Imaging the Magmatic Plumbing System beneath Askja Caldera, Iceland with Seismic Tomography

Askja caldera is a large central volcano located in the Northern Volcanic Zone in Iceland. It has experienced a number of eruptions in modern history with one of the largest taking place in 1875, from which tephra managed to travel as far as Germany. After its most recent erup- tion in 1961, GPS measurements have shown that deflation has continued since the 1970s, which may primarily be caused by the cooling of a magma body at a depth of around 2 km below sea level, as suggested by geodetic modelling. In ~September 2021, the Askja caldera switched from deflation to inflation. This has caused a lot of excitement in the seismology and volcano communities, with increased monitoring and data collection beginning in earnest in an attempt to better evaluate the progress and potential outcomes of this interesting phenomenon.

My study aims to image the magmatic plumbing system beneath the top ~10 km of the Askja caldera and the surrounding region using seismic tomography. The dataset consists of the first arrival picks of P- and S-waves from local earthquakes. The initial dataset is sourced from Greenfield et al. (2016), but additional picks from more recently collected data are also incorporated to enhance ray coverage. These arrival times are inverted using the FMTOMO package, which jointly constrains hypocenter location and 3-D Vp, Vs and Vp/Vs structure using an iterative non-linear approach, in which the forward problem of traveltime prediction is solved using the Fast Marching Method (Rawlinson et al., 2005).

The final tomographic results yield a variety of wavespeed anomalies that can be associated with the volcanic plumbing system. Of particular note is a low Vp, low Vs and high Vp/Vs anomaly at around 2 km depth below the caldera, a feature that has previously not been observed in seismic imaging results. A low wavespeed anomaly also connects the mid-crust with the surface below the edifice, which is consistent with the flux of melt through the crust. Synthetic checkerboard and spike tests indicate that these features are constrained by the data.

References

Rawlinson, N. and Sambridge, M. (2005). The fast marching method: an effective tool for tomographic imaging and tracking multiple phases in complex layered media. Exploration Geophysics, 36(4):341.

Greenfield, T., White, R. S., and Roecker, S. (2016). The magmatic plumbing system of the Askja central vol- cano, Iceland, as imaged by seismic tomography. Journal of Geophysical Research: Solid Earth, 121(10):7211– 7229.