Spatial coherency analysis of seismic motions from a hard rock site dense array in Busan, Korea

We conducted a spatial coherency analysis of ground motion using earthquake recordings from a hard rock outcrop dense array situated at a nuclear power plant site in Busan, located in the south-eastern coast of Korea. Utilizing data from a total of 16 events occurring from July 2021 to June 2022, we computed the plane-wave, lagged, and unlagged coherency functions of both horizontal and vertical components. We also provided comprehensive comparisons with other empirical functions developed for rock sites. Notably, all reported rock site curves exhibit clear distinctions, emphasizing the site-specific nature of these curves. The observed coherency tends to be larger for harder sites, that happen to have higher V_s30 as well, at separation distances less than 50 m. The Busan array, being the hardest among the available rock site arrays in existing literature, demonstrated the highest coherency at short distances and higher frequencies (e.g. above 25 Hz). This observation could be attributed to the presence of hard rock layers that have relatively higher spatial homogeneity at the Busan array site than the others. However, at larger interstation distances (e.g., above 50 m), relatively lower coherency is observed at Busan array. This could be attributed to the particular shape of the array as the farther away stations in a pair are located on more heterogeneous grounds, and experience direction-dependent, phase-shifted seismic waves.