This study examines the temporal variation of the seismicity in the Val d'Agri (southern Italy) and adjacent areas, for the current seismic hazard
evaluation. The temporal variation of the seismicity is expressed as time
series of the number of earthquakes,

Val d'Agri is the most seismically active sector of the central-western
Mediterranean region, having repeatedly been struck by destructive earthquakes in
1561 (

The Val d'Agri instrumental seismicity recorded in the last 30 years is low
and sparse showing only two small seismic swarms recorded between 1996
April–June and 2002 February–December. The first swarm was characterized
by low-magnitude events (

In this area there are many environmentally protected zones and natural parks, a sequence of shallow and deep aquifers and hydraulic network systems for storage and supply water for agricultural and civil purposes; therefore, the seismic hazard evaluation is a very important task. Usually to assess the seismic hazard, the most widespread and internationally accepted method is to estimate the peak ground motion expected in a given place by applying probabilistic or deterministic methodologies.

Nevertheless, the study of the properties of the time distribution of the seismicity can contribute also to the evaluation of the seismic danger in a given area. There are several methods that can statistically describe the properties of the time distribution of the seismicity. Among them the non-extensive statistical mechanics seem to be an interesting approach, for the description of the frequency–moment and the time and distance distributions between strong events. (Leonard et al, 2001; Parsons and Geist, 2012; Michas et al., 2013; Papadakis t al., 2013; Vallianatos and Sammonds, 2013; Antonopoulos et al., 2014).

The present work examines the temporal variation of the seismicity in the Agri valley (southern Italy) and adjoining region in the period 1983–2014 in order to evaluate the current status of the seismic danger and the time period between strong events, by means of FastBEE tool (Papadopoulos and Baskoutas, 2009, 2011).

This tool is suited to visualize simultaneously the temporal variation
curves of common seismicity parameters like the number of earthquakes
log

FastBEE algorithm, based on the character of the seismicity parameters described previously, assumes that the temporal variation curves of these parameters represent distinct phases of a phenomenological model for the strong earthquake preparation (Popandopoulos and Baskoutas, 2011) and can be explained according to the classical models for the preparation of earthquakes and especially the phases of the consolidation model (Dobrovolski, 1991).

The characteristic onset of the temporal change, where the amount of the
released seismic energy log

Characteristic FastBEE output schematic general trend of the temporal prognostic anomaly (solid blue lines) before a strong earthquake occurrence. The open rectangular parallelogram denotes the first low probability stage, since the prognostic anomaly beginning, followed by a second higher probability stage, which concludes with the strong earthquake occurrence. Vertical red arrow shows the earthquake origin time from Baskoutas and Popandopoulos (2014).

Based on the above consideration, the methodology for the current seismic
hazard evaluation consists of the following: first in the construction, by means of FastBEE tool, of the temporal
variation series of a set of seismicity parameters, like the number of
earthquakes

The magnitude of these events represents the lower magnitude that correlates better with the observed temporal changes and was determined by retrospective analysis of all available seismic data in the examined area. According to the FastBEE algorithm, these events depend on the seismotectonic characteristics of the area and represent, from the physical point of view, a representative response of the medium to the local tectonic stress acting in the area.

The number of earthquakes per unit time, log

Seismicity map of Val d'Agri area and surrounding in the period
1983–2013 (Rectangle denotes the limits of the study area. Solid stars show the strong earthquakes,

The standard error of the calculation is given by the relation

Estimates of

The standard error of the

Finally log

This analysis uses seismic data, in the period 1983–2013, from an area
bounded by the coordinates 39.7

Non-cumulative frequency–magnitude distribution, in the period 1983–2013, denoting the changes of catalogue completeness.

Temporal variation of the seismic parameters
log

Temporal variation of log

Figure 4 shows, from the top to the bottom, the temporal variation of the
seismic parameters log

The numbered arrows, perpendicular to the time axis, show the origin times of
all events with

Strong earthquakes,

Dates of the appearance of the relative maximum and
minimum of the log

Therefore the analysis will be based on the identification of the clear low
and high probability periods of the precursory seismicity pattern (Fig. 1)
on parameter log

Practically the appearance of the relative minimum, in the majority of the examined cases, signalizes the beginning of the alarm period lasting until the earthquake occurrence, unless this behaviour changes.

The identification of the relative maxima and minima that represent the beginning of the two probability periods can be measured directly on these graphs or by analysing the appropriate temporal variation series.

Table 2 reports the dates of the appearance of the relative maximum that
practically signalizes the beginning of the low probability period and
relative minimum which signalizes the high and their respective duration, in
months, which were measured using the temporal variation estimates of the
parameter log

Both qualitative and quantitative analyses show that six of the eight
cases (i.e. 75 % of the relative minima that appears on log

It is interesting to point out that the temporal variation analysis of the seismic parameters reveals a period of stable seismic activity, which started at the beginning of 2006 and ended in 2009.

The seismicity temporal variation analysis shows significant temporal changes in all three examined seismicity parameters, using data from the Val d'Agri region in the period 1983–2012.

The form of these changes, which fluctuates around parameter mean values in the examined over 30-year period of observations, shows clear regularity corresponding to phases of a phenomenological model of earthquake preparation process. Therefore, these changes were considered to depict the response of the medium in the examined period due to the stress acting in the wider area.

Six of the eight cases of earthquakes with magnitude

In accordance with the above findings, the evaluation of the temporal
variation of seismic parameters until 2013 shows that the region is going
through a period of low probability of earthquake occurrence with a
magnitude above

The authors are thankful to two anonymous reviewers for their comments that helped improve the manuscript. Edited by: F. Vallianatos Reviewed by: two anonymous referees