Investigations on the seismic attenuation properties of the High Agri Valley, southern Italy: preliminary results by coda attenuation method

Abstract

<p>The High Agri Valley (HAV) is an intermountain basin, located in the axial zone of Southern Apennines. It is characterized by a very high seismic hazard, as testified by the 1857 M<sub>W</sub> 7 Basilicata earthquake and by the present widespread seismic activity as well. The strong seismogenic potential of the area is strictly related to the geologic evolution of the Apennines and in turn to its present complex tectonic configuration.</p><p>In addition to the natural low-magnitude seismicity occurring therein, two anthropogenic seismicity clusters are documented, located NE and SW of the artificial Pertusillo lake, respectively: a) fluid-induced microseismic swarms due to the injection, through the Costa Molina 2 well, of the wastewater produced by the exploitation of the Val d’Agri oilfield; b) protracted reservoir induced seismicity (RIS) caused by the combined effects of the water table oscillations of the Pertusillo lake, of the regional tectonics and of the poroelastic/elastic stress due to aquifers in the carbonate rocks.</p><p>In the recent past, the elastic properties of the target area have been thoroughly investigated through velocity tomography studies; nonetheless, the attenuation crustal features are to date almost completely unexplored. In our work we thus aim at starting to fill this scientific gap.</p><p>We here present the preliminary results obtained by applying the coda attenuation method on the seismic dataset collected in the period 2001-2014 by the stations belonging to both the trigger-mode monitoring network managed by ENI Oil Company and the Italian National Seismic Network managed by INGV. The complete seismic catalogue consists of about 1800 earthquakes in the magnitude range Ml 0 – 3.3 recorded by an average number of 14 stations. We show the unprecedented Q-coda estimations in the area. This information, in the first order Born approximation, represents the total attenuation accounting for both the scattering (Q<sub>S</sub>) and the intrinsic quality factor (Q<sub>i</sub>). </p><p>The preliminary results retrieved in this study are intended to be the first step towards a complete comprehension of the attenuation properties in this area by including in the near future also the continuous data recorded by the recently installed dense seismic network belonging to the HAV geophysical Observatory (HAVO). The planned investigation will pave the way to the discrimination between the scattering effects and anelastic features acting on the seismic waves propagating within the seismogenic crust of the study area. In that way it will be possible to better identify structural discontinuities and to gain further insights into the role of fluids in the preparatory phase and nucleation of earthquakes in the HAV.</p>