Vertical Deformation of the Axial Seamount Summit from Repeated 1-m Scale Bathymetry Surveys Using AUVs

Abstract:

Axial Seamount is an intensely studied submarine hotspot volcano on the Juan de Fuca Ridge that erupted in 1998, 2011, and 2015. We report repeated Autonomous Underwater Vehicle (AUV) bathymetry surveys measuring vertical deformation of the summit region associated with the deflation of the subsurface magma reservoir during the 2015 eruption and the subsequent re-inflation. Within the caldera, the AUV-observed changes in seafloor depth are consistent with biannual pressure measurements at seafloor benchmarks by Nooner and Chadwick. The AUV method is less precise than point pressure measurements but can measure the deformation pattern over a large, spatially continuous area. From 2006-2019, multibeam sonar equipped AUVs have been used to map the summit region and the north and south rift zones of Axial Seamount at 1-meter scale. Repeat AUV surveys were used to map the thickness and extent of lava flows erupted in 2011 and 2015; flow thicknesses greater than 0.2 m were detected. Comparison of repeated components of the 2011 and 2014 surveys revealed uplift between the 2011 and 2015 eruptions that peaked at 1.8 m near the caldera center, and diminished away from this site. Similarly, comparison of the 2014 and 2015 surveys showed the caldera subsidence associated with the eruption, again largest (1.5 m) near the center. Since 2015 we have repeated a survey line set extending up to 5 km outside the caldera; this pattern combines radial lines emanating from the central caldera with connecting lines inside and outside the caldera. The survey pattern extent is 20 km NNW and 10 km ENE. The uplift since 2015 has a maximum of 1.5 m in the central caldera, drops to less than 0.3 m at the farthest locations along the north and south rift axis, and drops to near zero at the farthest locations to the east and west. There are distinct offsets in uplift of 0.4-0.7 m at the caldera walls not buried by recent lavas, indicating that motion on the west and north caldera ring faults is accommodating greater uplift inside the caldera than outside. Significant uplift of 0.7 m is observed to the SE of the caldera to the edge of the survey pattern. This region is underlain by a SE extension of the subsurface magma lens observed in multichannel seismic reflection data, confirming that the uplift pattern closely reflects the subsurface magma reservoir geometry.