The 23 July 1930 earthquake (MS=6.7) in the Southern Apennines (Italy)
was a catastrophic event that produced many effects such as surface faulting,
fractures, landslides, settlements, hydrological changes, variations in chemical/
physical activity related to the volcanic and/or thermal zones and also acoustic
and optical phenomena. It is the first great earthquake of the twentieth century that
was studied, thanks to the hydrological monitoring network of the Italian Hydrographic
Survey (IHS) set up from 1925 to 1929. For this earthquake we analysed
the initial IHS hydrometric and pluviometric data, looking for significant anomalies
in springs, water wells and mountain streams. Hydrological data relative to rivers,
springs and water wells indicate that some changes can be correlated with the
earthquake: a post-seismic excess discharge in some streams, pre- and co-seismic
decreases in stream flows and water levels in wells, pre- and post-seismic increases
in discharges. The pre- and co-seismic stresses and the tectonic deformations were
studied in order to find a possible model of interaction between stress state and
hydrological variations. The anomalies found in this work can be considered “rebound
anomalies”, which are the most common precursor reported by several authors
and related to increases in porosity and permeability caused by the fracturing
that precedes an earthquake. An estimation of the total excess discharge (0.035
km3) caused by the Ms=6.7 Irpinia earthquake is consistent with the excess discharge
of about 0.01 km3 determined for the Mw=6.9 Loma Prieta earthquake.
To define the normal hydrodynamic behaviour of the considered aquifers, an
analysis of the correlation between rainfalls and water levels and flow rates has
been carried out. The delay of significant peaks in the correlograms of figure 7
with respect to the start of the hydrological anomaly and/or its duration, can define
the space-time limits of the changes correlated with earthquake
