Vanja Kastelic and Michele M. C. Carafa (INGV, L’Aquila, Italy) recently published an article in the Bollettino di Geofisica Teorica e Applicata (an international journal of Earth sciences) entitled “Earthquake rates inferred from active faults and geodynamics: the case of the External Dinarides.” This article covers the area affected by the earthquake of Ml 4.7 (Mw 4.6) occurred on April 22, 2014.
The same authors also wrote a brief seismotectonic report dealing with such an earthquake. They share the report with us under the Creative Commons Attribution-ShareAlike 3.0 Unported License.
A quick seismotectonic report for the 22 April 2014 (Mw=4.6) earthquake in SW Slovenia
Vanja Kastelic1 and Michele M. C. Carafa1
1 INGV – Istituto Nazionale di Geofisica e Vulcanologia, L’Aquila.
This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.
The 22nd April 2014 earthquake occurred in the SW Slovenia, an area corresponding to External Dinarides, a structural unit characterized by geologically long lived NW-SE oriented structures, active in the current stress regime as dextral strike-slip faults (Kastelic et al., 2008; Kastelic and Carafa, 2012). Faults belonging to the External Dinarides unit that have a clear morphological imprint in the terrain are mostly Idrija, Rasa and Ravne faults, all characterized by low slip rates in the range of 0.10 to 0.35 mm/yr (Kastelic and Carafa, 2012) but associated to strong and medium-sized earthquakes in historic and recent times (Ribarič, 1982; Bajc et al., 2001; Fitzko et al., 2005; Kastelic et al., 2008; DISS WG 2010; Stucchi et al., 2012).
Figure 1: Western and Central Slovenia with surrounding areas; the broader region of the Pivka seismogenic fault, the hosting fault of the April 22nd 2014 earthquake. Orange lines represent upper edges of seismogenic faults in the region capable of hosting M≥5.5 earthquakes (taken from EDSF; Basili et al., 2013). Multicolour stars and triangles represent earthquake epicenters as reported by different seismological institutes and agencies (blue – ARSO; green – INGV; black – EMSC), while the blue triangles stand for early aftershock locations (ARSO). Red lines stand for maximum compressive stress orientations (Heidbach et al., 2008), while the thin black lines along the fault’s upper edges represent interpolated maximum compressive stress axis orientations (Carafa and Barba, 2013; Carafa et al., 2014). Inset in the lower right corner shows tectonic situation in the epicentral area.
The source of the April 22nd 2014 earthquake is Pivka fault (Figure 1), a fault that respect to above listed faults shows lesser morphological imprint, shorter along strike distance that can be followed at surface, but was still recognized as a seismogenic active fault, capable of hosting M5.5 earthquakes (Basili et al., 2013). Its geometric and kinematic parameters were investigated in the past years,and they are in a good agreement with focal mechanisms calculated from seismological records of the earthquake (for greater details the reader is referred to Kastelic and Carafa, 2012 as an example of such studies). In 2008,the area to the NW of Pivka town was interested from a two months seismic activity with several Ml ≤3.0 shocks. Focal mechanism solutions for the stronger event of that sequence (Ložar Stopar et al., 2009) is in good agreement with April 22nd 2014 earthquake and represent its NW continuation.
Figure 2: Number of expected earthquakes above the threshold magnitude of M>5.66 for the broader Pivka fault region (Carafa and Kastelic, 2014). The maximum (1.59E-20) equals to 0.5 earthquakes per 1000 km2 every one thousand years. The long-term earthquake number derives from a geodynamic finite-element model (Kastelic and Carafa, 2012) and a model of active faults (Basili et al., 2013). It can be seen that the number of expected earthquakes is higher for the area around Pivka fault (like it is also true for areas around other active faults in the region) respect to areas more to the south that are not interested by active seismogenic faults. Two focal mechanisms correspond to different solutions for the April 22nd 2014 earthquake (QRCMT solution available at http://autorcmt.bo.ingv.it/QRCMT-on-line/E1404220858A.html while the USGS/SLU Moment Tensor Solution is available at http://www.eas.slu.edu/eqc/eqc_mt/MECH.EU/20140422085827/index.html). Both solutions confirm dextral strike-slip movements along a NW-SE oriented steep fault plane. The black line to the north and north-east of the Pivka region corresponds to a limit of the study area in the Carafa and Kastelic 2014 study; earthquake rates on the outside are not realistic solutions and should not be considered as such.
The strong correlation between the Pivka faults geometric parameters and orientation of the most compressive stress axis orientation has been establiehed, favouring the fault’s dextral strike-slip activity in the present stress regime (Carafa et al., 2014). Moreover, calculations of expected earthquake rates above certain magnitude threshold for the area of External Dinarides has revealed higher expected seismic activity of the Pivka fault respect to surrounding areas, especially the ones to the south, not interested by active faults (Carafa and Kastelic, 2014). All these indications, well established in the scientific literature, testify that the 22nd April 2014 earthquake kinematics and related long-term activity of the Pivka fault were far from being unexpected.
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