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The hypothesis that earthquake foreshocks have a prognostic value has been challenged by models of the normal behaviour of seismicity, where no distinction between foreshocks, mainshocks and aftershocks can be made. In the former view, foreshocks are passive tracers of a tectonic preparatory process that yields the mainshock (i.e. loading by aseismic slip) while in the latter, a foreshock is any earthquake that triggers a larger one. Although both processes can coexist, earthquake prediction is plausible in the first case while virtually impossible in the second (for more details about different models, wait for the upcoming R&D project 'Earthquake forecasting physical theory'). Here, I wish to show that that the justification of one hypothesis or the other depends on the selected magnitude interval between minimum foreshock magnitude mmin and mainshock magnitude M and that this can be demonstrated by a meta-analysis of past studies on earthquake foreshocks (Mignan, 2014). From a literature survey (37 foreshock studies published between 1982 and 2013), anomalous foreshocks are indeed found to emerge when mmin < M - 3.0 [Fig. 1]. This suggests the following: Small events are key to improve our forecasting power of larger earthquakes and their number should thus be maximized for statistical learning of potential precursory patterns. For any given earthquake catalogue, this means, first, optimizing their number by correctly assessing the completeness magnitude mc and, second, maximizing their number by modelling incomplete seismicity data (representing up to 90% of the data and so far discarded in standard statistical analyses).

◀ Fig. 1 | Meta-analysis of 37 published studies in which the origin of observed foreshocks is determined. Mainshock magnitude M versus minimum foreshock magnitude mmin. Arguments are based on heuristic, statistical or physical considerations. A same study may include different values of M and mmin, yielding a total of 146 data points. The emergence of an anomalous foreshock behaviour (i.e. loading process due to aseismic slip) is observed once microseismicity is included in the analysis, roughly with mmin < M - 3.0; otherwise foreshocks are best explained by the normal behaviour of seismicity (i.e. earthquake triggering process) - Interactive version of Mignan (2014:Fig. 2).