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.

Measurements of ground tilt are a critical geodetic tool for monitoring active volcanoes because they provide multidimensional data that can resolve complex deformation signals. We are developing a Self-Calibrating Tilt Accelerometer (SCTA) for use in the marine environment and present results from two deployments: on land at the Scripps Institution of Oceanography Cecil and Ida Green Piñon Flat Observatory and on the seafloor at Axial Seamount on the Juan de Fuca Ridge. The SCTA utilizes a Quartz Sensor Solutions triaxial accelerometer on a gimbal system to periodically rotate the horizontal channels into the vertical to calibrate against the local g vector, achieving high precision and stability within 1 microradian. The SCTA tiltmeter has the added benefit of simultaneously measuring ground accelerations and recording seismic signals. We compare the SCTA performance at the center of the summit caldera at Axial Seamount against a co-located Jewell Instruments LILY tiltmeter on the OOI Cabled Array. The tilt measurements in one direction are consistent, but the data suggest that the deployment platform for the SCTA may be settling in the other direction. We are using data from the ensemble of 4 cabled pressure sensors and 5 tilt sensors at Axial, including the SCTA, to study its inflation behavior since its eruption in 2015. We have identified several significant, cm-scale deflation events of durations of tens of days. The tilt and relative elevations of instrument sites are asymmetric about their turning points, suggesting a more complex mechanism than a simple inflation reversal. We are conducting forward modeling of the deformation signals to determine if the geodetic signals are consistent with differential slip rates, normalized to the rate of inflation/deflation, on the caldera’s outwardly dipping ring faults between these periods. Another plausible mechanism that we plan to investigate is the lateral transport of magma from beneath the southern caldera to either the northern caldera or to a secondary reservoir, located 5 km to the east. These deflation events are potentially important for understanding the mechanisms of magma supply, storage, and transport at Axial Seamount, as well as for accurately forecasting future eruptions, which have been shown to be inflation-predictable.

Axial Seamount is the most active submarine volcano in the NE Pacific Ocean, and is monitored by instruments connected to a cabled observatory (the US Ocean Observatories Initiative Cabled Array), supplemented by autonomous battery-powered instruments on the seafloor (at ~1500 m depth). Axial is a basaltic hot spot volcano superimposed on the Juan de Fuca spreading ridge, giving it a robust and apparently continuous magma supply. It has had three effusive eruptions in the last 21 years: in 1998, 2011, and 2015. Deformation measurements have been conducted at Axial Seamount since the late 1980’s with bottom pressure recorders (BPRs) that can detect vertical movements of the seafloor with a resolution of ~1 cm. This monitoring has produced a 22+ year time-series including co-eruption rapid deflation events of 2.5-3.2 meters, separated by continuous gradual inter-eruption inflation at variable rates between 15-80 cm/yr. The overall pattern appears to be inflation-predictable, with eruptions triggered at or near a critical level of inflation. Using this pattern, the 2015 eruption was successfully forecast within a one-year time window, 7 months in advance. As of December 2019, Axial Seamount has re-inflated 1.98 m (~78%) of the 2.54 m it deflated during the 2015 eruption. We are exploring several methods to forecast the next eruption, including daily extrapolation of the average rate of inflation from OOI BPR data during the last 3 months forward in time until it intersects the threshold reached before the 2015 eruption. Using this method with the difference in inflation between two OOI BPR instruments located 3.5 km apart removes noise from tidal residuals and oceanographic signals that are common to both instruments. This method suggests the next eruption is likely between 2020 and 2024. However, this simple method is complicated by uncertainties in the next inflation threshold (the volcano inflated 20 cm higher before the 2015 eruption compared to 2011), changes in the rate of inflation with time, and by intermittent pauses in the inflation (and seismicity) observed since 2015 that have lasted from a week to several months.