Repeating Earthquakes During Multiple Phases of Unrest and Eruption at Mount Agung, Bali, Indonesia, 2017

Abstract

In 2017, Mount Agung produced a small (VEI 2) eruption that was preceded by an energetic volcano-tectonic (VT) swarm (>800 earthquakes per day up to M4.9) and two months of declining activity. The period of decreased seismic activity complicated forecasting efforts for scientists monitoring the volcano. We examine the time history of earthquake families at Mount Agung in search of additional insight into the temporal changes in the shallow crust prior to eruption. Specifically, we analyze the period of declining seismic activity about five weeks prior to the eruption when forecasting uncertainty was greatest. We use REDPy (Hotovec-Ellis and Jeffries, 2016) to build a catalog of 6,508 earthquakes from 18 October 2017–15 February 2018 and group them into families of repeating earthquakes based on waveform similarity using a cross-correlation coefficient threshold of 0.8. We show that the evolution of earthquake families provides evidence that Mount Agung was progressing toward eruption even though overall earthquake rates and seismic-energy-release declined. We find that earthquake families that dominated seismicity during the beginning of the crisis ceased near the onset of tremor on 12 November 2017. Then, earthquake families took on characteristics commonly observed during effusive phases of eruptions on 15 November—a full six days before the first phreatomagmatic eruption on 21 November 2017 and a full ten days before the actual onset of lava effusion on 25 November 2017. We interpret the transitions in seismicity as the manifestation of a three-phase physical model including an Intrusion Phase, a Transition Phase, and a Eruptive Phase. During the Intrusion Phase, seismicity was dominated by VT earthquakes with a relatively high percentage of repeaters (59%) grouped into numerous (65) simultaneous families. During the Eruptive Phase, seismicity included both VT and low frequency earthquakes that grouped into relatively long-lived families despite a low overall percentage of repeaters (14%). The Transition Phase exhibited characteristics of earthquake families between the Intrusion Phase and Eruptive Phase. We conclude that the time history of earthquake families provides insight into the evolution of the stress distribution in the volcanic edifice, the development of the volcanic conduit, and seismogenesis of magma effusion. Finally, we discuss the role that repeating earthquakes could play in real-time monitoring at restless volcanoes. Our work suggests eruption forecasts can be improved by incorporating automatic processing codes to assist seismologists during sustained periods of high earthquake rates, even at sparsely monitored volcanoes.

Return From Dormancy: Rapid Inflation and Seismic Unrest Driven by Transcrustal Magma Transfer at Mt. Edgecumbe (L’úx Shaa) Volcano, Alaska

“…REDPy utilizes a standard short‐time average/long‐time average (STA/LTA) algorithm (Allen, 1978) to trigger on events, before cross‐correlating events with successive triggers in an attempt to group them into families. REDPy has been used to quantify the temporal evolution of repeating seismicity at several other volcanic settings (e.g., Hotovec‐Ellis et al., 2022; Wellik et al., 2021). To focus on swarm‐like clusters of events, we opt for a relaxed STA/LTA trigger ratio of 4 on data filtered between 1 and 10 Hz, and a high cross‐correlation threshold of 0.7 to group up events.…”

High‐Resolution InSAR Reveals Localized Pre‐Eruptive Deformation Inside the Crater of Agung Volcano, Indonesia

“…It is possible for magma to intrude at such shallow depths (e.g., Cordón Caulle, Chile, and Usu, Japan, Castro et al., 2016; Delgado et al., 2019; Tobita et al., 2001). However, we propose a shallow hydrothermal system as the source of deformation based on: (a) the lack of magmatic gases emitted prior to eruption, pointing to extensive scrubbing by a hydrothermal system (Syahbana et al., 2019; Symonds et al., 2001), (b) the increased activity of the fumarolic field located in the crater during September‐October 2017, (c) the shallow depth of the modeled source (<200 m) and the lack of temperature anomalies at the summit until late 2017, which exclude the presence of a persistent high‐melt‐fraction magma body (possibly remnant from the 1963 eruption) in the shallow subsurface, (d) migration of seismic activity from October‐November, attributed to magma ascent (Sahara et al., 2021; Wellik et al., 2021), lagging behind the intra‐crater deformation starting in September, and (e) the difference in volume change associated with the September‐October deformation (∼1 × 10 4 m 3 ) and the erupted magma from 25 November to 18 December 2017 (27 × 10 6 m 3 ) (Andaru et al., 2021). While sub‐surface volume change and erupted volume are not expected to be equal (Kilbride et al., 2016; Yip et al., 2022), the orders of magnitude difference points to differing sources/mechanisms.…”

“…The signal taking place during 17–21 November 2017 (4 days–11 hr before eruption onset), provides valuable insights into the underlying mechanism of deformation. There is compelling evidence that magma was ascending to shallow depths by this time as indicated by: (a) the onset of tremor on 12 November 2017 (Syahbana et al., 2019), (b) a migration of seismic activity from the site of the dyke intrusion to the summit of Agung from October to November 2017 (Sahara et al., 2021), (c) a change in the behavior of earthquake families from intrusive (before 12 November 2017) to eruptive (after 15 November 2017) (Wellik et al., 2021), and (d) Anomalous CO 2 gas detected on the morning of the eruption (Syahbana et al., 2019).…”

“…Either this reservoir was exhausted during the 1963 eruption and no longer exists, or the magma is so gas‐rich (Syahbana et al., 2019) that it is highly compressible and the deformation is below the detection limit (Albino et al., 2019; Kilbride et al., 2016; Yip et al., 2022). Deformation associated with magma ascent is rarely observed and probably indicates that the ascent was rapid and occurred a few days before the onset of eruption (Wellik et al., 2021). Despite the lack of direct evidence of shallow magma movement, our observations suggest that precursory deformation signals do exist and can be detected using the right data at the right time.…”

“…The heightened seismic activity and displacement lasted approximately 4 weeks before diminishing in mid‐October (see Figure 2). Seismicity continued from October to November, but at a slower rate, introducing large forecasting uncertainties (Syahbana et al., 2019; Wellik et al., 2021). Eventually, CVGHM lowered the alert level to Siaga (3/4) on 29 October 2017.…”