Time-magnitude correlations and time variation of the Gutenberg-Richter parameter in foreshock sequences

Abstract

The time dependence of the parameter of the Gutenberg-Richter (GR) magnitude distribution is computed for foreshock sequences of earthquakes, correlated with the main shock, by using the geometric-growth model of earthquake focus, the magnitude distribution of correlated earthquakes and the time-magnitude correlations, derived recently. It is shown that this parameter decreases in time in the foreshock sequence, from the background values down to the main shock. If correlations are present, this time dependence and the time-magnitude correlations provide a tool of monitoring the foreshock seismic activity. We discuss the relevance of such an analysis for the occurrence moment and the magnitude of a main shock. The discussion is applied to a few Vrancea main shocks and the precursory seismic activity of the l'Aquila earthquake. The limitations of such an analysis are discussed.