HIGH LEVELS OF BACKGROUND SEISMICITY DURING A PERIOD OF ERUPTIVE QUIESCENCE AT GARELOI VOLCANO, AK

Seismology is the most reliable physical characteristic for identifying volcanic unrest and changes within a volcanic system. Rapid changes in earthquake locations, signal type, depth, magnitude, and quantity have been directly linked to impending volcanic eruptions, thus making volcano seismology the first line of defense in eruption forecasting. Although it has not erupted since 1989, Gareloi Volcano (Aleutian Islands) exhibits exceptionally high rates of volcanic seismicity with up to hundreds of long-period (LP) earthquakes per day. Because seismometers were not installed until 2003, we cannot determine how Gareloi’s pre-eruptive seismicity would differ from the high rates of background seismicity currently observed on the island. Using hypoDD, we relocate ~5,900 earthquakes located by the Alaska Volcano Observatory (AVO) between 2003-2019 to characterize the type of seismicity occurring on the island, explore potential source mechanisms, and identify what signals might indicate potential unrest and eruption.

From 2003-2007, LP earthquakes were the dominant signal type These locate between 2-10 km depth, with hypocenters heavily concentrated beneath the north end of the island. Frequent station outages from 2008-2015 leave a significant gap in our understanding of Gareloi seismicity. Seismicity recorded during 2016-2019 exhibit a higher percentage of high-frequency (HF) earthquakes, and a shift to depths of 6-16 km in a strong SW-NE trend across the island.

Aside from a swarm of multiplets recorded from April-July 2007, Gareloi earthquakes are long period, yet broadly dissimilar, indicating a plethora of seismic sources throughout the island. We postulate that Gareloi is composed of highly fractured, heterogenous material through which volcanic fluids may flow, generating LP seismicity. Weak zones in the edifice, as well as stresses from regional tectonics, contribute to HF activity. Lastly, the only instances of observed multiplet activity experienced at Gareloi occurred from April-July 2007 and May-July 2021; thus, the sudden appearance of highly-correlated earthquakes may be the most telling indication unrest is building within the Gareloi system.