### Rationale

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
`m _{min}`
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

`m`<

_{min}`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**(representing up to 90% of the data and so far discarded in standard statistical analyses).

`m`and, second, maximizing their number by modelling incomplete seismicity data_{c}**◀ Fig. 1 | Meta-analysis of 37 published studies in which the origin of observed foreshocks is
determined.**
Mainshock magnitude `M` versus minimum foreshock magnitude `m _{min}`. Arguments are
based
on heuristic, statistical
or physical considerations. A same study may include different values of

`M`and

`m`, 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

_{min}`m`<

_{min}`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).