Space geodetic data (GPS) and earthquake forecasting: examples from the Italian geodetic network

Some considerations are given about attempts at predicting impending earthquakes by the use of space geodetic (GPS) observations in the Italian peninsula. Both short (weeks to days) and middle/long-term (months to years) presumed precursors are considered. At present, none of the numerous published studies seems to identify significant and systematic earthquake precursors. At most, geodetic data may be used to quantify the rate of tectonic deformation and its lateral variations, in order to identify the zones where strain accumulation is fastest. However, the connection between the level of strain and the probability of earthquakes is not clear yet. We argue that the most promising way to use geodetic data is monitoring post-seismic relaxation, that is the perturbation of the strain field triggered by a major shock. The examples recognized so far of interaction between seismic sources suggest that the highest probability of induced earthquakes at a mature fault corresponds to the arrival time of the highest strain rate values. The numerical simulation of post-seismic relaxation indicates that the expected variation of velocity, strain and strain rate in the zones surrounding the triggering shock may be significantly higher than the resolution of GPS observations. Thus, one may expect that geodetic monitoring, integrated by a deep knowledge of the ongoing tectonic setting, may allow the identification of the zones where the probability of an induced earthquake is undergoing a significant increase.