Real-time discrimination of earthquake foreshocks and aftershocks: An update

Abstract:

The relative earthquake size distribution, or b-value, can act as an direct stress meter in the earth crust, a finding confirmed in numerous laboratory studies but also in diverse natural systems. Based on the analysis of the average size-distribution of aftershocks of about 60 earthquakes sequences, with main-shock magnitudes above 6.0, we have proposed that it may, in many cases, be possible to discriminate if an ongoing sequence is representing a decaying aftershock sequence or foreshocks to an upcoming large event. We proposed in 2019 a simple traffic light classification to assess in real-time the level of concern for subsequent larger event, and tested it against 58 sequences, reaching a classification accuracy of 95%. Since then, we also applied the Foreshock Traffic Light System (FTLS) in a pseudo-prospective test to the 2019 Ridgecrest sequence. Results are fully in line with the hypothesis:In this Ridgecrest case study, after analysing carefully the magnitude of completeness of the sequences, we find that in the hours after the first Mw6.4 Ridgecrest event, the b-value drops by 23% on average, when compared to the background value, resulting in a red foreshock traffic light. Spatially mapping the changes in b, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. After the second, magnitude-7.1 mainshock, which did occur in that location as anticipated, the b-value increased by 26% over the background value, triggering a green traffic light. Here we will report on these findings, discuss additional case studies and criticisms raised and discuss physics-based mechanics that may allow us to understand and model the observations.