Soil gas distribution in the main coseismic surface rupture zone of the 1980,M_s = 6.9, Irpinia earthquake (southern Italy)

Abstract

Soil gas measurements of different gas species with different geochemical behaviors were performed in the area of the Pecore Plain, a 200 m × 300 m sized, fault-bounded extensional basin located in the northern Mount Marzano massif, in the axial belt of the southern Apennine chain. The Pecore Plain area was affected by coseismic surface faulting during the M s = 6.9, 1980 Irpinia earthquake, the strongest and most destructive seismic event of the last 30 years in southern Italy. The collected data and their geostatistical analysis provide new insights into the control exerted by active fault segments on deep-seated gas migration toward the surface. The results define anomalies that are aligned with the NW-SE trending coseismic rupture of the 1980 earthquake along the western border of the plain, as well as along the southern border of the plain where a hidden, E-W striking fault is inferred. Geospatial analysis highlights an anisotropic spatial behavior of 222 Rn along the main NW-SE trend and of CO 2 along the E-W trend. This feature suggests a correlation between the shape and orientation of the anomalies and the barrier/conduit behavior of fault zones in the area. Furthermore, our results show that gas migration through brittle deformation zones occurs by advective processes, as suggested by the relatively high migration rate needed to obtain anomalies of short-lived 222 Rn in the soil pores. IntroductionThe Pecore Plain is a small, fault-bounded basin located in the northern Mount Marzano massif, in the axial belt of the southern Apennine chain, Italy (Figure 1). The area was affected by coseismic surface faulting related to the M s = 6.9, 23 November 1980 Irpinia earthquake, the strongest and most destructive (I 0 = X Mercalli-Cancani-Sieberg scale (MCS)) seismic event of the last 30 years in the southern Apennines [e.g., Pantosti and Valensise, 1990;Porfido et al., 2002;Serva et al., 2007;Locati et al., 2011;Rovida et al., 2011]. The earthquake struck the epicentral Mount Marzano area and the neighboring Irpinia region, which was affected by a large number of secondary geological effects (e.g., landslides, ground cracks, liquefaction, and variations in the discharge rate of major carbonate springs) [Porfido et al., 2002] and by more than 3000 coseismic ruptures within the region recording the largest intensity (I ≥ IX MCS) [Carmignani et al., 1981].The main coseismic fault trace of the 1980 earthquake consists of approximately 7 km long, up to 1 m high, NE facing fault scarp with an average NW-SE trend that crosses the Pecore Plain basin. This fault, which was associated with rupturing caused by the main earthquake, lowered the basin floor relative to its outlet. At present, surface drainage has not been recovered and the fault scarp still acts as a dam and occasionally causes ponding during the wet season [Pantosti and Valensise, 1990;Ascione et al., 2003, and references therein].As soil gas surveys conducted in the years after the 1980 earthquake highlighted large helium anomalies along the faul...

New insights into crustal structure, Cenozoic magmatism, CO2 degassing, and seismogenesis in the southern Apennines and Irpinia region from local earthquake tomography

“…We hypothesize that high-fluid pressure in the deep CO 2 reservoir (the low-Vp/Vs volume) played a key role in the rupture that enucleated just under the AP. This hypothesis is in agreement with the recent study of Ciotoli et al [2014] that relates soil gas anomalies along the 1980 surface ruptures to deep-seated gas (including deep origin CO 2 ) migrating along the fault segments. Then, the main shock rupture propagated preferentially within high-Vp main asperities located inside the Apulian carbonates.…”

“…This hypothesis is in agreement with the recent study of Ciotoli et al . [] that relates soil gas anomalies along the 1980 surface ruptures to deep‐seated gas (including deep origin CO 2 ) migrating along the fault segments. Then, the main shock rupture propagated preferentially within high‐ Vp main asperities located inside the Apulian carbonates.…”

Radiogenic helium degassing and rock fracturing: A case study of the southern Apennines active tectonic region

“…A recent geochemical investigation involving measurements of soil gases coupled to the geospatial analysis of the behavior of the volatiles along the main faults linked to the 1980 Irpinia earthquake revealed an absence of impermeable seals that could prevent the migration of gas from depth [ Ciotoli et al , ]. This confirms that an effective advective fluid migration from depth toward the surface occurs even through the strongly anisotropic and rheologically layered crust of the southern Apennines.…”

Seismic imaging of a fluid storage in the actively extending Apennine mountain belt, southern Italy

“…Active fault segments at surface are decoupled from seismogenic deep‐seated structures by the mélange zone and the Pliocene shales overlying the Apulian Platform carbonates. The mélange, and the shales, although allowing CO 2 migration toward the surface [ Ciotoli et al ., ], act as a seal preventing the migration of deep‐seated aqueous fluids – as well as oil in the Basilicata region to the SE [ Shiner et al ., ] – toward the surface. This is inferred from the geochemical signature of fluid inclusions from fault rocks [ Cello et al ., ] and thermal waters in the investigated area [ Duchi et al ., ].…”