PKIKP Phase Back-Projection and Its Application on Southern Hemisphere Earthquake Imaging

Standard Back-Projections (BPs) use P phase recordings at large aperture arrays within teleseismic distances (30°–90°) to image earthquake sources. However, the majority of sizable arrays are in the northern hemisphere, leaving many southern hemisphere earthquakes beyond the teleseismic range. This paper studies the effectiveness of the PKIKP phase traveling through the Earth's core in the BP method. We identify the optimal distance range for the PKIKP BP as 150°–180°. We evaluate its resolution using array response functions and theoretical seismograms and test uncertainties by adding noise to coherent waveforms. Tests show that, within the optimal distance range, PKIKP BP achieves resolutions and uncertainties comparable to P phase BP. We validate the method using synthetic models of point sources and unilateral ruptures and demonstrate accurate recovery of rupture location, length, and speed. Applying PKIKP BP to the 2010 Mw 8.8 Chile, 2021 Mw 7.2 Haiti, and 2021 Mw 7.4 and Mw 8.1 Kermadec earthquakes, we compare our results with published BPs and/or slip models, confirming the feasibility and reliability of the method. We also find that waveform and wavefront distortions of the PKIKP phase within an array are minor and comparable to those of the P phase, further supporting the robustness of the method. These results suggest that PKIKP BP can serve as a powerful tool for source imaging and hazard assessment in regions such as the southwest Pacific, the western coast of South America, the South Sandwich Islands, and the southern Indian Ocean, where P-phase BP is limited or unavailable.