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

IntroductionThe degree of openness of a volcanic system is recognized as a major determinant of behavior (e.g., Vergniolle & Métrich, 2021; Seropian et al., 2021; Roman et al., 2019). Whether magma or gas can escape determines the state of stress, and ultimately the eruptibility. Similarly, when an eruption begins, the opening of the vent is the most important factor in any subsequent dynamics. Eruptions are often prolonged and complex sequences of events with eruptive plumes separated by quiescent periods, that might be indicative of resealing, changes in magma fragmentation, or exhaustion of supply. Characterizing the eruptive processes is difficult as high temporal resolution of co-eruptive observations requires techniques that can track behavior continuously during one of the most difficult-to-observe periods of the volcanic cycle.Volcano seismicity provides a promising road into solving the problem since often the earthquake record provides continuous and highly resolved information. However, seismicity during long-lived, explosive eruptions is practically invisible because of the extreme noise levels of the eruption itself. In a companion paper, we have set out a workflow to solve the detection problem utilizing modern earthquake detection methods (Garza-Giron et al., submitted). As we showed in that paper, we can use a combination of traditional, machine learning and template matching approaches to expand the co-eruptive seismic catalog of the 2008 Okmok Volcano eruption by about a factor of 10. In this paper we now use that catalog to address the first-order questions about co-eruptive seismicity that have not been previously accessible:When does the overall earthquake rate increase or decrease in context of the eruption? How does the seismicity evolve as the volcano opens and reopens to erupt material?Okmok Volcano is a 10 km wide basaltic-andesitic caldera located on Umnak Island, in the Aleutian arc of Alaska (Figure 1). For over a century, most of the eruptions, the last of which occurred in 1997, had their source at an intra-caldera cone (Cone A; Figure 1 inset) and were mostly Hawaiian to Strombolian (Coats, 1950; Grey, 2003). The 12 July 2008 eruption, which was given a scale of 4 in the Volcanic Explosivity Index (VEI), marked a change in this behavior because of the interactions between magma and water, making new intra-caldera maars and developing a new tephra cone during a large phreato-magmatic eruption (Larsen et al., 2015). Since the island has a topographical regional-scale tilt toward the northeast, the northern sector is characterized by larger bodies of surface and groundwater, with approximately 1010 kg of water available for the 2008 eruption (Unema et al., 2016). Multiple geophysical studies, most of which focused on modeling the source of geodetic deformation, have found the location of a shallow (2-4 km) magma reservoir at approximately the same location in the caldera (Figure 1; Mann et al., 2002; Fournier et al., 2009; Biggs et al., 2010; Freymueller and Kaufman, 2010; Lu and Dzurisin, 2010; Masterlark et al., 2010; Albright et al., 2019; Xue et al., 2020; Wang et al., 2021). Furthermore, different authors have shown that inflation cycles started immediately after the end of the deflationary eruptive periods in 1997 and 2008, suggesting a quick replenishment of the shallow magma reservoir to compensate the pressure gradient (Lu et al., 2005; Lu and Dzurisin, 2010; Freymueller and Kaufman (2010); Wang et al., 2021).During the 6 months preceding the eruption, the Alaska Volcano Observatory (AVO) detected only 9 low magnitude (M<=2.6) earthquakes, although many of the stations in the region had outages during those months. Most of the inter-eruptive seismicity is localized in a geothermal field on the isthmus of Umnak Island inland from Inanudak Bay (Figure 1). The only precursory activity to the eruption came on 12 July 2008, when the seismic network at Okmok Volcano recorded the onset of a ~4.5 hour-long earthquake swarm (Larsen et al., 2009; Johnson et al., 2010) after which explosive activity commenced. The short sequence of precursory earthquakes was reanalyzed by Ohlendorf et al. (2014) using the AVO catalog, and the earthquakes originated at approximately 3 km depth beneath the intra-caldera cone known as Cone D (Figure 1 inset). The beginning of the eruption was marked by a large-scale sub-Plinian explosion that released a ~16 km above sea level (ASL) high dark plume, consistent with a VEI 4 eruption (Newhall and Self, 1982). This plume was accompanied by more than 12 hours of continuous high-amplitude seismic eruption tremor (Larsen et al., 2009). Tremor continued at variable levels throughout the 40-day-long eruption and emanated mainly from a new intra-caldera cone (Haney, 2010; Haney, 2014). This new cone, to the north of Cone D and built during the 2008 eruption, was subsequently named Ahmanilix, which means ‘surprise’ in the language of the Unangan people whose ancestral lands included Umnak Island (Larsen et al., 2015). After the initial plume, the activity continued by the opening, and perhaps widening, of new vents in a westward alignment from the north-west of Cone D. On July 19, the network recorded high-amplitude continuous tremor that lasted ~20 hours, and is thought to be related to the initiation of the temporary drainage of the perennial North Cone D Lake (hereby called North Lake) (Figure 1). The drainage of the lake was verified by satellite imagery until August 1 when standing water was observed again at the lake (Larsen et al., 2015). Whether the lake refilled before August 1 is unknown. Moreover, Larsen et al. (2015) reported that between July 24 and August 1 the North Vent structure, directly to the north of Ahmanilix, widened and there was an increase in number and size of reflectors observed in Synthetic Aperture Radar (SAR) images, accompanied by an increase in ash production from August 1 until August 3, confirmed by AVO staff in the field. From August 3 until August 19, when the last emission of ash was reported and the eruption officially ended, the plumes decreased in number and size.