The contribution of fluid geochemistry to define the structural pattern of the 2009 L’Aquila seismic source

Abstract

Field investigations performed in the epicentral area within the days following the April 6, 2009 L’Aquila earthquake (Mw 6.3) allowed several researchers to detect evidence of coseismic ground rupturing. This has been found along the Paganica Fault and next to minor synthetic and antithetic structures. Although a lot of geo-structural and geophysical investigations have been recently used to characterize these structures, the role of the different fault segments – i.e. as primary or secondary faults – and their geometrical characteristics are still a matter of debate. In light of this, we have here integrated data derived from fluid geochemistry analyses carried out soon after the main-shock with field structural investigations. In particular, we compared structural data with CO2 and CH4 flux measurements, as well as with radon and other geogas soil concentration measurements (see details in Voltattorni et al., this issue). Our aim was to better define the structural features and complexities of the activated Paganica Fault. Here, we show that, in the near rupture zone, “geochemical signatures” could be a powerful method to detect earthquake activated fault segments, even if they show subtle or absent geological-geomorphological evidence and are still partially “blind”. In detail, a clear degassing zone was identified just along the San Gregorio coseismic fracture zone – i.e., the surface deformation related to the "blind" San Gregorio normal fault. Indeed, CO2 and CH4 flux maximum anomalies were aligned along the Northern sector of the San Gregorio fault, in the Bazzano industrial area. This area also corresponds to the depocenter of the maximum coseismic deformation highlighted by DInSAR analysis (ATZORI ET AL., 2009). Here, maximum radon concentration values in soil gases were also found. As a whole, these results corroborates the hypothesis of BONCIO ET AL. (2010) who suggested that the San Gregorio fault probably represents a synthetic splay of the Paganica Fault, being thus connected with the main seismogenic fault at depth.Moreover, another maximum in CO2 flux anomaly has been measured along the southernmost tip of the earthquake rupture zone, close to the San Gregorio village. Minor or absent soil gas and flux anomalies were instead located along antithetic structures as the Bazzano and Fossa faults, while some anomalies in CO2 flux or radon concentration in groundwater have been found within transfer zones, such as the step-over zone between the central segment of the Paganica fault and the San Gregorio fault and in the zone which separates the Paganica fault from the i) Middle Aterno Valley- Subequana Valley and ii) Barisciano-S. Pio delle Camere-Navelli fault systems. Our results corroborate the power of fluid geochemistry in investigating the structural features of active tectonic structures, being particularly helpful in discerning blind faults. More specifically, our data suggest that the youngest fault splays, as in the case of the San Gregorio fault, may represent preferential sites for degassing.