There are two extreme opinion on the subject – l Pessimistic: “… in the case of seismic data, most of the observed variations are, in fact, related to changes in the system for detecting and reporting earthquakes and not to actual changes in the Earth.” l Optimistic: Among existing data seismic catalogs remain the most reliable record on distribution of earthquakes in space and time.
			Sixth Workshop on Non-Linear Dynamics and Earthquake Prediction The Abdus Salam ICTP, Miramare
	15/10/2001				16