First hydroacoustic recording of ebullition events in an active volcanic lake – Poás, Costa Rica

This study demonstrates how the sounds of subaqueous gas seeps can be used to measure the volume of volcanic gas being released into an active crater lake, weeks before an eruptive period. A hydrophone placed in Poás crater lake recorded changes in the subaqueous soundscape over the course of one month. Using new passive acoustic inversion techniques, we were able to measure the volume of gas being released from the lakebed at a sample rate of 5 min, far higher than traditional sub aerial gas sampling techniques. Comparing these findings to local seismic measurements allowed us to observe variations in gas flux driven by both volcanic and non-volcanic factors. Non-volcanic causes consist of small-scale diurnal variations of ~2 L min-1 driven by local atmospheric pressure conditions. We also see a large and abrupt aseismic mass bubbling event releasing 18,000 ± 3000 L of gas in just 15 hours (compared to a daily average of 3600 ± 500 L) likely resulting from the collapse of gas pocket(s) in the sediment underlying the lake. Alongside an even larger mass bubbling event releasing 30,000 ± 5000 L of gas in 24 hours correlated with local seismic activity, presumed to be triggered by excess volatiles being released from deeper within the volcano, which preceded a new eruptive period at Poás volcano. This work paves the way for future studies to quantify subaqueous volcanic gas emissions via hydroacoustics, a potential new volcano monitoring technique.