May engineering activities located in Sicily (Southern Italy) "potentially" induce seismicity?

Abstract

Earthquakes caused by human engineering activities are commonly termed as “triggered” or “induced”. This class of earthquakes, though characterized by low-to-moderate magnitude, have significant social and economical implications since they occur close to the engineering activity responsible for triggering/inducing them and can be felt by the inhabitants living nearby, and may even produce damage. One of the first well-documented examples of induced seismicity was observed in 1932 in Algeria, when a shallow magnitude 3.0 earthquake occurred close to the Oued Fodda Dam (Gupta, 1985). By the continuous global improvement of seismic monitoring networks, numerous other examples of human-induced earthquakes have been identified (see Davies et al., 2013 for an overview). Induced earthquakes occur at shallow depths and are related to a number of human activities, such as fluid injection under high pressure (e.g. waste-water disposal in deep wells, hydrofracturing activities in enhanced geothermal systems and oil recovery, shale-gas fracking, natural and CO2 gas storage), hydrocarbon exploitation, groundwater extraction, deep underground mining, large water impoundments and underground nuclear tests (Davies et al., 2013). Despite the presence in the Sicilian territory of a large amount of engineering activities “potentially capable” of inducing seismicity, no extensive researches on this topic have been conducted to date. Hence, in order to improve our knowledge, and correctly assess the hazard at a specific location in the future, we started a preliminary study on the main engineering activities located on- and off-shore of Sicily (Southern Italy). To this end, in a first step we collected all the useful information coming from available on-line national and regional catalogues. The compiled database includes 46 dams, 598 quarries and 839 oil and gas wells for a total of 1483 engineering activities. Among these, 175 are located along the southern Sicilian coastal off-shore while the remaining 1308 are located inland. As a second step, we performed a detailed compilation of instrumental seismicity striking the investigated area. Continuous seismic monitoring of the whole Italian territory started in the 90s and is currently performed by the National Seismic Network managed by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). In addition, monitoring and systematic analysis of seismic activity in eastern Sicily, by means of a dense local network, is performed also by the “Osservatorio Etneo” (INGV-OE), an INGV-branch located in Catania, close to Mt. Etna. Since 1983, earthquakes occurred in the entire Italian territory have been analysed and archived in the catalogue managed by the INGV headquarters in Rome (INGV-CNT; Castello et al., 2005; ISIDe Working Group - INGV, 2010), while since 1999, earthquakes occurred in eastern Sicily have been analysed and archived in the database of the INGV-OE (Alparone et al., 2009; Gruppo Analisi Dati Sismici, 2016). Because the INGV-OE catalogue covers with great details only the eastern sector of Sicily, while the INGV-CNT catalogues, extends back in time since 1983, in order to identify possible prospective effects of the human activities on the seismicity, here we took into account both catalogues. We considered only the seismicity occurred within the first 10 km of the crust because the induced seismicity should be likely confined in the shallower crust. As a final step, in order to identify prospective effects of the human activities on the seismicity, we investigated the spatio-temporal relationships between engineering activities and earthquakes, by adopting a statistic approach aimed to the detection of anomalous seismicity densities. Finally, we identified 46 engineering activities (2 dams, 16 wells and 28 quarries) characterized by anomalous seismicity density. These activities are mainly located in Western Sicily and on the eastern sector of the Hyblean Plateau, while a few number of activities are locate in northern Sicily and on the Island of Vulcano. Currently, we are performing detailed analyses on the nature of the observed seismicity activity in proximity of these engineering activities.