Spatiotemporal distribution of low-temperature thermal anomalies at volcanic calderas: The case of Okmok volcano, Alaska

Volcanic eruptions present serious risk to human life and infrastructure. This risk can be minimized by improving eruption forecasts, which in turn requires increasing our capabilities to detect volcanic unrest and a better understanding of the physicochemical processes governing magma-hydrothermal interactions. The improvement of eruption forecasting techniques is especially important as some volcanic eruptions can occur with little to no precursory warning signs. That was the case of the most recent eruption at Okmok caldera, which took place in 2008 between July 12 – August 23, with a volcanic explosivity index of 4. This eruption highlighted the need to develop new methods to detect precursory activity and unrest.

Recently, through the analysis of satellite-based thermal spectroscopy data from MODIS instruments, Girona et al. (2021) found that low-temperature thermal anomalies along the flanks of volcanoes can predate their eruptions. In this work, we use an updated version of the method presented in Girona et al. (2021) to analyze the spatiotemporal distribution of low-temperature thermal anomalies at Okmok Caldera between July of 2002 and November of 2021. Preliminary analysis shows ~1-1.3 degrees of warming at Cone A in the ~3 years leading up to the 2008 eruption. This analysis also shows a warming trend in the caldera at several cones (D, E, A, and Ahmanilix), peaking in 2014, with brightness temperatures increasing by ~1-1.4 degrees for ~2 years (correlating with an observed inflation event); along with current warming at the same cones of ~0.8-1.2 degrees beginning in ~2017.

We propose that the low-temperature thermal anomalies observed at different cones of Okmok caldera are linked to the latent heat released during the condensation of magmatic and/or hydrothermal water vapor in the subsurface. In particular, we design a 1-dimensional thermal diffusion model to quantify how long it will take for the surface ground temperature to increase by one kelvin in response to the subsurface condensation of water vapor. Our preliminary analysis shows that, for realistic values of the parameters involved, the surface requires ~3.3 years to increase its temperature by one kelvin in response to a diffuse H2O flux of 161.5 kg/s condensing at 30m depth, and ~21.7 years for the surface to increase by one kelvin in response to the same gas flux condensing at 60m depth. The observed low-temperature thermal anomalies at Okmok are therefore consistent with the condensation of magmatic and/or hydrothermal water vapor at no more than a few tens of meters depth below the surface.

This work provides further insight into how volcanic hydrothermal subsurface processes manifest as thermal anomalies on the surface, and how these thermal anomalies can be used to detect unrest at Okmok and other active volcanoes. In the future, we aim to integrate the spatiotemporal distribution of low-temperature thermal anomalies with deformation, seismic signals, and diffuse gas emissions prior to and during eruptions.

 

Girona, T., Realmuto, V. & Lundgren, P. Large-scale thermal unrest of volcanoes for years prior to eruption. Nat. Geosci. 14, 238–241 (2021). https://doi.org/10.1038/s41561-021-00705-4.