Holocene surface rupturing earthquakes on the Dinaric Fault System, western Slovenia

Abstract

Abstract. The Dinaric Fault System in western Slovenia, consisting of NW-SE trending, right-lateral strike-slip faults, accommodates the northward motion of Adria with respect to Eurasia. These active faults show a clear imprint in the morphology and some of them hosted moderate instrumental earthquakes. However, it is largely unknown if the faults also had strong earthquakes in the Late Quaternary. This hampers our understanding of the regional tectonics and the seismic hazard. Geological evidence of co-seismic surface ruptures only exists for one historical event, the 1511 Idrija Earthquake with a magnitude of ~M6.8, but the causative fault is still disputed. Here we use geomorphological data, near-surface geophysical surveys, and paleoseismological trenching to show that two of these faults, the Predjama Fault and the Idrija Fault ruptured in strong earthquakes in the Holocene. In a paleoseismological trench across the Predjama Fault we found at least one earthquake with a minimum magnitude of MW6.1 that occurred between 13–0.7 ka, very likely not earlier than 8.4 ka. At the Idrija Fault, a surface-rupturing earthquake with a magnitude of at least MW6.1 happened in the last ~2.1 ka. This event could correspond to the 1511 Idrija earthquake. Our results show that the faults rupture in rare, but strong earthquakes, which dominate the seismic moment release. We show that instrumental and historical seismicity data do not capture the strongest events in this area. The fact that many of the NW-SE trending, parallel faults are active implies that the deformation in western Slovenia is distributed, rather than focussed on one major structure.

Surface deformation analysis in Northeast Italy by using PS-InSAR and GNSS data 

“…These tectonic structures are currently being investigated to better define their characteristics (geometry and kinematics) and their seismogenic potential. Indeed, despite their low rates, these active dextral transcurrenttranspressive tectonic structures characterized by complex geometries are capable of moderate/strong earthquakes, as the 1998 Mw 5.6 and 2004 Mw 5.2 Bovec-Krn earthquakes (NW Slovenia) (Kastelic et al, 2008;Kastelic and Carafa, 2012;Vičič et al, 2019;Atanackov et al, 2021;Grützner et al, 2021). Although it is not possible to discriminate the contribution of every single structure, the signals detected in our profile suggest the activity of these transcurrent and transpressive structures.…”

The Ljubljana 1895 earthquake: source parametres from the inversion of macroseismic data

“…Although the system generates moderate earthquakes, it is largely unknown if the faults also had strong earthquakes in the Late Quaternary. Grützner et al (2021) point out a difficulty in understanding the regional tectonics and the seismic hazard of the area. Žibret and Vrabec (2016) indeed, report two main thrust characteristic for this area and at least four post-Paleocene stress tensor groups.…”

“…In western Slovenia, the Dinaric Fault system, consists of a NW-SE trend of active right-lateral strike slip faults (Žibret and Vrabec 2016;Atanackov et al 2021;Grützner et al 2021). This system, formed during the Cenozoic, is the junction of the Dinaric chain with the Alps and accommodates the northward motion of Adria with respect to Eurasia.…”