Possible Observations of PKJKP in the AlpArray Seismic Dataset

Understanding the elastic properties of Earth's inner core is crucial for unraveling its role in the planet’s evolution and dynamics. Seismic shear waves provide a direct means to constrain the shear modulus of the solid inner core at high frequencies. However, their detection has been challenging due to their extremely weak amplitude and interference from other seismic arrivals (Doornbos, 1974). This study aims to provide direct observations of inner-core shear waves through a systematic search using the AlpArray Seismic Network (AASN), a large European seismic array. The approach combines 3-C polarization filtering and slant-stacking techniques. The inspection focuses on events between 2015 and early 2022 within the epicentral distance range of  ~110-150° from the AASN. This source-receiver geometry is close to that of previous PKJKP observation reported using the Gräfenberg array (Cao et al., 2005).

Our systematic search and classification reveal multiple potential observations of PKJKP at frequencies > 0.1 Hz, consistent in both time and slowness with the 1-D Earth model ak135, as well as previous body-wave-based observations, particularly Wookey and Helffrich (2008). The new evidence of PKJKP demonstrates a path forward for formalizing a method for the repeatable detection of inner-core shear waves for different source-receiver geometries. Additional PKJKP observations and comprehensive modeling are essential for gaining insights into the intricate inner-core structure and phenomena, such as anisotropy and focusing effects, which could explain the limited number of observations to date.