ULF signals as earthquake precursors in various tectonic regions

Ultra-Low-Frequency (ULF) geomagnetic variations in the Pc2-Pc4 bands were examined in association with moderate-to-great earthquakes in New Zealand, Indonesia, Japan, Taiwan, and Peru. High-resolution magnetometer data from the INTERMAGNET and MAGDAS networks were analyzed using a dual-station framework comparing near-epicentral sites with distant references to identify potential pre-seismic ULF anomalies. Two diagnostic parameters were applied: the polarization ratio and the Residual Standard Deviation (RSD) of vertical geomagnetic (Z-component) amplitude differences. The analysis reveals distinct pre-seismic signatures, with some events showing clear polarization ratio enhancements at near-epicentral stations, while others display marked increases in Z-component RSD, particularly when polarization changes were less evident. RSD is introduced as a novel and sensitive metric for detecting localized ULF perturbations. The results indicate that ULF anomaly characteristics vary across tectonic settings and event-specific conditions. Overall, the study demonstrates the value of combined-parameter analysis and highlights the Z-component as a key indicator for identifying seismogenic ULF disturbances relevant to short-term earthquake monitoring.