Monochromatic Long-Period Seismicity Prior to the 2012 Earthquake Swarm at Little Sitkin Volcano, Alaska

Abstract

Detection of the earliest stages of unrest is one of the most challenging and yet critically needed aspects of volcano monitoring. We investigate a sequence of five unusual long-period (LP) earthquakes that occurred in the days prior to the onset of a months-long volcano-tectonic (VT) earthquake swarm beneath Little Sitkin volcano in the Aleutian Islands during late 2012. The long-period earthquakes had two distinctive characteristics: their signals were dominated by a monochromatic spectral peak at approximately 0.57 Hz and they had impulsive P and S-wave arrivals on a seismometer located on Amchitka Island 80 km to the southeast of the volcano. In each case, the monochromatic earthquakes ended with a higher-frequency event after approximately 2 min of duration. We find evidence that the five monochromatic LP earthquakes resulted from the resonance of a tabular magma body at middle crustal depths (15 km) on the western side of Little Sitkin. Based on the resonant frequency and quality factor of the monochromatic LP earthquakes, we infer the magma body to have a lateral extent of 500 m and a thickness of 9 m. We interpret that a magmatic intrusion excited the monochromatic LP earthquakes and subsequently increased the stress beneath the volcano, leading to the onset of the shallow (<10 km depth) VT swarm five days later.

Earthquakes Record Cycles of Opening and Closing in the Enhanced Seismic Catalog of the 2008 Okmok Volcano, Alaska, Eruption

“…LP earthquakes in volcanic systems have been attributed to effects such as resonance in fluid‐filled cracks (Chouet, 1996; Haney et al., 2021; Kumagai & Chouet, 1999), brittle failure in melt (Goto, 1999; Neuberg et al., 2006), slow rupture in either heated material (Harrington & Brodsky, 2007) or in unconsolidated material (Bean et al., 2014) and magma degassing (Melnik et al., 2020). Migration and burstiness of long period earthquakes in volcanic systems have been previously reported for the preparatory stage of volcanic eruptions and associated with pore‐pressure transients in a hydraulically connected plumbing system (Frank et al., 2018; Shapiro et al., 2017).…”

A Specific Earthquake Processing Workflow for Studying Long‐Lived, Explosive Volcanic Eruptions With Application to the 2008 Okmok Volcano, Alaska, Eruption

“…The former are related to the brittle failure of rocks and usually contain energy in a broader range of the frequency spectrum, but have the majority of their energy at relatively high frequencies (>5 Hz) (Roman, 2005; Roman & Cashman, 2006; Zobin, 2012). The latter are known for producing energy in the lower band of the spectrum (<5 Hz) and are thought to be related to volumetric deformations of fluid fill cracks, the resonance in volcanic conduits, and/or the slow brittle failure of magma in the unconsolidated or warm materials of volcanoes (Bean et al., 2014; Chouet, 1996; Collier & Neuberg, 2006; Haney et al., 2021; Harrington & Brodsky, 2007; J. W. Neuberg et al., 2006).…”

Long-period and volcano-tectonic earthquakes record cycles of opening and closing in the enhanced seismic catalog of the 2008 Okmok Volcano, Alaska, eruption

“…Long-period earthquakes in volcanic systems have been attributed to effects such as resonance in fluidfilled cracks (Chouet 1996;Kumagai and Chouet, 1999;Haney et al, 2021), brittle failure in melt (Goto, 1999;Neuberg et al, 2006), slow rupture in either heated material (Harrington and Brodsky, 2007) or in unconsolidated material (Bean et al, 2014) and magma degassing (Melnik et al, 2020). Migration and burstiness of long period earthquakes in volcanic systems have been previously reported for the preparatory stage of volcanic eruptions and associated with pore-pressure transients in a hydraulically connected plumbing system (e.g.…”