Testing the Sensitivity of Shear Wave Splitting to Volcanic Inflation, A Case Study from Askja, Iceland

Askja is an active volcano situated in the Northern Volcanic Zone of Iceland that last erupted in 1961. Since then, long-term geodetic studies of Askja’s caldera complex have tracked the deflation at a decaying rate associated with a shallow source. However, in August 2021, a rapid reversal of this trend indicated the onset of re-inflation, which, as of January 2023, has resulted in 45cm of uplift near the centre of the primary caldera. While several techniques have been used to measure the geodetic signal associated with this inflation, including gravity and InSAR data, there has yet to be a detailed examination of the seismic response. We observe a definitive increase in the rate of seismicity associated with the onset of re-inflation in August 2021. In this study we examine the sensitivity of shear wave splitting, a phenomenon arising due to seismic anisotropy in the crust, to the changing stress state of the crust within and surrounding Askja associated with this new phase of inflation. We estimate the fast orientation and delay time, which parameterise the orientation and magnitude of seismic anisotropy respectively, from split shear wave arrivals across our local network of seismometers. We leverage an extensive catalogue of microearthquakes in and around Askja spanning 2007 to 2022 in order to compare the variation in pre- and post-inflation delay times and strength of anisotropy, to better understand the sensitivity of shear wave splitting to stress changes during periods of volcanic inflation. This will give valuable information on whether shear wave splitting can be used as a proxy for stress changes when other geodetic observations cannot be performed in volcanic and other settings, as well as the role shear wave splitting has in combination with these other techniques.