La zona di faglia Ravne è situata in un area di interazione fra due sistemi regionali di faglie con differente cinematica, entrambi collegati alla convergenza fra Adria e Eurasia: le faglie dinariche orientate NW-SE e le faglie del Sud-alpino orientate E-W. L’analisi di dati di geologia strutturale e di due sequenze sismiche recenti che hanno colpito l’area, ci permette di proporre un modello sismotettonico per la faglia di Ravne, che è stata interessata da diverse fasi tettoniche. La geometria originale e la storia evolutiva della zona di faglia svolgono un ruolo cruciale nella distribuzione recente dell’attività sismica e del potenziale sismogenetico dell’intera struttura. Infatti, la configurazione attuale della faglia Ravne, caratterizzata da fagliazione trascorrente su piani ad alto angolo a profondità crostali, è il risultato dell’iniziale geometria di un thrust orientato NW-SE e avente immersione verso NE, e della sua interazione con i piani di thrust diretti essenzialmente E-W. Partendo dai dati raccolti e tenendo in considerazione sia il quadro geodinamico che le relazioni empiriche, proponiamo tre possibili scenari con relativi potenziali sismogenetici per la possibile futura attività della faglia di Ravne

Burrato, P.

Kastelic, V.

Vrabec, M.

Earth-prints Repository

Rendiconti online Soc. Geol. It., Vol. 5 (2009), 108-110, 2ff. 
RIASSUNTO 
L’influenza delle geometrie ereditate e della storia della faglia sulla 
segmentazione e il potenziale sismogenetico dei sistemi trascorrenti in 
Slovenia nord-occidentale: la Faglia di Ravne. 
La zona di faglia Ravne è situata in un area di interazione fra due sistemi 
regionali di faglie con differente cinematica, entrambi collegati alla 
convergenza fra Adria e Eurasia: le faglie dinariche orientate NW-SE e le 
faglie del Sud-alpino orientate E-W. 
L’analisi di dati di geologia strutturale e di due sequenze sismiche recenti 
che hanno colpito l’area, ci permette di proporre un modello sismotettonico per 
la faglia di Ravne, che è stata interessata da diverse fasi tettoniche. La 
geometria originale e la storia evolutiva della zona di faglia svolgono un ruolo 
cruciale nella distribuzione recente dell’attività sismica e del potenziale 
sismogenetico dell’intera struttura. Infatti, la configurazione attuale della faglia 
Ravne, caratterizzata da fagliazione trascorrente su piani ad alto angolo a 
profondità crostali, è il risultato dell’iniziale geometria di un thrust orientato 
NW-SE e avente immersione verso NE, e della sua interazione con i piani di 
thrust diretti essenzialmente E-W. Partendo dai dati raccolti e tenendo in 
considerazione sia il quadro geodinamico che le relazioni empiriche, 
proponiamo tre possibili scenari con relativi potenziali sismogenetici per la 
possibile futura attività della faglia di Ravne.  
 
Key words: fault geometry, fault growth and linkage processes, 
reactivation, seismicity.  
 
INTRODUCTION 
In this work we study the influence of the inherited 
structural pattern and the fault history on the segmentation and 
seismogenic potential of the Ravne Fault, a fault zone located 
in the eastern Southern Alps. We also present a model of 
development of the Ravne fault, as a fault zone, which has 
undergone multiple tectonic phases. The emphasis of this work 
is on the fault's seismic activity and its related seismogenic 
potential. For this purpose we analyzed surficial geological and 
structural data to study the fault geometry and the 
microkinematic indicators present within the fault zone. We 
also carried out a detailed study of the spatial, temporal and 
kinematic characteristics of the 1998 and 2004 seismic 
sequences. 
The Ravne Fault is a NW-SE trending, right-lateral strike-
slip fault that lies in the western Julian Alps of NW Slovenia. 
In this area the NW-SE oriented faults, typical of the Dinaric 
domain, meet and interact with the E-W oriented faults of the 
South Alpine domain (e.g. CASTELLARIN & CANTELLI, 2000; 
DOGLIONI, 1987; PAMI et alii, 2002). The activity of both 
structural trends is controlled by the Adria-Europe relative 
convergence, that in the study area results in about 2 mm/a of 
N-S oriented shortening (WEBER et alii, 2006, GRENERCZY et 
alii, 2005). This deformation is seismically released by thrust 
faulting earthquakes along the Eastern Southern Alps fronts, 
and by right-lateral strike-slip faulting along the Dinaric system 
(e.g. BURRATO et alii, 2008; DISS WORKING GROUP, 2007). 
STRUCTURAL SETTING 
The morphological expression of the Ravne Fault can be 
observed in the field and on satellite and digital topographic 
imagery over a distance of approximately 30 km in a NW–SE 
direction (Fig. 1) between Kal Koritnica in the NW and Cerkno 
at the SE end (KASTELIC et alii, 2008). The fault trace is 
discontinuous, interrupted by local transtensional basins 
arranged in a right stepping manner. The fault zone consists of 
individual NE-dipping fault planes with different dip angles. 
The shallower dipping planes, with values between 40°-60°, 
usually represent the contact between Cretaceous flysh rocks in 
Influence of inherited geometry and fault history on the seismogenic 
activity and potential of strike-slip fault systems in NW Slovenia: 
the case study of the Ravne Fault. 
VANJA KASTELIC (*, °), PIERFRANCESCO BURRATO (*) & MARKO VRABEC (°) 
 
Fig. 1 – Map of  the main faults in the NW Slovenia, with the emphasis on 
the geometry of Ravne Fault. In the upper left corner a regional tectonic 
framework with depicted main structures is shown; blue rectangle showing 
area of investigations of this study. 
_________________________ 
 
(*) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Sismologia e 
Tettonofisica, Roma, Italy 
(°) Department of Geology, Faculty of Natural Sciences and Engineering, 
University of Ljubljana, Askerceva 12, 1000 Ljubljana, Slovenia 
109INFLUENCE OF INHERITED GEOMETRY AND FAULT HISTORY ON THE SEISMOGENIC ACTIVITY AND POTENTIAL OF STRIKE-SLIP FAULT 
SYSTEMS IN NW SLOVENIA: THE CASE STUDY OF THE RAVNE FAULT 
the foot-wall and Triassic limestones in the hanging-wall. In 
places, where such planes are present within the limestones, 
shear zones are also present. The geometry of the planes and 
the shear bands indicate uplift of the hanging-wall blocks. Well 
developed cataclastic zones up to a few centimetres wide are 
developed adjacent to some planes, whereas in other places, 
sheared surfaces of polished tectonic breccia occur in the 
innermost fault zone. Fault-related deformation on moderatly 
dipping fault planes is not confined only to the steep trace of 
the Ravne Fault, but is rather well recorded also on fault planes 
of the foot-wall block. Where preserved, microkinematic 
indicators on these planes prove dip slip displacements. Planes 
with steeper dips reaching values of more then 75 degrees in 
most cases lack of microkinematic indicators. Throughout the 
outcrops along the trace of the fault zone, only individual 
subvertical planes with visible striation marks and horizontal 
grooves showing strike-slip movement are present. In places, 
also (sub)vertical fractures cutting through the more shallowly 
dipping thrust planes can be observed at the surface, indicating 
the propagation towards the surface of the steep subvertical  
planes forming at depth (KASTELIC et alii, 2008). 
Recent seismicity recorded in the Ravne Fault zone shows 
that ongoing seismic activity is confined to shallow crustal 
levels and does not exceed depths beyond 10 km. The 1998 
(MW=5.7) and 2004 (MW=5.2) earthquake sequences were 
characterized by focal mecanisms ranging from dextral strike-
slip to almost pure reverse faulting (KASTELIC et alii, 2006; 
Fig. 2). The main shocks of both earthquakes delineate 
prevailing dextral strike-slip movements on steep SW dipping 
planes. The 1998 earthquake cluster follows a NW–SE trend, 
with the main shock located approximately at 4.5 km from the 
fault’s NW tip, whereas the SE end of the cluster reaches the 
Tolminka Springs basin. Focal mechanism solutions for 
stronger aftershock events indicate thrust or reverse 
movements on N dipping fault planes on both ends of the 
cluster, all within the first 7 km of the crust (ZUPANI et alii, 
2001). The earthquake did not cause surface faulting, but it did 
(re)activate a 12 km long and 7 km wide rectangular fault plane 
with an average slip of 18 cm (BAJC et alii, 2001). The 2004 
earthquake was weaker and also did not cause surface faulting. 
The epicentre of the main shock was located about 1 km from 
the 1998 main shock location. This event and its aftershock 
sequence were also confined to shallow crustal levels. The 
spatial distribution of aftershocks is not as homogeneous in 
direction as for the 1998 cluster. Two distinctive branches of 
aftershocks can be observed for the 2004 sequence: one line 
continues in a NW–SE direction rupturing an area further NW 
of the 1998 cluster, while the other line branches off in an E–W 
direction (Fig. 2). The computed focal mechanisms for the 
stronger aftershocks (KASTELIC et alii, 2006) show similar 
kinematics as the main shock, the difference being a larger dip-
slip component for some aftershocks, indicating oblique 
dextral-reverse movements on steep to moderate-steep NW–SE 
oriented fault planes and oblique reverse-dextral and reverse 
movements on E–W oriented, moderate-steep faults. 
 
DISCUSSION 
Knowing and understanding the geometry, time history and 
recent seismic destribution is essential in providing a reliable 
seismotectonic model and seismogenic potential assesment of a 
fault that has undergone multiple tectonic phases. In the case of 
Ravne Fault it was proven that the original geometry of the 
fault plane connected to SW verging thrusting phase plays an 
important role also in the recent seismic behaviour of the fault 
zone (KASTELIC et alii, 2008). During both the recent seismic 
sequences recorded in the Ravne Fault zone the main shocks 
occurred on SW-dipping subvertical fault planes, while the 
aftershocks were distribued on NE to N dipping fault planes 
related to the older thrusting phase. Of important significance 
is also the presence of the E-W thrusting phases within the 
fault zone, that caused disintegration and displacements of the 
NW-SE oriented thrust zone, and formation of local 
geometrical barriers that are governing the seismic distribution 
acting as segment boundaries. Examples of such behaviour can 
be found at both tips of the Ravne Fault. To the SE, the 
Tolminka Springs basin, an E-W trending thrust system meets 
the NW-SE oriented fault zone causing its displacement and 
formation of a transtensional basin that acted as a stopping 
barrier for the SE propagation of the 1998 earthquake 
sequence. From this location further to the SE the fault trace is 
getting more and more discontinous and slowly fading away, 
although some reports show its continuation in the SE direction 
for further 15 km (GRAD & FERJANI, 1968). At the NW tip 
of the fault zone, the surface expression of the fault trace is lost 
in the Bovec basin under the Quaternary sediment infill, while 
in the NW slopes above the basin no fault trace can be 
observed. Close to the location of NW-SE oriented fault trace 
entering the Bovec basin is also the location of the intersection 
with the E-W oriented fault plane that was reactivated during 
the the 2004 earthquake sequence. 
Given such structural setting three different seismotectonic 
scenarios can be envisaged: 
 
Fig. 2 – Spatial distribution of aftershock sequences of the 1998 and 2004 
Ravne Fault earthquake events. Grey triangles depict the 1998 MW= 5.6, 
black circles the 2004 MW= 5.2 earthquake seismic sequences, respectively. 
The grey and light-greyish stars represent the 1998 and 2004 main shock 
locations, respectively. Grey-coloured focal mechanism belong to the 1998 
(ZUPANI et alii, 2001), the black solutions to the 2004 events (KASTELIC et 
alii, 2006). Red boxes represent the sources for both main shocks of the two 
seismic sequences, with the geometries obtained from their  focal  
mechanisms and dimensions in agreement with geometrical empirical 
relationships (WELLS & COPPERSMITH, 1994). 
110 V. KASTELIC ET ALII 
 
a) if the existence of the SE continuation of the Ravne 
Fault zone with a length of about 15 km is established, the 
coseismic rupture of this new segment would result in an 
earthquake of magnitude similar to that of the 1998 event; 
b) with the same assumption, the rupture of the entire 
Ravne Fault zone by breaching of the Tolminka Springs basin 
segment boundary, would produce a stronger earthquake. 
Taking into consideration the length of the fault trace and 
keeping the geometry and kinematics of the 1998 source, a 
Mw=6.1 earthquake is proposed by empirical relationships 
(WELLS & COPPERSMITH, 1994); 
c) the coseismic rupture of the E-W oriented thrust plane 
already activated during the 2004 earthquake sequence, would 
result in a Mw=5.5 earthquake. 
The model of segmentation applied in this case study can 
be applied also to the other fault systems of the region, which 
share the same geometry and the same structural development 
as the Ravne Fault. One of these fault systems is the NW-SE 
oriented Idrija Fault, responsible for the 1511 M=6.9 
earthquake (FITZKO et alii, 2005) that is the strongest 
earthquake recorded in the region. Given the continuation of 
the fault trace to the SE, a higher magnitude earthquake with 
strike-slip motion is predicted by our model that is in 
accordance with the data of known seismicity. By taking into 
consideration all the data known we can improve the 
understanding of recent and future seisimic processes of a 
particular region and fault systems and therefore contribute to 
more realistic and reliable seismogenic potential assesments. 
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Influence of inherited geometry and fault history on the seismogenic activity and potential of strike-slip fault systems in NW Slovenia: the case study of the Ravne Fault

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Influence of inherited geometry and fault history on the seismogenic activity and potential of strike-slip fault systems in NW Slovenia: the case study of the Ravne Fault

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