ULF geomagnetic anomaly associated with the Sumatra-Pagai Islands earthquake swarm during 2020

Observations of earthquake precursors via geomagnetic anomalies from ultra-low frequency (ULF) could provide an expectation for short-term prediction. However, there are still several obstacles in determining this precursor, one which is the presence a precursor bias if occurs at close time and location. To cope with problem, we analysed six earthquakes magnitude >5 on Pagai Island, Mentawai, Sumatra, during 2020. These have epicentres to each other occur within short (one month). This study used data recorded by Magnetic Data Acquisition System (MAGDAS) magnetometer network Sicincin station (SCN), West Kepahiyang (KPY), Bengkulu, span month before earthquake. The anomaly ULF emission was using power spectral density method 0.012 Hz H Z components data. onset determined standard deviation value (pz + σ, pz – σ) SZ/SH ratio polarization. disturbance storm index (Dst) ensure that occurred not caused storm. Also, single transfer function calculate azimuth, empirical formula between amplitude as validation source. Several were observed, especially KPY station. estimated azimuth shows good accuracy compared earthquake's average 97.8%. Furthermore, calculated agreement actual magnitude, 98.13%.