Repeated Short-Term Deflation Events Observed During Long-Term Inflation at Axial Seamount

Abstract:

Axial Seamount is a basaltic hot spot volcano with a summit caldera at a depth of ~1500 m, 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 instruments connected to the US Ocean Observatories Initiative Cabled Array. Geodetic 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 shown co-eruption rapid deflation events of 2.5-3.2 m, separated by gradual inter-eruption inflation at variable rates between 15-80 cm/yr. In this study, we use the difference in inflation between two OOI cabled BPR instruments located 3.5 km apart to remove noise from tidal residuals and oceanographic effects that are common to both instruments. Since its last eruption in April 2015, Axial Seamount has re-inflated to ~85% of its pre-eruption level. During that time, we have identified 7 discrete, short-term deflation events (1-3 cm over 1-3 weeks), that occurred quasi-periodically, about every ~4 months between August 2016 and May 2019. During each short-term deflation event, the rate of earthquakes dropped abruptly to background levels, and then did not return to higher levels until reinflation had resumed and surpassed its previous high. This suggests that a decrease in pressure within the shallow magma supply system may explain both observations. In addition, each short-term deflation event appears to change the average long-term inflation rate between these events. This suggests that the short-term deflation events represent some perturbation in the deep magma supply system, perhaps due to magma re-distribution within the stacked sill complex imaged by multi-channel seismic data at 2.5-4.5 km depth below the seafloor. The most dramatic and unusual example of this pattern is the short-term deflation event that began in May 2019, which was followed by 2 months of “flat-line” behavior (neither inflation nor deflation) and low background levels of seismicity. Since inflation resumed in July 2019, the long-term inflation rate is about half of its previous magnitude. Also, since then, we have not seen any additional short-term deflation events over the last year (as of July 2020).