On artificial daily and weekly periodicities of recorded earthquake frequencies

Levels of artificial seismic noise are typically lower at night, when road and train traffic, industries and other human activities are decreased. Such a variable noise hampers detection of small earthquakes preferentially during daytime, so typically they are more frequently recorded at night. Small earthquakes are recorded in higher numbers during the weekends too, also due to the lower artificial noise. Daily variations of earthquake frequencies might also have natural causes, but higher numbers of earthquakes recorded during weekends are unequivocally artificial.

These variations of detection capabilities are usually not well taken into account when looking for natural periodicities of earthquake frequencies, for example when searching for correlations of earthquake occurrence with diurnal or semidiurnal tides.

Featuring examples from different, regional, earthquake catalogues, this presentation shows that using a magnitude of completeness (M_c) calculated from the whole catalogue can be misleading. The reason is that such a value is actually an average between lower M_c values reached during the night (and weekends) and higher ones reached during the day (and working days).

The solution proposed here is to use a high enough M_c value such as the artificial (daily and weekly) periodicities of earthquake frequencies are removed, considering not only the best estimate of M_c but also its uncertainty range.