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.

中文翻译：

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印度尼西亚苏拉威西岛 2018 年 7.5 级帕卢地震前的即时前震活动

本研究旨在解释 2018 年 7.5 级帕卢地震发生前的地震活动记录，涉及大型走滑超剪切地震的地震成核时间。主震之前有两个不同系列的前震序列，主震前七个月和主震前三个小时。前震地震活动发生在即将发生的主震震中周围的一个显着孤立带上，靠近总破裂的北边缘。区域地震图的互相关表明前震不是由断层蠕变引起的同一块块的重复破裂。在即将发生的破裂之前，既没有观察到低频信号，也没有观察到延长的起始阶段。前震归因于级联模型，即，帕卢主震是由一连串前震触发的，而不是由慢滑瞬变触发的。这可以通过最大前震 (Mw 6.1) 赋予的库仑应力变化模型来解释。然而，也有可能是最后一次前震的动态地震波瞬间触发了主震的初始破裂。