Martijn van den Ende , Jean Paul Ampuero 

Earthquake foreshocks may provide information that is critical to short-term earthquake forecasting. However, foreshocks are far from ubiquitously observed, which makes the interpretation of ongoing seismic sequences problematic. Based on a statistical analysis, Trugman & Ross (2019) suggested that as much as 72% of all mainshocks in Southern California is preceded by foreshock sequences. In this study, we re-assess the analysis of Trugman & Ross (2019), and we evaluate the impact of the assumptions made by these authors. Using an alternative statistical approach, we find that only 15 out of 46 mainshocks (33%) are preceded by significantly elevated seismicity rates. When accounting for temporal fluctuations in the background seismicity, only 18% of the analysed foreshock sequences remain unexplained by the background seismicity. These results imply that even in a highly complete earthquake catalogue, the majority of earthquakes do not exhibit detectable foreshock sequences.