Observations of Inner Core Shear Waves with AlpArray

Although the solidity of Earth’s inner core is evidenced by normal mode data, the direct observation of inner core shear waves (J-waves) has remained challenging for decades due to their small amplitudes. Previous studies have presented evidence of J-waves in different seismic datasets (e.g., Okal and Cansi Y, 1998; Deuss et al., 2000; Cao et al., 2005; Wookey and Helffrich, 2008), however, the observability seems to be highly dependent not only on distance, but also on the location of the source and receiver, suggesting that amplification from specific 3D structures in the deep Earth is necessary to elevate the phase above noise for certain ray paths. Waszek and Deuss (2015) and Tkalčić and Phạm (2018) also found J-waves in global stacks and global correlation wavefield respectively, but these average over all possible source-receiver geometries and inner core structure.

To improve phase identification and discrimination, we use an approach that combines the array method of slant stacking and polarization filtering to enhance linearly polarized signals with the expected slowness and incident angle. We apply this technique on the data of the AlpArray Seismic Network, a large-scale seismic network in Europe that consists of over 600 broadband stations with a mean station spacing of 30-40km. An arrival consistent with PKJKP (in reference travel time, slowness, and polarization) is found from events in the source region reported by Cao et al. (2005). We present an overview of PKJKP candidate paths over distance based on observations with AlpArray. We also examine whether these observations correspond to specific depths or azimuths and investigate the effects of anisotropy or other three-dimensional earth structures​​​​​​.