To Erupt or Not to Erupt? That is the Question: Extremely High Levels of Background Seismicity and Lack of Eruptivity from 2003-2019 at Gareloi Volcano, Aleutian Islands, Alaska

Mount Gareloi, one of the westernmost volcanoes in the Aleutian arc, has not erupted since 1989, yet it continuously exhibits extremely high levels of background seismicity. Hundreds of volcanic earthquakes are recorded per day on the island since seismometer installation in 2003. I analyzed and classified seismic data collected from 2003-2019 to explore the geophysical processes causing this consistent seismicity with no subsequent eruptive activity. Analysis of waveform and corresponding spectra indicate the vast majority of Gareloi earthquakes are characterized by frequencies between 1 and 5 Hz, which are typical of long-period (LP) events, and these events are particularly dominant from 2003-2007.I relocated 5,884 earthquake hypocenters calculated by the Alaska Volcano Observatory (AVO) using the algorithm hypoDD. From 2003-2007, hypocenters locate shallowly between 2 and 10 kilometers depth and are primarily beneath the north half of the island. Meanwhile, seismicity from 2016-2019 was typically deeper, between 6 and 16 kilometers depth and included more high-frequency (HF) events, characterized by frequencies above >5 Hz. Although relocated hypocenters have similar spatial distribution to the initial locations, seismicity from 2003-2007 clusters more tightly and vertically beneath the island, whereas earthquakes from 2016-2019 are scattered throughout the region with no obvious clustering. Cross correlation of the data reveals multiplet activity occurred from April to July 2007 with minimum correlation coefficients of 0.7, tightly clustered at the northern end of the island and may be the result of heated fluids repeatedly moving through a single fracture within the edifice. This multiplet activity may have been a failed eruptive event.Overall, Gareloi seismicity is broadly distributed and dissimilar, indicating a plethora of seismic sources within the volcanic edifice. I attribute Gareloi’s constant LP seismicity to the movement and exsolution of volcanic fluids and gases within a highly fractured, heterogenous volcanic edifice and magmatic system. I ascribe the sporadic HF activity to brittle failures that result from an accommodation of magmatic stresses on surrounding rock and regional tectonics. Further studies and imaging of Gareloi are necessary to fully understand the magmatic system and geophysical processes producing these high levels of background seismicity and lack of eruptivity in order to better predict signs of volcanic unrest.