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
This paper is concerned with Holocene surface-rupturing earthquakes on strike-slip faults belonging to the Dinaric Fault System (DFS) in western Slovenia (Fig. 1). Following the established nomenclature in the literature (e.g. Slejko et al., 1989), the faults are named Dinaric faults because they strike parallel to the trend of the Dinarides. However, they must not be confused with the now inactive, SW-vergent Dinaric thrusts. It is well understood that the Dinaric right-lateral faults are active and that they accommodate a share of the relative motion between Adria and Europe (Poljak et al., 2000; Vrabec and Fodor, 2006; Placer et al., 2010; Moulin et al., 2016; Atanackov et al., 2021). However, so far very little is known about their earthquake record. In this rather slowly deforming region, strong but rare earthquakes may dominate the overall seismic moment release. Identifying the strongest earthquake events in the late Quaternary will, therefore, help to better understand the regional active tectonics, shed light on the role of individual faults in the deformation of the crust, and better inform seismic hazard assessments.
Regions of slow continental deformation pose a challenge for active-tectonics studies for several reasons. (i) In low-strain settings, geodetic techniques that cover only a few decades of monitoring (GPS, InSAR) have to deal with very small amounts of total crustal deformation. This hampers a proper recognition of active structures. (ii) Instrumental seismicity is usually low. Microseismicity studies will, therefore, often not be able to detect active faults. Earthquakes strong enough to compute reliable focal mechanisms, for example by teleseismic body waveform modelling, may not have occurred during the instrumental era. Thus, our understanding of fault mechanisms and seismogenic faults may be limited. (iii) Large earthquakes are rare. Historical catalogues contain felt events but do not necessarily record the strongest possible earthquakes on the local active faults. This is due to the faults' long earthquake recurrence intervals. Also, historical earthquakes can rarely be tied to their causative faults. Earthquakes strong enough to leave their imprints in the landscape must have very large recurrence intervals on slowly moving faults. For the DFS, no long-term recurrence intervals are known. Areas with similar strain rates of a few millimetres per year are known to have recurrence intervals in the order of thousands to tens of thousands of years (e.g. Grützner et al., 2017). Chances are high that their traces were modified or obliterated by erosion and sedimentation before any targeted geoscientific investigations. Notwithstanding all those issues, studying slowly deforming regions and diffuse plate boundaries is important both from the perspectives of active tectonics (e.g. Stein et al., 2009; Landgraf et al., 2017) and seismic hazard (England and Jackson, 2011).
In our study area at the transition zone between the eastern Southern Alps
and the Dinarides we are confronted with the above-mentioned problems to
varying extents. GPS studies show that there is about 3 mm/a of convergence
between Adria and Europe at the longitude of the Eastern Alps (D'Agostino et
al., 2005, 2008; Weber et al., 2010; Metois et al., 2015), but the station
network is too sparse to assess individual faults. Only few reliable moment
tensor solutions are available for moderate earthquakes (Anderson and
Jackson, 1987; Herak et al., 1995; Bajc et al., 2001; Pondrelli et al.,
2002, 2011; Kastelic et al., 2008), and they only occurred on some of the
main faults, for example the Ravne Fault earthquakes in 1998 and 2004 (Fig. 1). For historical earthquakes of
In this paper we present data from paleoseismological trenches dug across two of the large strike-slip faults in western Slovenia: the Idrija and Predjama faults. We show evidence for Holocene surface-rupturing earthquakes on those faults and support our interpretation with geomorphological data, geophysical profiling, and radiocarbon dating. We then discuss the implications of our findings in the light of the regional tectonic setting and seismic hazard.
Our study area is located in western Slovenia in the External Dinarides, close to the transition zone between the eastern Southern Alps and the Dinarides (Figs. 1, 2). The area was shaped by the collision of the Adriatic microplate and Europe during the Cenozoic (Schmid et al., 2008; Placer et al., 2010; Ustaszewski et al., 2010; Handy et al., 2015). Shortening in NE–SW direction and the related SW-directed thrusting of Mesozoic carbonates along NW–SE-striking faults lasted from the Cretaceous to the Eocene (Fig. 3). This phase has shaped the large-scale geological picture as we see it today. From the Oligocene to the early Miocene, S-directed transport on E–W-striking faults characterised the phase of south Alpine thrusting. Žibret and Vrabec (2016) used paleostress analyses to distinguish three phases of deformation post-dating the late Eocene NE–SW shortening in western Slovenia: early to middle Miocene back-arc extension in the Pannonian Basin led to NE–SW extension and normal faulting on NW–SE-striking structures. Then, a short pulse of late Miocene E–W contraction led to a left-lateral reactivation of the NW–SE-striking faults and N–S extension. Since the Pliocene, N–S shortening is taken up by right-lateral motion on a NW–SE-trending strike-slip fault system known as the Dinaric Fault System (e.g. Poljak et al., 2000; Vrabec and Fodor, 2006; Placer et al., 2010; Moulin et al., 2016; Atanackov et al., 2021; Figs. 1–3). This phase lasts until today and is corroborated by seismological data (e.g. Herak et al., 1995; Pondrelli et al., 2002; Vičič et al., 2019), remote sensing, and field studies (e.g. Cunningham et al., 2007; Kastelic et al., 2008; Gosar et al., 2011; Moulin et al., 2014, 2016).
The study area in the Alps–Dinarides transition zone. Main faults
of interest (red lines) are from Atanackov et al. (2021). Historical and
instrumental seismicity are from the SHEEC database (Stucchi et al., 2013;
Grünthal et al., 2013); the 1511 Idrija earthquake is marked (note that the
location is uncertain; the earthquake probably occurred either on the Idrija
Fault or on the SW side of the mountain front in Italy). Beach balls with
black outlines are derived from
The study sites in NW Slovenia. Faults are from Moulin et al. (2016) and our own mapping.
Geological setting of the study sites in NW Slovenia; same extent as Fig. 2. The geology is compiled from the 1 : 250 000 geological map of Buser (2009) and the 1 : 150 000 geological map of the Friuli Venezia Giulia region (Carulli, 2006).
The largest faults of the strike-slip Dinaric Fault System in the western
part of this system are the Raša, Predjama, Idrija, and Ravne faults
(Figs. 1–3). They accommodate N–S shortening by right-lateral strike-slip
motion since the Pliocene (Žibret and Vrabec, 2016). The more than 120 km long Idrija Fault has the most prominent morphological imprint in the
study area (Cunningham et al., 2006, 2007). It has a total right-lateral
offset of around 10–12 km (Šušteršič, 1996; Placer et al.,
2010) and an almost straight fault trace, pointing to its predominant
strike-slip mechanism. Lower offset estimates (
The
Geological map of the Predjama Fault trench site. Modified from the 1 : 250 000 geological map of Buser (2009). Same extent as the inlet in Fig. 2.
GPS studies show that there is about 2–3 mm/a of N–S convergence of Adria
vs. Europe at the longitude of the Eastern Alps (D'Agostino et al., 2005,
2008; Weber et al., 2010; Fig. 1) and a anticlockwise rotation of Adria
about an Euler pole in the Western Alps of not more than 0.52
Large instrumental earthquakes with well-constrained fault plane solutions
in the study area are rare. The 1976 Friuli sequence occurred on a
N-dipping, E–W-trending thrust fault (e.g. Aoudia et al., 2000; Peruzza,
2002) at the southern Alpine front. Three earthquakes (beach balls with red
outlines in Fig. 1) with magnitudes of 6.5, 6.0, and 6.1 (Aoudia et al.,
2000) caused widespread damage. No surface rupture occurred, but a few
centimetres of uplift were inferred from high-precision levelling and
triangulation (e.g. Cheloni et al., 2012). Two shallow earthquakes of
magnitude
Historical seismicity in western Slovenia includes several earthquakes
exceeding magnitude 5.5 (Ribarič, 1982; Albini et al., 2013, Fig. 1). None of
the reported events can be conclusively tied to a specific fault, and there
is no apparent clustering on any of the known active structures. Reliable
historical data are mainly confined to the last 200 years (e.g. Herak et
al., 2009, 2017, 2018; Cecić, 2015). Modelling of the macroseismic
effects of the 1895
In order to identify the most promising sites for paleoseismological trenching, we first analysed the 1 m digital elevation model (DEM) of Slovenia (Ministry of the Environment and Spatial Planning, Slovenian Environment Agency, ARSO, 2019). These data are freely available for the entire country and were collected using lidar technology (light detection and ranging; airborne laser scanning). We manually inspected the mapped traces of the Predjama and Idrija faults and searched for offset geomorphic markers, breaks in slope, scarps, and similar indicators of recent tectonics. Then we identified sites with sediment archives that could have recorded large past earthquakes. For selected sites we acquired aerial images using a DJI Phantom 4 drone, which allowed us to compute high-resolution DEMs with up to 5 cm resolution using the structure-from-motion technique (SfM).
Geophysical surveys were then used to image the shallow subsurface and to precisely map the fault traces. All geophysical data are available online (Grützner et al., 2020). We used a ground-penetrating radar (GPR) system from Geophysical Survey Systems Inc (GSSI) with monostatic 100, 270, and 400 MHz antennas and a Pulse EKKO Pro Sensors & Software system equipped with bistatic 250 MHz antennas. All data were processed with the ReflexW software (Sandmeier Geophysical Research). Processing included frequency bandpass filtering, background removal, gain adjustments, and topographic corrections. The topographic data were extracted from the 1 m DEM.
Electric resistivity tomography (ERT) was performed with a 4point light system (Lippmann Geophysikalische Messgeräte). We used up to 80 electrodes with varying electrode spacing for Wenner, Schlumberger, and dipole–dipole arrays, depending on target resolution and depth. Data inversion was done with Res2DInv (Geotomo Software) and included manual de-spiking and topographic corrections.
Along several profiles we measured the vertical gradient of the geomagnetic field and the total magnetic field strength with a proton magnetometer GSM–19T (GEM Systems). The system consists of a rover and a base station, which allows correcting the data for diurnal variation. We used 1 m point spacing along several long transects.
Based on the geophysics results we selected one trench site at the Predjama
Fault and one at the Idrija Fault. The trenches were excavated with a
backhoe and their walls were cleaned and straightened. We installed a
In the area of the Čepovan Canyon the Predjama Fault mainly runs through Triassic and Jurassic carbonates (Fig. 4). Further to the northwest of the canyon, Maastrichtian calcarenites are present, which belong to the Cretaceous to Eocene foredeep sequence. The Predjama Fault trench site is located on the Banjšice Plateau, northwest of the Čepovan Canyon. In this area, a single fault trace is clearly visible in the DEM (Fig. 2). At Avče in the Soča Valley to the northwest of our trench site, Moulin et al. (2016) described a prominent change in valley topography caused by the fault (Fig. 2). The fault bifurcates just SE of our trench site and the two strands offset the cliffs of the Čepovan Canyon (Moulin et al., 2016). These authors also described a prominent vertical offset across the two fault strands. We found that slickensides in two quarry outcrops on the two strands (Fig. 2) show almost pure right-lateral strike slip on fault planes dipping steeply to the southwest (Fig. 5a–c). The fault zone is about 30 m wide in the second quarry (Fig. 5d). Both quarries exhibit Kimmeridgian–Oxfordian coralliferous limestones. To the southeast of the Čepovan Canyon, the two strands of the Predjama Fault cross the Trnovski Gozd Plateau, but only the southern fault trace continues further to the southeast (Fig. 2). We chose our trench site partly based on these observations in order to capture the entire slip of the fault on a single trace and to not miss significant portions due to its branching. About 70 m south of our trench site, Kimmeridgian–Oxfordian coralliferous limestones crop out in a large road cut, which runs parallel to the fault strike. These limestones are not intensely fractured and do not exhibit a fault zone. They only have a thin soil cover. According to the geological maps, Maastrichtian calcarenites are juxtaposed with the Jurassic limestones by the fault at this site (Fig. 4; Buser, 2009). We did not find any outcrop of the calcarenites near our trench site, but we encountered weathered and deformed calcarenites in the trench. We will describe these rocks in detail in Sect. 4.1.4. These observations indicate that the fault must run between the road outcrop and the trench site.
Quarries along the Predjama Fault; see Fig. 2 for location. Both
quarries exhibit Jurassic coralliferous limestones.
At the trench site we observed a break in slope on a gently NE-dipping
meadow (Fig. 6). The break in slope is detectable for
Morphology of the Predjama Fault at our trench site.
We used GPR, ERT, and magnetic measurements on parallel profiles perpendicular to the scarp in order to check the subsurface conditions and to select the most promising trench site (Fig. 7). Georadar did not provide good data quality, which during trenching turned out to be due to highly conductive clayey sediments present near the surface. The geomagnetic data showed neither indications of the fault nor of metal objects in the ground, which indicates the lack of potentially dangerous war remains.
The dipole–dipole ERT data with 1 m electrode spacing show mainly
resistivities of less than 70
Map of the trench site at the Predjama Fault and ERT data.
Paleoseismic trench 1 across the Predjama Fault on the
Banjšice Plateau.
Trench 1 was 10 m long and on average 1.5 m deep (Fig. 8). Deeper excavation was prohibited by the shallow groundwater level. We distinguished seven units in both trench walls, most of which represent a rock weathering profile. Since both trench walls show comparable features, we use the northwestern trench wall for the detailed description in the following. We use the terms and weathering grades defined by ISRM (1981).
Unit U1 at the base of the trench consists of highly weathered calcarenites in a clayey matrix (grade IV). The rocks are intensely fractured; coherent bedding is only occasionally visible. While the U1 mainly consist of large clasts in the first 4 m of the trench, the weathering products dominate the upper parts of the unit between 5 and 8 m. There is no sharp boundary between these two zones in the central part of the trench. However, the large amounts of fractured calcarenites there and the relationship to unit U2 require mapping it as U1. Occasionally, large and coherent but intensely fractured blocks of calcarenites are also present in the upper parts of U1. Because of their distinct appearance we mapped them as unit U2, representing moderately weathered rocks of grade III. Both units show a dip towards the northeast, parallel to the slope. On top of U1 and U2 sits a thin layer of compact grey clay: unit U3. This unit is a residual soil of grade VI. U3 is overlain by a thin red ribbon of sandy clay (unit U4) that mainly consists of moderately weathered calcarenites (grade III). Both U3 and U4 dip to the northeast. We interpret Units U1–U4 as a sequence of intensely weathered bedrock.
Most of the upper part of the trench is formed by a residual soil (grade VI) that forms massive brown clays (unit U5). The contact between U4 and U5 is sharp. Where U5 is in contact with U4, it contains small amounts of sand and fine gravels as well as fragments of charcoals. Small (fine gravel) calcarenite and limestone clasts are present throughout U5 in small amounts. Around 5 m in the trench, U5 is in contact with U1. Here the units U3 and U4 appear as lenses within U5, although they overlay U1 and U2 in the southwestern part of the trench. The pale, bleached clayey unit U6 with up to 30 cm thickness occurs just beneath the recent soil (unit U7). We note that unit U6 is not a sedimentary body, but a residual soil. The distinct pale appearance is likely just due to a modification of U5 by increased water content, or it may result from compaction due to the occasional use of the track by the farmers.
No sharp fault trace was found in the trench. Instead, we observe a zone of localised deformation at around 4–5 m distance from the southern end of the trench. Unit U1 can be easily identified in the first 5 m of the trench. Around 5 m distance from the southern end of the trench, this unit bends down together with the overlying units U3 and U4. In a 40 cm wide gap unit U5 reaches the bottom of the trench. Unit U1 then occurs again between 5–6 m and between 6–8 m with up to 0.6 m thickness. It becomes gradually more weathered upwards and the contact with unit U5 is not sharp. The larger blocks of intact calcarenites that make up unit U2 are present throughout the trench and show no deformation.
Unit U3 is only visible in the southern section of the trench for about 2 m. It has a more constant thickness of 10 cm in the SE wall of the trench and varies between 10 and 20 cm thickness in the NW trench wall. While this unit parallels the top of unit U1 for about 2 m from its southern end, it then bends down and dips steeply towards the northeast beneath the break in slope. Here we observe small folds in the tip of the layer.
A similar feature is observed in unit U4, which also parallels U1 but then
bends downward and becomes folded. Here we identified small horizontal
faults with
Two scenarios could explain the observations in this trench: non-tectonic or tectonic processes. In the non-tectonic scenario, the shape of U3 and U4 as well as the vertical offset in units U1 and U2 could be due to shallow-seated, slope-parallel motion, combined with phases of pedogenic development above the weathered bedrock units and perhaps additional oblique run-off processes. Arguments in favour of this scenario are the lack of a sharp fault zone in the trench and the fact that vertical motion is not expected on the strike-slip Predjama Fault. Arguments in favour of a tectonic process are (i) the lack of evidence for slope-parallel motion in the trench, such as shallow sliding planes, (ii) that the vertical separation of units U1 and U2 is not easily explained by gravitational processes only because the downslope side is uplifted, and (iii) the absence of evidence for run-off processes or slope-parallel motion in the two high-quality DEMs (1 m lidar and drone DEM). The slope is very smooth apart from the scarp; there are no landslide scars or “wrinkles” that may point to the toes of slides, etc. Since at least the upper units in the trench are of Holocene age as we will show in the following, we might expect to see such features in the morphology. Furthermore, although prevailing vertical motion is not expected, a vertical component of slip can be due to an irregular, bending fault trace.
The bending, the folding, and the faulting observed in the trench could be interpreted as tectonically induced (scenario 2). This interpretation is supported by the observation that U3 and U4 are intensely deformed in a relatively narrow zone, across which a vertical separation of U1 and U2 is observed. This zone at around 5 m would then represent the fault. This view is strengthened by the geomorphological and geophysical observations.
A sample of organic material from the contact between U4 and U5, sample SLO18_BAN3, gave an age of 8406–8311 cal BP (Table 1). Note that we sampled a fragment of charcoal, but during sample preparation in the lab it turned out to be too small, but it was possible to date the organic sediment around it.
We dated unit U5 with a bulk sample (SLO18_BAN2) from 10 cm above U4. This sample gave an age of 7795–7669 cal BP. A charcoal sample (SLO18_BAN4) from right below U6 provided an age of 3695–3565 cal BP. All these ages are in stratigraphic order. On the opposite trench wall, we also dated U5 with a bulk sample (SLO18_BAN6) that returned an age of 12 976–12 727 cal BP and a stratigraphically higher charcoal sample (SLO18_BAN7) that showed an age of 800–694 cal BP. These two samples are significantly older and younger than those of U5 on the NW trench wall, respectively.
Unit U6 can only be observed for around 4 m on either trench wall. It probably represents compacted residual soil of U5 resulting from the occasional use of the break in slope as a dirt road. Its age is constrained by samples SLO18_BAN4 and SLO18_BAN7 to post-800 cal BP. The full high-resolution trench orthophotos are presented in the Supplement.
Trench 2 exhibited a different stratigraphy. Here, we observed massive brown
and grey, clayish residual soils beneath the modern soil but no
calcarenites. Instead, the base of the trench at
The Idrija Fault trench site is located 4.5 km NW of the town of Idrija, in the valley of the Kanomljica River (Fig. 2). Here, the fault juxtaposes Upper Triassic with Lower–Middle Triassic rocks, mainly dolomites (Fig. 9; Mlakar and Čar, 2010). Cunningham et al. (2006) noted that several drainage anomalies around the Kapa hill just to the southwest of our trench site point to late Quaternary or even Holocene fault motion (Figs. 9, 10). Bavec et al. (2012, 2013) chose the river valley at the SW foot of the Kapa hill for a geophysics study and for a paleoseismological trench. Moulin et al. (2014) analysed the geomorphology in detail and reported between 340 and 380 m of right-lateral offset based on the correlation of the modern Kanomljica river and abandoned valleys. The precise trace of the Idrija Fault in the vicinity of our trench site is subject to some debate (Fig. 10; also see Moulin et al., 2014). The NW flank of the Kapa hill exhibits a wide shear zone with (ultra-)cataclastic rocks. This wide shear zone is in line with the observation that the Idrija Fault has a large cumulative offset (Čar, 2010; Placer et al., 2010). The trace of the fault in the wide valley of the Kanomljica is obscured by flat-lying fluvial sediments. Outcrops on the north side of the valley show that at least one fault strand clearly runs along the base of the hills (Moulin et al., 2014).
Geological map of the trench site at the Idrija Fault. Simplified from Mlakar and Čar (2010). Same extent as the inset in Fig. 2.
Map of the trench site at the Idrija Fault and selected ERT data.
We concentrated our geophysical investigations on the flat area of the Kanomljica valley because we expected to find deformed sediments if the fault ruptured the surface in the late Quaternary (Fig. 10). Additional surveys were performed on the southern bank of the Kanomljica. The geomagnetics survey was not successful. Not only did the data quality suffer from the presence of power lines and other anthropogenic disturbances, but also in areas not affected by these effects we could not identify a signal pointing to a change in the subsurface structure. Georadar data suffered partly from low penetration depths owing to clay-rich sediments and a low groundwater table. Although some information on the geometry of the fluvial units could be extracted, we did not see any clear evidence for faulting that would have allowed for picking a promising trench site.
The ERT profiles on the flat surface north of the Kanomljica showed the
clearest indication of the location of the Idrija Fault. For these profiles we
used 3 m electrode spacing in order to reach great depths (Fig. 10). The
uppermost part of the subsurface is made up of a medium-resistivity layer
(200–700
Below the medium-resistivity layer, the central parts of the profiles are
characterised by an up to 30 m thick low-resistivity unit (
Based on the ERT data we decided to open a trench across the sharp contact between the high-resistivity units and the low-resistivity units in the western part of the survey area (Fig. 10). The trench was 20 m long, 2 m deep, 2 m wide, and placed on the track of profile 1. We did not dig any deeper because the nearby Kanomljica stream incised about 2 m into the flat valley bottom, and the groundwater table was located just beneath the trench floor. In the following we only describe the central part of the trench where we encountered traces of recent deformation. The full high-resolution trench orthophotos are presented in the Supplement.
In general, the trench is characterised by fine to coarse gravel deposits. In the centre of the trench, a channel exhibits clayey to sandy units. In the north wall, the channel can be seen between 5.5 and 12.5 m distance, reaching down to the trench floor (Fig. 11a). In the southern trench wall, the channel is observed between 4 and 14 m distance at up to 1 m depth (Fig. 11b).
We observed 11 different units in the trench. Unit U1 consists of clast-supported, fine-coarse fluvial gravels in a brown clayey matrix. This unit is present throughout the entire trench. Unit U2 is made up of mostly medium gravels in a bright red, clayey matrix. This layer is situated within unit U1 and only visible in the northern wall. Fine to coarse gravels in a light red clay matrix (unit U3) can be found as pockets in both trench walls within U1. We cannot rule out that U2 and U3 are the same unit, since it is only their colour that is different. At the base of the channel, we identified U4, consisting of sandy silt with yellow flakes. This unit is in vertical contact with massive grey clays (unit U5) and overlain by yellowish clayey gravels (unit U6). In the southern trench wall, U4 partly overlies U5, which may indicate that it has eroded into the latter. Only in the northern trench wall is a small pocket of grey clayey sands (unit U7) present at the contact between U1 and U8/U10. In the southern trench wall, silty–sandy, dark-grey unit U9 cuts through units U5 and U8. Both units U4 and U6 terminate at U9. Unit U9 is overlain by the darkest parts of U10, to which there is no sharp boundary.
Unit U10 consist of gravels in a matrix of grey clays. This unit overlies U7 in the northern trench wall and units U5 and U6 in the southern trench wall. The upper part of the channels is made up by unit U10. The lowest part of U10 is dark and rich in organics and made up of sandy clays. It changes upwards into massive grey and brownish clays with a minor gravel component. It is not possible to draw a sharp boundary between the dark, sandy clays and the greyish–brownish clays with gravel because the transition is gradual. However, these deposits must be mapped as one single unit because they clearly cover all the other fine and coarse units in the trench and because the internal changes are so gradual. We found white speckles at the base of U10 between 7 and 9 m distance (N wall) and between 11 and 12 m distance in the southern trench wall. Unit U10 is overlain by the recent soil (unit U11).
We interpret most of the units in the northern trench as undisturbed fluvial
sediments. Between 8 and 9 m distance, however, we observe several
features that point to tectonically induced deformation. Units U4 and U6
show vertical terminations at their eastern ends, where they are in contact
with U5. A charcoal sample dates U5 to 2140–1987 cal BP. The base of U5
fills the space between larger blocks (Fig. 11c). It remains unclear whether
this resembles a fissure fill because of the lack of horizontally layered,
fine-grained material here. Two bulk sediment samples from these lowest
parts of U5 returned radiocarbon ages of 2645–2489 and 1528–1376 cal BP, respectively. Next to the base of U5 we encountered vertically
aligned pebbles (Fig. 11c). This is the only occasion of aligned pebbles in
the entire trench. Right above these pebbles sits a large (
Results from the Idrija Fault trench at Srednja Kanomlja.
In the southern trench wall, only a narrow zone shows evidence for tectonic deformation between 11 and 12 m distance. Similar to the north wall, we observe vertical terminations of units U4 and U6. The most striking feature is the organic-rich, dark-grey U9. This unit is funnel-shaped and extends from the base of U10 about 1 m downwards. It is in vertical contact with U4 and U6, and it cuts through U5 and U8 (Fig. 11d). We interpret U9 as a fissure fill, with the filling material sourced from the base of U10. Unit U10 is the only one in the trench that also yields this dark layer rich in organics. This indicates that the fissure was filled from above and later became covered by the thick clays that make up the top part of unit U10.
If the fissure were instead a sand dyke that propagated upwards due to seismic shaking as opposed to surface rupture, we should see the source material of the dark fissure fill at its base, which is not the case. Furthermore, there is no evidence for liquefaction or soft sediment deformation structures elsewhere in the trench. These features may include flame structures, warped layers, mushroom structures, pseudonodules, broken layers, ball-and-pillow structures, etc., but none of these were observed here (also see the high-resolution photomosaics in the Supplement). For liquefaction to occur, a sequence of liquefiable, water-saturated layers is needed, but in our case the base of the trench exhibited coarse high-energy deposits. Our geophysics data hint at a rather thin sediment layer above fractured bedrock only. A similar observation was reported by Bavec et al. (2013) from their nearby trench site. We excavated the fissure U9 further into the trench wall to check if it may be due to a large root, but we found that the feature is elongated and not round as would be typical for roots. A bulk sediment sample from the fissure fill returned a radiocarbon age of 2378–2306 cal BP. A charcoal fragment from U5 was dated to 2067–1924 cal BP. At the base of U10 we collected a charcoal that gave an age of 2440–2315 cal BP.
Summary of all dated samples. Probability was calculated after Bronk Ramsey (2009). Calibration was done with INTCAL13 (Reimer et al., 2013).
Paleoseismological trenching at the Predjama Fault provided evidence that
could be interpreted as Holocene activity because the weathered units are
deformed. Thus, the deformation can be interpreted as post-dating the
formation of the base of U5. Alternatively, slope processes and pedogenesis
could have led to the features observed in the trench. Combined with
geomorphological data and geophysical prospection two main observations
support the view in favour of a recent surface-rupturing seismic event: (i) the uphill-facing scarp and (ii) the folded and faulted units in the trench.
The detectable break in slope is rather short (
In addition to the morphological evidence, the trench (Fig. 8) exhibits
features that could be interpreted as fault-perpendicular shortening. In the
northern trench wall, we observe a vertical step of
The observed deformation could also be explained by shallow-seated mass movements or soil creep. Pedogenic development above the weathered bedrock units could have contributed to the apparent deformation, perhaps aided by oblique run-off processes. However, there is no geomorphological observation that would support this view. The geophysics show that there is a fault zone beneath the trench site, although this does not necessarily mean that it caused the deformation in the trench. Although we cannot pin down the exact fault trace, the sharp breaks in unit U4 point to rapid shortening. We argue that there are better arguments in favour of tectonic deformation and that the fault has had at least one recent surface-rupturing earthquake. The lack of clearly identifiable, thin sedimentary layers makes it impossible to decide whether only one event or several earthquakes were recorded in the trench.
In this section we first consider the tectonic interpretation of the
trenching results. In the northern trench wall, all three radiocarbon
samples are in stratigraphic order (Fig. 8, Table 1). The stratigraphically
lowest sample from U4, sample SLO18_BAN3, was submitted as a
charcoal, but in the lab, it turned out that the sample size was very small
and that only the surrounding organic sediment could be treated. This
sample, however, gave an age of 8406–8311 ka cal BP (Table 1) and dates
the start of pedogenesis of the weathered bedrock. Bulk sample
SLO18_BAN2 is located just a few centimetres above U4 and
returned a slightly younger age of 7795–7669 cal BP. Taken together, these
two samples indicate that the deformation happened after
In the southern trench wall, a bulk sample from unit U5 close to the deformation zone gave an age of 12 976–12 727 cal BP (SLO18_BAN6). The stratigraphically higher sample SLO18_BAN7 (charcoal) is much younger with an age of 800–694 cal BP. While samples SLO18_BAN2 and SLO18_BAN3 are consistent, sample SLO18_BAN6 is much older than those two but should give a similar age. This can either be explained by the inherent large uncertainties related to the dating of bulk samples (e.g. Wang et al., 1996; Howarth et al., 2018; Langridge et al., 2020) or by the (unknown) lateral component of fault motion that juxtaposes units of different ages. Likewise, the two charcoal samples SLO18_BAN4 and SLO18_BAN7 are 3 kyr apart, although they were expected to give similar ages. One explanation is that the older sample has a residence time issue; that is, the charcoal could stem from an old tree, and it could have rested somewhere upslope before deposition in the present-day location (see discussion in Zachariasen et al., 2006). Another explanation is again the unknown lateral component of fault motion. Post-glacial pedogenic processes might be responsible for the widely scattered bulk radiocarbon ages, but this does not apply to the dated charcoals. The most conservative approach for bracketing the age of the observed deformation is to use the oldest and the youngest samples, which would place the earthquake(s) between 13–0.7 ka. However, the good agreement of the samples in the northern trench wall leads us to speculate that the deformation actually happened after 8.4 ka. The young age of sample SLO18_BAN7 could explain why the scarp is still preserved.
In the non-tectonic scenario, the deformation could have happened in many phases since, or even partly prior to, 13 ka.
Our ERT data show that the strong vertical resistivity contrasts are situated along-strike of the mapped Idrija Fault trace, and we found deformation structures at this very location in the paleoseismological trench. This leads us to conclude that we rightly identified the recently active fault trace. Four main observations from the trench are evidence for earthquake faulting in the Holocene: (i) the filled fissure, (ii) the vertical terminations of fluvial sediments, (iii) vertically aligned pebbles, and (iv) a large, fractured clast. These four features were all found in a narrow zone that aligns with the mapped fault trace.
The fissure in the southern trench wall cuts through units U5 and U8. Units U4 and U6 terminate at the fissure. This geometry cannot be explained by sedimentary processes. The termination of layers in the southern trench wall probably results from lateral shifts of sedimentary bodies due to strike-slip motion on the fault. The fill consists of dark organic material that is also present above the fissure at the base of unit U10. Therefore, we interpret that an open crack formed due to faulting in a strong earthquake and that the material present at the surface fell in and filled the crack relatively quickly. We consider it unlikely that the fissure was caused by lateral spread or a failure of the riverbanks, perhaps due to seismic shaking because the fissure is only localised and lacks a sliding horizon at its base. Also, no indications of liquefaction were observed. The trench lacks any evidence for typical secondary soft sediment deformation structures such as flame structures, warped layers, mushroom structures, pseudonodules, broken layers, or ball-and-pillow structures. Recent examples of lateral spread show extensive cracks with large offsets that occur due to the shaking and often also in combination with liquefaction (e.g. Papathanassiou and Pavlides, 2007; Hayes et al., 2010; Cubrinovski et al., 2012). Instead, the fissure that we observe could resemble en echelon ruptures in a strike-slip earthquake, as has been frequently observed in recent events (cf. Sylvester, 1988; Treiman et al., 2002; Talebian et al., 2004; Duman et al., 2005; Audemard, 2006; Barrell et al., 2011; Choi et al., 2018; Little et al., 2021). This would explain why we did not observe a similar feature in the northern trench wall.
In the northern trench wall, we observed not only the vertically terminated layers but also aligned pebbles and a large, fractured clast. Aligned pebbles are described from many faults cutting through sediments world-wide (e.g. Hooyer and Iverson, 2000; Vanneste and Verbeeck, 2001; Sapkota et al., 2013; Patyniak et al., 2017; Zabcı et al., 2017). However, they are sometimes also related to liquefaction (e.g. McNeill et al., 2005), for which we do not find evidence. Although we found aligned pebbles only in one place in our trench, their location fits perfectly with the other indicators of active faulting. Right above these aligned pebbles we found the only ruptured clast of the trench. Such ruptured clasts in a fault zone surrounded by intact clasts have been described from faults around the world (e.g. Atwater, 1992; Radjai et al., 1998; Alfaro et al., 2001; Thakur and Pandey, 2004; Silva et al., 2009; Agosta et al., 2012; Tokarski and Strzelecki, 2020; Tokarski et al., 2020). If the broken clast were due to gelifraction, we would expect to find many fractured rocks and not only one right in the fault zone (see Bertran et al., 2020). The fact that only one clast broke indicates that the fractures were not caused by the regional stress regime (cf. Ramsay, 1964; Eidelman and Reches, 1992). Similarly, it is unlikely that the river transported only one clast with cemented voids that later dissolved at the trench site. We conclude that all the observations described above prove a tectonic origin of the observed features.
In the northern trench wall, we obtained two radiocarbon ages from bulk organic material below the fractured clast (Fig. 7). Sample SLO18_SK8 gave an age of 1528–1376 cal BP; sample SLO18_SK9 returned an age of 2645–2489 cal BP. These two ages differ by at least 1 kyr, which we attribute to the inherent uncertainties of dating bulk material. Charcoal sample SLO18_SK10 from unit U5 dates to 2140–1987 cal BP. This is slightly younger than SLO18_SK9, which is in line with its stratigraphic position. Thus, sample SKO18_SK8 likely underestimates the age of the unit below the fractured clast. A charcoal sample from U10 right above U5, SLO18_SK11, is about 1.6 kyr younger (492–315 cal BP) than U5, which is again in stratigraphic order. However, another sample from the base of unit U10 gives an age of 2307–2056 cal BP (sample SLO18_SK13). This rather old age could be explained by considering that unit U10 consists of washed-in material, which could include older charcoal fragments with a complex history. However, we consider the young age of sample SLO18_SK11 to be an outlier because it is significantly younger than all other samples. From the northern trench wall, we can conservatively deduce that the deformation occurred after the deposition of units U1–U6, that is, after 2645 cal BP (U5).
We obtained three samples from the southern trench wall. Sample
SLO18_SK2 is from a charcoal from unit U5 and returned an age
of 2067–1924 cal BP. This fits perfectly well with the age of U5 in the
northern trench wall. Unfortunately, only bulk organic material could be
sampled from the fissure fill. Sample SLO18_SK14 gave an age
of 2378–2306 cal BP. This is older than unit U5, which was cut by the
fissure. We attribute this discrepancy to the uncertainties related to
All samples with the exception of young charcoal sample SLO18_SK11 cluster in a period between ca. 1.5–2.5 ka. We interpret this cluster as the time span in which most of the sedimentary units were emplaced. The younger age of sample SLO18_SK11 is most likely an outlier.
Taking the results from both trench walls together leads us to conclude that the observed deformation can be bracketed to 2645–2306 cal BP: the oldest depositional age in the affected units is 2645 cal BP from the material of unit U1; the fissure fill is not younger than 2306 cal BP as indicated by samples SLO18_SK6 and SLO18_SK14. This time span excludes the 1511 Idrija earthquake as the causative event and contradicts the results of Bavec et al. (2013), who trenched the fault close to our site (Figs. 9, 10) and found evidence for deformation dating to about 900–360 cal BP.
Holocene surface-rupturing earthquakes on the NW–SE-trending faults in the
area have been proven for the Colle Villano Thrust on the mountain front in
Italy (Falcucci et al., 2018), for the Predjama Fault (this study), and for
the Idrija Fault (Bavec et al., 2013; this study). Historical earthquakes on
the Idrija Fault and in the Ljubljana Basin exceeded
A simple thought experiment can be used to estimate an upper bound of the
slip rates in the DFS. The GPS velocity of Adria with respect to stable
Eurasia is 3 mm/a in a northerly direction (Fig. 1; Weber et al., 2010; Metois et al.,
2015). If the entire northward motion of Adria were accommodated by the
right-lateral motion on the DFS striking
Simple maximum slip rate estimation for the DFS in western
Slovenia. A northward velocity of Adria with respect to Europe of 3 mm/a
(Weber et al., 2010) would lead to 4.7 mm/a of right-lateral strike slip on
the Dinaric Fault System if this were the only structure to accommodating
the deformation. Hence, the combined slip rates of the DFS cannot exceed 4.7 mm/a. Atanackov et al. (2021) estimate a combined right-lateral slip rate of
Vičič et al. (2019) reported earthquake swarms on the NW–SE-striking
strike-slip faults in Slovenia. On the southern Predjama Fault, more than
3000 earthquakes occurred between late 2009 and early 2011. The strongest
event was
From a seismic hazard perspective, it must be assumed that all of the large NW–SE-trending strike-slip faults could host surface-rupturing earthquakes, although recurrence intervals on the individual faults are likely long. Thus, instrumental seismicity and even historical data may not adequately depict the actual hazard.
With a combination of geomorphological investigations, near-surface
geophysical surveys, and paleoseismological trenching we show that
surface-rupturing earthquakes happened on the Predjama and Idrija faults in
western Slovenia in the Holocene. At least one earthquake on the Predjama
Fault resulted in a vertical displacement of
SRTM data are distributed by the Land Processes Distributed Active Archive
Center (LPDAAC), located at USGS/EROS, Sioux Falls, SD,
The supplement related to this article is available online at:
CG, KR and KU designed the study. JW and CG undertook the remote sensing research. CG led the field investigations. SA, NS, JW, BV, PJR, MV, and CG ran the field campaign. SA and NS analysed the geophysics results. CG compiled the data and wrote the initial draft of the paper. All authors reviewed and edited first and final drafts.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This article is part of the special issue “New insights into the tectonic evolution of the Alps and the adjacent orogens”. It is not associated with a conference.
This study was undertaken in the framework of SPP 2017 – Mountain Building Processes in 4D. We thank our field crew Alexander Krämer, Andrea Viscolani, Hamid Sana, and Wahid Abbas for their support. This work benefitted from discussions with Jure Atanackov, Jernej Jež, and Tomaž Fabec, who also provided a 19th-century cadaster plan from the Banjšice site. Elena Makorič (Pokrajinski arhiv v Novi Gorici) is thanked for finding the historical photographs from World War I. We thank the reviewers for their very detailed, thorough, and constructive comments, which helped us to improve our paper.
This research has been supported by the Deutsche Forschungsgemeinschaft (grant nos. GR 4371/1-1 and GR 4371/3-1) and the Javna Agencija za Raziskovalno Dejavnost RS (grant no. P1–0011).
This paper was edited by Christian Sue and reviewed by Riccardo Vassallo and Juan M. Insua-Arevalo.