VOLCANIC GEODESY: FROM MOUNT ST. HELENS TO AXIAL SEAMOUNT

Monitoring of ground deformation at Mount St. Helens in the early 1980s lead by Don Swanson showed the value of: (1) making basic geologic observations, (2) adapting to opportunities that present themselves, (3) developing creative ways to make useful time-series measurements, even if they weren’t pretty or sophisticated, and (4) looking for patterns in these observations that reveal information about hidden volcanic processes, and that might even aid in eruption forecasting. I have taken the lessons I learned from Don at Mount St. Helens and the fascination I discovered in volcano geodesy there, and have adapted them to a very different environment at Axial Seamount. Axial Seamount is the most active submarine volcano in the NE Pacific, located on the Juan de Fuca Ridge, offshore Oregon and Washington. Axial is a basaltic hot spot volcano atop a spreading ridge and its most recent eruptions were in 1998, 2011, and 2015. It has a summit caldera that is 8 x 3 km across and up to 100 m deep, underlain by a shallow magma reservoir that has been imaged seismically. However, none of the deformation monitoring techniques used on land work at the bottom of the ocean. Instead, precise pressure sensors have been used at Axial to document vertical movements of the seafloor for the last several decades. This unique time-series has shown that Axial has a continuous magma supply and a repeatable pattern of gradual, slow inflation between eruptions (at rates of 10-90 cm/yr) and sudden, rapid deflation during eruptions (of up to 2 to 4 m). The recognition of this apparently repeatable deformation pattern at Axial has inspired efforts to forecast eruptions based on the hypothesis of inflation-predictable behavior. This has been surprisingly successful: in September 2014 the next eruption at Axial Seamount was forecast to occur sometime in 2015 and that eruption occurred in April 2015. The level of inflation reached before the 2015 eruption was only 30 cm higher than the level reached at the start of the previous eruption in 2011. The inflation time-series at Axial has also shown that the rate of inflation (and presumably magma supply) increased by more than four times after the 2011 eruption, which greatly shortened the recurrence interval between eruptions. Axial Seamount is now part of a cabled observatory providing real-time monitoring data to shore.