Abstract
A study of ultra-low-frequency (ULF) geomagnetic signals has been carried out prior to the onset of Nias earthquake M5.3 that occurred at 07:13:49 LT (Local Time) on September 6, 2018, in West Nias regency, North Sumatra, Indonesia. We analyzed three components of geomagnetic data H, D, and Z measured in Gunung Sitoli (GSI) geomagnetic station which is about 108 km from the epicenter, from January 2018 until December 2018. We used the polarization ratio method in terms of power spectrum density within 0.027-0.033 Hz frequency range to evaluate the geomagnetic data by applying Fast Fourier Transform (FFT). This work presents evidence of ULF geomagnetic anomalies exceeding the statistical threshold associated with Nias earthquake M5.3 which appeared starting from 3.5 months until 3 weeks before the earthquake. When these anomalies occurred, the value of disturbance storm time (Dst) index shows that there is no high global geomagnetic activity. Thus, it is concluded that the clear precursory anomalies detected are related to the earthquake.
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