Manual Detection of P and S Wave Arrival Times and Re-localization of the 2021 Toba Swarm Earthquake from Temporary Seismic Network

Abstract The epicenter of the 2021 swarm earthquake was previously determined automatically using the STA/LTA method. Although, the results obtained show that the distribution of earthquake epicenters still needs to be accurate. This is shown by errors in determining the arrival times of P and S waves. Therefore, in this study, we try to improve the location of the earthquake epicenter manually. This manual detection method is done by re-analyzing the arrival times of P and S waves, which are recorded at each station, one by one. This study aims to determine the pattern of the 2021 Toba swarm earthquake to be used as a preliminary study, mainly in seismic hazard mitigation in Toba. The processing step starts by selecting the P and S arrival times using XQUAKE and then localized using HYPO71. We selected and only used earthquakes with azimuth angles ≤ 200 degrees and a minimum distance from the earthquake to the station < 100 km. The final results show that the epicenter distribution of the Toba swarm earthquake is clustered on the west side of Samosir Island toward the northwest-southeast and not along the Sumatra Fault. The results of this study indicate that the 2021 Toba swarm earthquakes are suspected to be caused by an unidentified geological structure.