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|Authors: ||Quattrocchi, F.*|
|Title: ||Geochemical anomalies during the 2009 l’Aquila seismic sequence (Central Italy): transverse lineaments inside the activated segments?|
|Issue Date: ||Nov-2009|
|Abstract: ||The 2009 L’Aquila seismic sequence, whose main shock (Ml 5.8, Mw 6.3) occurred on April 6th at 1:32 UTC, is still ongoing (August 2009) along the central Apenninic Belt (Abruzzo region, central Italy). The main earthquake was destructive and caused 300 casualties; its epicenter has been located at 42.35°N, 13.38°E, at a depth of around 10 km. The main shock was preceded by a long seismic sequence, started several months before (i.e., December 2008, with a total of 300 earthquakes with Mlmax = 4.0). After the April 6th main event, two other earthquakes struck the area on April 7th and 9th, with Ml 5.3 and 5.1, respectively. A lot of evidences stress the role of the pore-pressure evolution of deep fluids in generating the L’Aquila sequence (e.g. Vp/Vs anomalous ratio, Chiarabba C., 2009 personal communication) as occurred for the Umbria Marche (central Italy) 1997-1998 seismic sequence (Quattrocchi, 1999 and references herein). The entire sequence highlights more than one seismogenic segment activated along a main NW-SE-trending Apenninic lineament (Fig. 1). Soon after the strongest event, our group (UF “Fluid Geochemistry, Geological Storage and Geothermics”, Department Rome 1, INGV) carried out a geochemical study throughout the seismically activated area by sampling around 600 soil gas points and around 70 groundwater points (springs and wells). The main goal of this study was the comprehension of both the behaviour and the geometry of the activated fault segments by means the application of specific geochemical methods, already exploited in other Italian seismic and volcanic areas (Quattrocchi et al., 2000; Pizzino et al., 2004; Quattrocchi et al., 2008; Voltattorni et al., 2009).In particular, here we discuss only the results gathered by measuring soil gases, considering fluxes of CO2 and CH4 as well as concentrations of radon, CO2, CH4, He, H2, N2, H2S, O2, and other minor geogas (i.e. light hydrocarbons) in the main sectors of the activated seismic sequence (see the red box in figure 1).
The geochemical measurements were addressed to more than one objective. One was to update a GIS of the co-seismic effects (associated to the earthquakes with magnitude greater than ML=5.0 and surveyed by our INGV EmerGeo Working Group) and their spatial and geometrical parameters in the local geological framework. More than 400 observation sites (fractures mainly) have been detected in an area of ~ 900 km2, part of which coupled with geochemical measurements in soils (fluxes and concentrations). Most of the surface effects have been observed also as regards the presence/absence at surface of deep fluids uprising (hot water, gas pools/fluxes, vapours, etc….) along and close to the previously mapped active faults (INGV Catalogue of Strong Historical earthquakes). Geochemical measurements in soils are very powerful to discriminate the numerous local surface effects (landslides, differential compaction, rock falling, etc) with respect to the real signatures of the expression at surface of the activated seismogenic segment. For earthquakes of moderate magnitude, such as the L’Aquila 2009 event, where the superficial effects could be absent or not evident, our geochemical method demonstrated to be strategic and potentially applicable in other worldwide seismic areas.
Most ruptures with a structural significance have been observed along the Paganica Fault (elongated NW-SE); also the Bazzano and Monticchio-Fossa faults have been geochemically analysed; we searched the different behaviour of these structures as a whole, each having a different tectonic role (passive, active, transfer, crossing point, etc.), in determining the degassing observed at surface.
The results highlight the maximum geochemical anomalies just along a minor anti-apenninic NE-SW transverse lineament; here, CO2 (up to 2000 [gm-2day-1]) and CH4 (up to 300 [gm-2day-1] anomalous fluxes were found soon after the main shock, remaining anomalous in the following months, but with lower values. Furthermore, just in correspondence of this lineament highest values of radon (up to 40.000 Bq/m3) were found. The transects perpendicular to the Paganica Fault clearly highlighted the role of the main fracture at surface (masked in few days) as preferential pathway for gases escaping from depth, as revealed by geochemical methods.
The measured values are in any case not dangerous for the people’s health and minimise the problem of CO2-CH4 burst or explosions during strong earthquakes when these gases are stored naturally underground (km), as under these activated faults (as showed by the deep wells drilled in the area). Finally, the soundest sites to install 3 continuous monitoring stations, measuring the CO2 fluxes, were selected and the stations installed (Paganica, Bazzano and S. Gregorio sites) in cooperation with colleagues coming from the INGV geochemical department of Palermo (Sicily, southern Italy). The considered geochemical methods could be exploited along other faults in Italy and abroad by performing pre-main shock geochemical surveys to i) highlight earthquake preparation discovering anomalous degassing; ii) draw a picture of fault degassing before strong seismic events and, later, during a seismic sequence; iii) to highlight transverse lineaments among main fault segments, adding information where geo-structural expressions at surface are hidden.|
|Appears in Collections:||Conference materials|
05.08.01. Environmental risk
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