Expanding the Range of Back-Projections: Core Phase Approach for Southern Hemisphere Earthquake Imaging

Abstract

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. We extend the BP method by utilizing seismic waves traveling through the Earth’s core, expanding our capability to image earthquakes worldwide. Our core phase BPs incorporate PKIKP (150°-180°) and PKP (145°-175°) phases. We evaluate their theoretical resolutions using 1-D and 2-D array response functions and test uncertainties by adding white noise to coherent waveforms. Tests show that core phase BPs achieve resolutions and uncertainties comparable to P phase BP. We validate the method using a synthetic model of a unilateral rupture (Mw 7.45, 2 km/s) and demonstrate accurate recovery of rupture direction, length, and speed. Applying core phase BPs to the 2010 Mw 8.8 Chile and 2015 Mw 7.1 southeast Indian Ridge earthquakes, we compare our results with published BPs and/or slip models, confirming the feasibility and reliability of core phase BPs. We then apply core phase BPs to five understudied earthquakes in the southwest Pacific region, providing insights into these pelagic earthquakes. Core phase BPs play a crucial role in scenarios where teleseismic arrays are unavailable, and have weaker array-dependent effect and better performance in bilateral rupture imaging. Finally, we discuss the limitations of core phase BPs and outline potential avenues for future research.