Geodetic Monitoring at Axial Seamount Since its 2015 Eruption Reveals a
Waning Magma Supply and Tightly Linked Rates of Deformation and
Seismicity
Abstract
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.