Vp/Vs Temporal Variation as Precursor Anomalies for the 2018 Palu Earthquake

The Palu earthquake on September 28, 2018 with a magnitude of Mw 7.5 was one of the most destructive earthquakes in Indonesia. This earthquake also caused an underwater avalanche and caused a tsunami along the coast of the Palu bay. The source of the earthquake was the tectonic activity of the Palukoro fault. Until now, several research methods on earthquake precursors around the world have tried to find out whether there are long-term and short-term natural clues before the occurrence of a large and destructive earthquake. In this study, we examined the precursor anomaly of the earthquake in the Palukoro fault area using the least squares calculation method to obtain Vp/Vs by referring to the Wadati diagram. We used detailed earthquake catalog data from the nearest BMKG Station from the source, namely data on the arrival time of P and S waves from all earthquake events in the Palukoro Fault area from 2017 to 2017. 2020. The results show an anomaly pattern on the monthly Vp/Vs chart. The decrease in the value of Vp/Vs by an average of 6% occurred within a span of 9 months before the main Palu earthquake. Vp/Vs variations are an indication of the presence of tectonic stress and strain forces before an earthquake so that it can be used as an indication of earthquake precursors.