This study aims to explain the records of seismicity before the onset of the 2018 Mw 7.5 Palu earthquake, concerning the timing of earthquake nucleation of a large strike-slip supershear earthquake. The mainshock was preceded by two distinct series of foreshock sequences, seven months before the mainshock and three hours before the mainshock. The foreshock seismicity took place on a distinguishing isolated zone surrounding the epicenter of the impending mainshock, near the northern edge of the total rupture. Cross-correlating the regional seismogram indicates that the foreshocks were not repeated ruptures of the same patch by fault creep. Neither low-frequency signals nor an extended initiation phase were observed immediately before the impending rupture. The foreshocks were attributed to a cascade model, that is, the Palu mainshock was triggered by a cascade of foreshocks, not by a slow-slip transient. This is explained by the Coulomb stress changes model imparted by the largest foreshock (Mw 6.1). However, it is also possible that the mainshock initial break was instantaneously triggered by the dynamic seismic wave of the very last foreshock.
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The regional seismograms are retrieved and available from Badan Meteorologi, Klimatologi, dan Geofisika (BMKG), upon request. The earthquake catalog used in this study can be downloaded from BMKG repository (https://repogempa.bmkg.go.id/repo_new/repository.php). The author thanks Anne Socquet, Jin Fang, Xiaogang Song, Yongzhe Wang, and Qi Li for sharing the slip model of the Palu earthquake based on their works. Figures are created by using the Generic Mapping Tools (Wessel et al. 2013) and Seismic Analysis Codes (SAC). Comments and suggestions from Daryono (BMKG) are greatly appreciated. The author thanks the editor and two anonymous reviewers for their comments and suggestions that improved the quality of this paper.
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Sianipar, D. Immediate Foreshocks Activity Preceding the 2018 Mw 7.5 Palu Earthquake in Sulawesi, Indonesia. Pure Appl. Geophys. 177, 2421–2436 (2020). https://doi.org/10.1007/s00024-020-02520-1
- Earthquake nucleation
- Palu earthquake