An Algorithm for the Real-Time Recognition of Foreshocks: Application in Greece

Foreshock activity is one of the most important precursory phenomena occurring before the mainshock. The number of foreshocks increases with the inverse of time while the b-value usually drops with respect to the b- value of the background seismicity. Earthquake satatistics in Greece indicates that at least 50% of the mainshocks of M ¡Ý 5.5 are preceded by foreshocks and that foreshock sequences occur as a rule within a time interval no longer than four months before the mainshock. The recognition of the onset of a foreshock activity in near real-time is of importance to issue warning for a forthcoming stronger mainshock. To this aim we developed a computerized procedure for the automatic recognition of foreshock activity. The system consists of an earthquake data basis, an algorithm which performs the statistical tests, and a decision matrix which indicates the level of alert. The data basis is automatically updated from the results of the daily analysis of the national monitoring seismograph system. The algorithm updates calculations regarding seismicity rate and b-value changes and performs statistical tests of change significance. The decision matrix calculates the level of alert by taking into account the level of significance for both the seismicity rate change and the b-value change. Actual application of the system on real-time basis is scheduled to start after the phase of testing which is expected to terminate by the end of 2007. This is a contribution to the EU research project SAFER, contract n. 036935, FP6- 2005-Global-4, Reduction of seismic risks.