The optimum frequency for detecting earthquake precursors based on ultra-low frequency (ULF) geomagnetic data from Sukabumi (SKB) station

An earthquake precursor study has been conducted using normalized polarization ratio analysis based on Fast Fourier Transform (FFT). Three earthquakes in 2020 that have the magnitude (M) ≥ 5 and epicenter distance (R) ≤ 100 km were selected to be analyzed. The night-time geomagnetic signals of X, Y, and Z components from the Sukabumi geomagnetic station (SKB) were extracted at frequencies 0.01 Hz until 0.06 Hz to analyze the SDR value of the SZ/SG component and determine the optimum frequency for detecting earthquake precursors. The results show that frequency 0.06 Hz is optimum to detect a possible earthquake precursor 1 day before the M5 earthquake on March 10, 2020 (EQ1). Meanwhile, the frequencies 0.03 Hz, 0.04 Hz, and 0.06 Hz are optimum to detect the possible precursors for the M5.1 earthquake on July 7, 2020 (EQ2) in a similar period, about 9.5 - 4 weeks before EQ2.