV) bathymetric surveys covering 2016-2020. We show that implementing the MCS-derived 3D main magma reservoir (MMR) geometry with uniform pressure in a finite element model poorly fits the geodetic data. To test the hypothesis that there is compartmentalization within the MMR that results in heterogeneous pressure distribution, we compare analytical models using various horizontal sill configurations constrained by the MMR geometry. Using distributed pressure sources significantly improved the Root Mean Square Error (RMSE) between the inflation data and the models by an order of magnitude. The RMSE between the AUV data and the models was not improved as much, likely due to the relatively larger uncertainty of the AUV data. The models estimate the volume change for the 2016-2020 inter-eruptive inflation period to be between 0.054-0.060 km3 and suggest that the MMR is compartmentalized, with most magma accumulating in sill-like bodies embedded in crystal mush along the western-central edge of the MMR. The results reveal the complexity of Axial’s plumbing system and demonstrate the utility of integrating geodetic data and seismic imagery to gain deeper insights into magma storage at active volcanoes.

Axial Seamount is a basaltic hot spot volcano with a summit caldera at a depth of ~1500 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 2011-2015, 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.