  The identification of patterns in space, time, and magnitude, which could
potentially encode the subsequent earthquake magnitude, represents a
significant challenge in earthquake forecasting. A pivotal aspect of this
endeavor involves the search for correlations between earthquake magnitudes, a
task greatly hindered by the incompleteness of instrumental catalogs. A novel
strategy to address this challenge is provided by the groundbreaking
observation by Van der Elst (Journal of Geophysical Research: Solid Earth,
2021), that positive magnitude differences, under certain conditions, remain
unaffected by incompleteness. In this letter we adopt this strategy which
provides a clear and unambiguous proof regarding the existence of correlations
between subsequent positive magnitude differences. Our results are consistent
with a time-dependent b-value in the Gutenberg-Richter law, significantly
enhancing existing models for seismic forecasting.
