**1. Introduction**

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292 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

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> Moderate and strong (M5) earthquakes that take place in active tectonic crustal structures as South Apennines Chain, generate temporary or permanent environmental changes (geophysical, geochemical, geomorphic, hydrogeological and structural coseismic features). The repeated occurrence of these features leaves a geological signature in the recent stratigraphy and topography of an area, unequivocally related to the intensity of the local seismicity.

> Moreover, many anomalous behaviours of aquifers have been noticed before, during and after a seismic event: sudden increase/decrease of spring flows, changes of mountain streams (streamflow level and/or flow rate) and water table level in wells, and also increase of the emanation of deep gases.

> The study of the geochemical and hydrodynamic characteristics of aquifers and of their changes is considered a valid contribution to the knowledge of the natural processes connected to earthquakes, mostly because the changes in the water-rock interaction are caused by the seismic stresses in the area where the tectonic deformation leads to the seismic event.

> Hydrological anomalies concomitant and/or preceding seismic events has been widely known for at least 2000 years. Only in the last three decades such variations have been related to the characteristics of seismic sources. Also in Italy possible correlation between earthquakes and hydrological and geochemical anomalies have been recognised. Many other documents of the eighteenth and nineteenth centuries report information regarding variations of water levels and spring flows in occurrence with earthquakes.

> Various interpretative models indicate how the fluctuations of water level in wells and the variations in the flow of springs and/or mountain rivers can be used as earthquake precursors and, moreover, to characterise the mechanisms of the seismic source, because they are significantly influenced by the deformation field associated to the earthquake. In particular, King and Muir-Wood (1993) proposed a model of the deformation associated to

dislocation phenomena in complex systems of faults with different mechanisms and orientations. According to this model the coseismic dislocation during strong earthquakes produces a notable deformation of the superficial crust (surface layers) which influence directly the surface aquifers. Therefore, the hydrological regime and the spatial variation of anomalies are correlated to the spatial variation of the volumetric deformation produced by the phenomena of the coseismic dislocation in a wide area.

Here we resume the hydrological changes associated with some of the big earthquakes that occurred in the XX century in the Southern Apennines (Italy): 1930, 1980 and 1984. For the 23 July 1930 (MS=6.7) in Eastern Irpinia, 23 November 1980 (MS=6.9) in Irpinia-Lucania and 7 May 1984 (MS=5.8) in Southern Abruzzo earthquakes (figures 1a,b and 2a,b,c). We collected an abundance of well founded hydrological information, that cannot be ascribed to environmental or anthropical causes. We analysed also hydrometric and pluviometric data monitored from the hydrological network of Italian Hydrographic Survey (IHS), looking for significant changes in wells, springs and mountain streams. Hydrological data relative to wells, rivers and springs indicate that many changes can be correlated with the earthquakes. For these earthquakes, the pre- and co-seismic stresses and the tectonic deformations have been studied in order to find a possible model of interaction between stress state and hydrological anomalies.

In this chapter a description of Southern Apennines, the three earthquakes and the types of hydrological effects for each earthquake are reported.
