Estimation of completeness magnitude considering daily variation in earthquake detection capability

Earthquake detection capability has been shown to be lower in daytime than nighttime owing to human activity; accordingly, the daily variation in detection capability must be taken into account for the reliable estimation of the completeness magnitude (M-c) of earthquakes. Here, we present estimation results of M-c in which we consider this daily variation in earthquake detection capability. To evaluate such daily variation quantitatively, we introduced a statistical model representing a magnitude-frequency distribution covering the entire magnitude range. Then, the temporal variation in the model parameter, which corresponds to the earthquake detection capability, was estimated by adopting a Bayesian approach with a piecewise linear approximation. This method was applied to the 2006-2010 Japan Meteorological Agency catalogue to estimate the spatial distribution of M-c in and around Japan and to examine the statistical significance of the daily variation. We found significant daily variation in detection capability in 4391 of the 8144 gridpoints examined (i.e. more than 50 per cent) with an interval of 0.1 degrees, which highlights the importance of accounting for this daily variation. The values of M-c we obtained by considering the daily variation were around 2.5 or 3.0 during 2006 and 2010; these values are more conservative than the magnitude threshold suggested by an earlier study.