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Imaging the Seismic Velocity Structure beneath Askja Caldera, Iceland with Traveltime Tomography

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  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. After its most recent eruption in 1961, GPS measurements have shown that deflation has continued since the 1980s, which may primarily be caused by the cooling of a shallow magma body as suggested by geodetic modeling. 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 seismic velocity structure beneath the top 10 km of the Askja caldera and the surrounding region using traveltime 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 conduit beneath the caldera, which is consistent with the flux of melt through the crust. Outside the conduit, high-velocity anomalies align with the layout of the caldera. This may reflect the presence of solidified intrusive rocks. Southeast of the caldera is dominated by a low Vp, high Vs anomaly that is likely associated with the geothermal area. 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 volcano, Iceland, as imaged by seismic tomography. Journal of Geophysical Research: Solid Earth, 121(10):7211– 7229.

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