Earthquake Early Warning System in Southern Italy

Abstract

The origin of the term early warning probably goes back to the first decades of the last century. However, the first practical use of an early warning strategy was military and it was developed during the cold war years as a countermeasure to the potential threat from intercontinental ballistic missiles. The objective of these systems was to give an alert to target areas as soon as a missile was detected by a radar system or a launch was detected by a satellite system. In this context the term lead time was defined as the time elapsing between the detection of the missile and the estimated impact on the target. In the last decades the use of the term early warning greatly expanded. It is used with small, but significant, variations in various types of risks, from epidemiological, to economic, social, and of course all the types of natural and environmental risks. In fact, in these contexts, including some natural risks such as hydro-geological and volcanic, the warning is not given at the onset of the catastrophic phenomenon, but after the occurrence of some precursory phenomena which can trigger a catastrophic event (for instance intensive rainfall for hydrological risk, earthquakes and/or ground deformation for volcanic risk). The main consequence of this difference is an increase in the probability of issuing false alarms. The case of earthquake early warning is similar to missile early warning. The alert is given after an earthquake is detected by a network of seismometers. An earthquake early warning is based on the fact that most of the radiated energy is contained in the slower traveling phases S- and surface waves traveling at about 3.5 km/s or less) which arrive at any location with a delay with respect to small amplitude higher velocity phases (P-waves, travelling at about 67 km/s) or to an electromagnetically transmitted (EM) signal giving the warning.