Abstract
Phreatic and phreatomagmatic eruptions at volcanoes often present no short term precursory activity, making them a challenge to forecast. Poas 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, SO2 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 Poas, sending blocks up to 1 m in length to distances greater than 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 1300 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 hours 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.
Highlights
Phreatomagmatic eruptions at volcanoes occur when magmatic fluid or gases migrating toward the surface interact explosively with ground or surface water
A large phreatomagmatic eruption occurred at Poás volcano, Costa Rica, on 23 April 2017 at 04:12 UTC, following approximately one month of unrest, potentially as a result of pressure build up beneath a partially sealed, chilled margin or hydrothermal seal at depth
We suggest that the VT families are indicative of further fracturing of the volcanic edifice and the seal, creating new fluid pathways, and that the LF seismicity reflects the movement of magmatic fluid through these pathways toward the surface
Summary
Phreatomagmatic eruptions at volcanoes occur when magmatic fluid or gases migrating toward the surface interact explosively with ground or surface water. Precursors are identifiable but may not be able to be processed and analyzed in an appropriate time frame to give an alert, for example, the 1990 eruption of Kelut volcano, Java, which was preceded by spasmodic tremor in the hours prior to a number of phreatic explosions (Lesage, 1995). These types of eruptions are difficult to forecast in terms of timing and intensity, especially when lacking precursory activity, principally as groundwater reservoir locations and heat flow within the volcano are often poorly constrained. Renewed phreatic activity was observed at Poás between June and August 2016
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