Long-Period Seismicity At Shishaldin Volcano, Alaska

Abstract

Since it last erupted in 1999, Shishaldin Volcano, Aleutian Islands, Alaska, has been characterized by a continuous and extremely high level of seismicity. The activity consists of many hundreds to thousands long-period (LP; 1-2 Hz) earthquakes per day. The rate of one LP event every 0.5-5 minutes has remained more or less constant for the last 7 years. A high rate of LP seismicity has been associated with pre-eruptive activity at many other volcanoes presented in the volcano seismology literature. Shishaldin, however, shows no other signs of volcanic unrest except for a ~200 m high steam plume that nearly always emanates from the volcano's summit and occasional weak thermal anomalies observed in satellite imagery. This thesis investigates the nature of Shishaldin's unusual volcanic behavior, and provides a case-study that mainly focuses on seismic data recorded by the short-period monitoring network surrounding the volcano, but also integrates local infrasound data, visual observations and SO2 measurements. The observations suggest a steady-state volcanic process within an open conduit system that is capable of releasing a large amount of energy, approximately equivalent to at least one magnitude 1.8-2.6 earthquake per clay. Shishaldin infrasound signals recorded by a pressure sensor co-located with a seismic instrument are used to confine the source locations of the LP events to a depth of 240 +/- 200 m below the crater rim. The seismo-acoustic data suggest that the LP earthquakes are associated with degassing explosions, created by complex gas volume ruptures from a fluid-air interface. Measurements of the SO2 flux within the puffing summit plume have revealed low values (58 tons/day), suggestive of a hydrothermal system. Four time periods of increased earthquake amplitudes, which each lasted about 1-2 months; have been analyzed. The periods of elevated seismicity are characterized by an abundance of LP events with highly similar waveforms that represent a spatially confined, repetitive, and non-destructive source process. A mechanism, known as choked flow, fulfills all the requirements implied by the observed repeating events and provides a plausible trigger mechanism for Shishaldin's LP events. The observations suggest that the hydrothermal system at Shishaldin is multi-fractured, regulating a pressure gradient within the gas flow through the uppermost conduit.