Normal focal mechanisms in the Julian Alps and Prealps: seismotectonic implications for the Italian-Slovenian border region

The instrumental seismicity (earthquakes with 2.4 ≤ MD ≤ 5.6, collected by the Seismometric Network of the Friuli-Venezia Giulia Region) that hit the Italian-Slovenian border region in the period 1974-2000 was analysed and discussed. The study area, which is characterised by medium-high seismicity, is located in the north-eastern portion of the deformed margin of the Adria microplate. Here the SSE-verging Neogene to Quaternary thrust-belt of the Southalpine chain joins with the Paleogene SW-verging thrust-belt of the External Dinarides. In the western Slovenian region, a set of dextral NW-SE strike-slip faults, Neogene to Quaternary in age, seems to act as the easternmost tectonic release for the Southalpine chain. The focal mechanism of 48 events was constructed: their distribution confirms that two distinct seismotectonic zones coexist above the same maximum stress tensor (σ1 horizontal and about N-S striking). 1) The Gemona area, that shows fault-plane solutions typical of low-angle WNW-ESE trending reverse faults, confirming that it pertains completely to the eastem Southalpine thrust-belt which is propagating southwards deforming the Quaternary deposits of the Friuli plain. 2) The Cividale - NW Slovenia area, that shows two different sets of fault plane solutions. The first indicates NW-SE trending dextral strike-slip faults. This system is connected to the main seismicity of the Extemal Dinarides as testified by the Bovec earthquake of April 1998 (M_D = 5.6) and the Sneznik -Klana events of May 1995 (M_L = 4.7) and February 1992 (M_L = 4.1). The second set presents normal or transtensional fault plane solutions with high angle NNW-SSE trending planes. The inversion technique of Gephart and Forsyth confirms a heterogeneous stress field for the Cividale -NW Slovenia area. The structural model proposed shows that extensional strike-slip duplexes develop together with the main strike-slip fault-system of Cividale - NW Slovenia area. Moreover, it is hypothesised the seismogenic activity of this region is linked to the reactivation of deep inner structures probably Early Neogene in time.