Renewed Explosive Phreatomagmatic Activity at Poás Volcano, Costa Rica in April 2017

Abstract

Phreatic and phreatomagmatic eruptions at volcanoes often present no short term precursory activity, making them a challenge to forecast. Poás volcano, Costa Rica, exhibits cyclic activity with phreatic and some phreatomagmatic eruptions separated by times of quiescence. The latest phreatomagmatic stage began in March 2017 with increases in crater lake temperatures, SO 2 flux, and the rate of seismicity, as well as accelerated ground inflation near the active crater. On 23 April 2017 at 04:12 UTC, a large phreatomagmatic eruption occurred at Poás, sending blocks up to 1 m in length to distances >1 km. Hindsight analysis revealed a precursory seismic sequence from 25 March to 22 April of similar seismic events (in terms of their frequency and waveform characteristics). Fourteen families of similar seismic events (containing ≥10 events per family) were identified during this precursory sequence, totaling over 1,300 events. An acceleration within the dominant family of LF (low frequency) waveforms was identified, suggesting that a forecast for the onset of the eruption may have been possible using the Failure Forecast Method (FFM). However, no confidence could be placed in the forecast generated, reiterating that not all accelerating trends are suitable for analysis using the FFM, in particular in conjunction with a least-squares linear regression. Our residual analysis further supports the concept that using a least-squares linear regression analysis is not appropriate with this dataset, and allows us to eliminate commonly used forecasting parameters for this scenario. However, the identification of different families of similar seismicity allows us to determine that magmatic fluid on its way to the surface initially became stalled beneath a chilled margin or hydrothermal seal, before catastrophically failing in a large phreatomagmatic eruption. Additionally, we note that 24 h prior to the large phreatomagmatic eruption, all LF families became inactive, which could have been falsely interpreted in real time as the waning of activity. Our results suggest that identifying families of seismicity offers unique opportunities to better understand ongoing processes at depth, and to challenge conventional forecasting techniques.

Insights on Hydrothermal‐Magmatic Interactions and Eruptive Processes at Poás Volcano (Costa Rica) From High‐Frequency Gas Monitoring and Drone Measurements

“…A permanent MultiGAS was installed again once eruptive activity had diminished in November 2017. Details of the seismic and GPS networks and methods are available in Salvage et al (2018).…”

“…Three phases of eruptions are identified since 2012 ( Figure 1). Phase 3 (12 April 2017 to 24 September 2017) produced steam-rich eruption columns to~4 km ( Figure S1b) and culminated in the first significant expulsion of magma at Poás since 1953-1955(Salvage et al, 2018. Phase 2 (5 June 2016 to 17 September 2016) produced~80 phreatic eruptions, with column heights ≤400 m. Phase 1 and 2 eruptions were similar, consisting of explosive expulsion of lake water, sediments, and altered bombs in cypresoidal columns accompanied by radial steam-rich base surges.…”

“…The first evidence for rupture of the seal was suggested by a change in SO 2 /CO 2 slope in November 2016, which was accompanied by deflation and followed by increasing LP seismicity after January 2017 (Figure 1; Salvage et al, 2018). The notable but brief increase in SO 2 /CO 2 during small phase 2 eruptions (2016) is interpreted as a response of the system to sealing of the dome fumarole conduit, diverting emissions to the lake vents and driving small phreatic eruptions.…”

“…Phase 2 (5 June 2016 to 17 September 2016) produced~80 phreatic eruptions, with column heights ≤400 m. Phase 1 and 2 eruptions were similar, consisting of explosive expulsion of lake water, sediments, and altered bombs in cypresoidal columns accompanied by radial steam-rich base surges. Phase 3 (12 April 2017 to 24 September 2017) produced steam-rich eruption columns to~4 km ( Figure S1b) and culminated in the first significant expulsion of magma at Poás since 1953-1955(Salvage et al, 2018.…”

“…The crater typically hosts a warm hyper-acid lake (pH < 1,~50°C; Martínez et al, 2000;Rowe et al, 1992), which frequently produces small phreatic eruptions. In April 2017 the volcano produced larger VEI 2 phreatic to phreatomagmatic eruptions, the most significant activity since 1955 ( Figure S1 in the supporting information; Salvage et al, 2018). Poás was visited by~500,000 tourists per year (before the 2017 eruption) and is located just 22 km north of the heavily populated Central Valley (~3.5 million people) and SJO international airport.…”