Recently, Trugman & Ross (2019) performed an in-depth statistical analysis of the seismicity rates preceding 46 mainshock events in Southern California employing a highly complete earthquake catalogue (the QTM catalogue; Ross et al., 2019). By comparing short-term seismicity rates with the background rate over one year prior to a selected mainshock, Trugman & Ross (2019) concluded that over 70% of all analysed mainshocks were preceded by a statistically significant increase in seismicity rates. These authors further alluded to the possibility that in practically all cases foreshock sequences may be detected, provided by that the earthquake catalogue is sufficiently complete. If this claim holds true, this has the implication that the nucleation process of (large) earthquakes emits detectable information with potential application in short-term earthquake forecasting.
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.
The Python notebook analyse_QTM_catalogue.ipynb reproduces the analysis of the QTM catalogue and figures presented in van den Ende & Ampuero (EarthArxiv).