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Seismotectonic Model of the Kraljevo3 November 2010 Mw 5.4 EarthquakeSequenceby Sanja Knezevic Antonijevic, Pierre Arroucau, and Gordana Vlahovic

Online Material: Additional Kraljevo sequence data, details.

INTRODUCTION

AnMw 5.4 earthquake occurred on 3 November 2010 (Fig. 1)near the city of Kraljevo, Serbia, the population of which ismore than 100,000 people. By the end of March 2011, theearthquake had been followed by a sequence of more than650 aftershocks of magnitude greater than 1.0. Despite themoderate magnitude of the event, two people were killed,many others were injured, and the total damage to the city hasbeen evaluated as more than 100 million EUR (UnitedNations, Kraljevo Earthquakes Report, 2010). Liquefactionand rockslides occurred in several places (Serbian SeismologicalSurvey (SSS) Report, 2011). The earthquake also causedchanges in groundwater circulation: some existing wells havetotally dried out and new geothermal springs have appeared.

The Kraljevo 3 November 2010 earthquake was recordedby seismological stations worldwide. Although different seis-mological agencies show consistency in determination of mag-nitude (Mw 5.3 or 5.4), proposed locations for the mainshockvary greatly. In particular, proposed depths vary from 2 to30 km. In this article, we jointly determine the location of themainshock and aftershocks that occurred between 3 Novemberand 31 December 2010, along with a new P- and S-wave 1Dvelocity model for that area. Focal mechanism solutions areobtained for selected events using both local and regional data.The relocated events and associated focal mechanism solutionsare further used to propose a seismotectonic model for thatearthquake sequence.

GEODYNAMIC SETTING

The Kraljevo earthquake sequence occurred on the BalkanPeninsula, in southeastern Europe. The present-day geodynam-ics of the region is controlled by the relative motions of theEurasian and African plates (Dilek, 2010). The collision be-tween those two plates, driven by the northward drift of Africatoward the Eurasian plate, controlled the Neogene evolutionof the southern part of Central Europe. The formation of

the Alps, Dinarides, Pannonian basin, and Carpathian belt(Fig. 1) is related to the Alpine orogeny that started in the lateCretaceous. The collision caused the subduction of the Adri-atic microplate under the Alps, Apennines, and Dinarides(Panza et al., 1982, 2003); these three belts are currently active,although at different rates (D’Agostino et al., 2005; Devotiet al., 2008).

The area of Kraljevo where the earthquake sequenceoccurred belongs to the southern rim of the Serbian part of thePannonian basin and represents the border between the Peri-Pannonian realm and the Dinarides (Marović et al., 2002).During Neogene uplift of the Dinarides several depressionsformed, with one of the largest being the southeast–northwest-trending Čačak–Kraljevo basin (Fig. 2). This basinwas formed by activation of several deep faults, the most sig-nificant of which being theTrans-Serbian fault at the southernrim of the Čačak–Kraljevo basin (Fig. 2; Marović et al., 2002).

SEISMICITY OF THE AREA

The study area is characterized by moderate seismicity (Mar-ović et al., 2002). Earthquake records for the region (Catalog ofEarthquakes, UNDP/UNESCO Survey of the Seismicity ofthe Balkan Region, 1974) indicate earthquake depths rangingfrom 1.5 to 40 km and magnitudes as high as 6. Earthquakeswith ML ≥ 3 that have occurred in the area since 1900 areshown in Figure 2. Note that the grid pattern present on theearthquake epicenter map most likely reflects the precision lim-its of the catalog. Previous focal mechanism solutions pub-lished by the Serbian Seismological Survey (SSS) and theEuropean Mediterranean Seismological Center (EMSC) revealstrike-slip and reverse movements along faults striking mostlyin east-southeast–west-northwest, east–west, and northeast–southwest directions (Marović et al., 2002).

DATA

The seismological data for the Kraljevo earthquake sequencewere provided by the seismological networks of Serbia, Monte-negro, Croatia, Thessaloniki, Athens, and by the Virtual

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Regional Network (Fig. 1). Two hundred and thirty-nineevents have been successfully extracted and the first P- andS-wave arrival times along with the first-motion polarities,when possible, were manually picked. For the relocation of themainshock, 59 P- and 40 S-wave arrival times were used, with amaximum azimuthal gap in station coverage of 33 degrees anda distance to the closest station of 15 km. The estimated errorfor P-wave picks is 0.02–0.06 s for impulsive signals and isgreater for emergent signals. For larger aftershocks (withML ≥ 3, for which magnitudes were estimated by the SerbianSeismological Survey), the number of P and S arrivals variedfrom 24 to 73 and 10 to 19, respectively, with a maximumazimuthal gap ranging from 28° to 60° and distance to the clos-est station ranging from 14 to 20 km respectively.

VELOCITY MODEL

Previous crustal velocity studies in the area include deep sound-ing profiles (Dragašević and Andrić, 1973/1974) and regionalvelocity models of the Balkans and Albania (Herak and Herak,1995; Ormeni, 2011). However, velocity models for Serbia andthe Kraljevo area have not yet been published. In order tolocate more accurately the Kraljevo earthquake sequence, anew 1D velocity model has been developed in two steps. First,a grid search was used to obtain a set of basic 3-layered velocity

models, which were subsequently used to determine a morecomplex velocity model with the program VELEST.

Grid SearchA grid search was done using the Hypocenter program (Lienertet al., 1986; Lienert and Havskov, 1995) to find a simple 2-layered velocity model. The following parameters were tested:crustal velocity ranging from 5.25 to 5:75 km=s (with an incre-ment value of 0:05 km=s ); mantle velocity from 7.2 to8:3 km=s (with an increment value of 0:1 km=s ); and Mohor-ovičić discontinuity depth from 31 to 42 km (with an incre-ment value of 1 km).

Several hypocenter starting depths were tried (1, 15, and29 km) in order to test the sensitivity of hypocentral locationsto initial depth. The Hypocenter program also allows the userto define the maximum epicentral distance at which stationswill be considered for earthquake location. Several maximumstation distances were tested (250, 350, and 450 km) in orderto see how this parameter affects earthquake distribution.Along with assigning different weights to define pick quality,it is possible to additionally define the minimum station dis-tance at which weighting will start, assigning closer stations ahigher weight. The goal was to find velocities and Moho depthvalues associated with low rms arrival-time residuals as well as alow rms variance, a lower variance reflecting a more stable sol-ution. Grid search results for a starting depth of 1 km, a mini-mum and maximum distance of 50 and 250 km, respectively,for distance weighting, and a VP=V S ratio of 1.72 are shown inFigure 3. The highest rms residual variance is associated with acrustal velocity of 5:25 km=s, for which rms residual values varyfrom 0.74 to 0.9 s. Rms residual variance decreases as crustalvelocity increases from 5.25 to 5:8 km=s. The mean rms resid-ual value is almost 0.8 s for a crustal velocity of 5:25 km=s,decreases with increasing velocity and reaches its minimumof about 0.7 s for a velocity of 5:55 km=s, after which rms val-ues slightly increase with increasing velocity. The relationshipbetween the mean rms value obtained for a particular velocityand standard deviation of rms values obtained for the samevelocity is more apparent in Figure 3b. In this particular test,crustal velocity values from 5.5 to 5:6 km=s show both smallmean rms residuals and small standard deviations.

The same analysis was carried out for mantle velocity(Vm) and Moho depth (ZMOHO) as shown in Figure 3c–e. Inthe case of mantle velocity and Moho depth, values with thesmallest mean rms residuals also have the smallest rms residualstandard deviation. Mantle velocities between 7.6 and 7:9 km=sand Moho depths between 34 and 37 km are associatedwith smallest mean rms residuals and rms residual standarddeviation.

The same procedure was repeated for different startingdepths (1, 15, and 29 km) and different maximum station dis-tances (250, 350, and 450 km). After each test (9 tests in total)earthquakes were relocated using the new velocity model to seehow different parameters affect the earthquake distribution.A range of values for crustal velocity, mantle velocity, andMoho depth obtained from all tests are given in Table 1.

▴ Figure 1. Geodynamics of the Central Mediterranean region:Major plates (Africa, Eurasia, and Anatolia) and their boundaries,important fault systems, and orogenic belts (after Dilek, 2010).Thick black arrows, plate convergence directions; white star,the location of the 3 November 2010 Kraljevo earthquake. Symbols,seismological station networks from which data were used in thisstudy. Virtual Regional Seismic Network includes data from thefollowing networks: Italy, Romania, Croatia, Former YugoslavRepublic of Macedonia, Republic of Serbia, and Albania.

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Results obtained for a 2-layered velocity model were usedto find a 3-layered velocity model with an additional interfaceat 20 km depth, because an interface at this depth is mentionedby Herak and Herak (1995). Following the same procedure,values obtained from the grid search for 3-layered velocitymodels are given in Table 2.

Although results obtained for upper crust velocity (5:45–5:65 km=s), mantle velocity (7:5–7:95 km=s), and Moho depth(38–42 km) are well constrained, results for lower crust veloc-ity (5:6–6:25 km=s) vary greatly, probably due to the fact thatthe majority of earthquakes occurred shallower than 20 kmdepth. The results of grid searches were used in the programVELESTas starting a priorimodels. Because of a discrepancy inresults for Moho depth between the 2-layered and 3-layeredvelocity models, models with the Moho ranging from 33 to42 km depth were further tested with the program VELEST(Kissling et al., 1994).

VELESTProgram VELEST (Kissling et al., 1994) was used to invert fora more complex 1D velocity model. The starting model, as rec-ommended by Kissling et al. (1994), has interfaces at every2 km in the upper crust (in this article, from the surface to20 km depth), and at every 5 km in the lower crust (from20 km depth to the Moho discontinuity). Results from the gridsearches, described in the previous section, were used to con-strain the velocities in the starting model. We tested 20 differ-ent 3-layered starting models obtained from grid searches usingearthquake locations obtained with program HYPOCENTER(Lienert et al., 1986). These 3-layered a priori models areshown in Table 3. They are named as MIN when minimumvelocity values were used (upper crust velocity 5:45 km=s, lowercrust velocity 5:6 km=s, and mantle velocity 7:5 km=s) andMAXwhen maximum velocity values were used (upper veloc-ity 5:65 km=s, lower crust velocity 6:25 km=s, and mantle

▴ Figure 2. Earthquakes with ML ≥ 3 that occurred in the vicinity of Kraljevo, Serbia, from 1901 to 2010. Locations of earthquakes thatoccurred before 1974 are based on Catalog of Earthquakes (1974; UNDP/UNESCO Survey of the Seismicity of the Balkan Region). Theobserved grid pattern reflects the resolution of catalog locations. Locations of recent earthquakes are from the European–MediterraneanSeismological Center. Star, epicenter of the 3 November 2010 Kraljevo earthquake. A year is given next to events with magnitude 6. Solidlines, faults (after Marović et al., 2002), including the deep major Trans-Serbian fault at the southern rim of the Čačak–Kraljevo basin.

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▴ Figure 3. Results of grid search for a starting depth of 1 km, a minimum distance of 50 km and a maximum distance of 250 km fordistance weighting, and a V P= V S ratio of 1.72. (a, b) Obtained rms residual values for crustal velocity (V crust

P ); (c, d) for mantle velocity(Vmantle

P ); and (e, f) for Moho depth (ZMoho). Black dots represent mean rms residual for each velocity/Moho depth.

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velocity 7:95 km=s). In both cases, we tested Moho depthsranging from 33 to 42 km. Ten iterations were found sufficientto achieve convergence for all models.

Figure 4 shows P-wave velocity models obtained by VEL-EST and associated rms residuals. Velocity models presentedwith darker lines are associated with lower rms values. Regard-less of using 5.45 or 5:65 km=s as a starting velocity in theupper crust, the obtained result indicates higher velocities inthe upper crust. The lowest rms (0.711 s) has been obtainedfor the velocity model with 39 kmMoho depth and maximumstarting velocity values for all the layers. This model (Fig. 5)was chosen for relocation of the Kraljevo earthquake sequence.

DOUBLE-DIFFERENCE RELOCATION

The HYPODD program, based on the double-difference algo-rithm (Waldhauser and Ellsworth, 2000), was used to improvethe accuracy of relative hypocenter locations for the Kraljevoaftershock sequence. The 220 earthquakes were first locatedusing HYPOCENTER with the new Kraljevo 1D P-wavevelocity model presented in Figure 5 and fixed 1.72 VP=V Sratio obtained from a Wadati diagram (using Seisan package,Havskov and Ottemoller, 1999). These locations were furtherused for relative locations with HYPODD. A maximum inter-event distance of 10 km was used with a cutoff threshold of 6.Damping values between 0 and 100 were tested along withdifferent S-wave weights from 0 to 1 in order to find whichcombination of damping value and S-wave weight would givethe lowest rms residuals and still give a condition number inthe desired range (between 40 and 80, as recommended byWaldhauser [2001]). Results showed the lowest rms valuefor a damping value of 60 and an S-wave weight of 0.4.

Running HYPODD with the Kraljevo earthquake datasetresulted in the recognition of one cluster based on 88,091P-phase pairs and 56,914 S-phase pairs. A total of 195 earth-quakes (88% of initial sources) were relocated (Fig. 6b). Someshallower events were treated as so-called airquakes (21 events)and not considered as part of the cluster. Four other eventswere too far away from the cluster and therefore were notincluded. The mainshock was located at 43.743° N latitude,20.708° E longitude, 15.4 km depth, with an uncertainty of 2.8,2.2, and 4 km in latitude, longitude, and depth, respectively.The largest aftershock (ML 4.4) occurred on the next day at43.746° N latitude, 20.617° E longitude, 14.7 km depth, orabout 7–8 km west of the mainshock. Comparing the locationspublished by the Serbian Seismological Survey (Fig. 6a) withthe ones obtained in this study (Fig. 6b), a significant change isnoticeable. Original hypocenters by the Serbian SeismologicalSurvey are artificially aligned at every 2 or 3 km depth and themainshock is far away from the aftershock sequence. Relocation

Table 1Results of Grid Search for 2-Layered Velocity Model (See

Text for Explanation)

StartingDepth of

Earthquakes

MaximumStationDistance(km)

V c Range(km=s)

V m Range(km=s)

Z Range(km)

1 km 250 5.50–5.65 7.60–7.90 34–37350 5.50–5.65 7.60–7.80 35–37450 5.50–5.65 7.60–7.80 33–34

15 km 250 5.45–5.60 7.50–7.70 36–37350 5.45–5.60 7.50–7.70 36–38450 5.40–5.60 7.60–7.80 33–34

29 km 250 5.45–5.60 7.50–7.70 36–39350 5.45–5.60 7.50–7.70 36–39450 5.40–5.60 7.50–7.80 33–34

Range of crustal (V c), mantle velocities (Vm) and Mohodepth (Z ) obtained with grid search for 2-layer velocitymodel, for 1, 15, and 29 km starting earthquake depthsand 250, 350, and 450 km maximum station distances.

Table 2Results of Grid Search for 3-Layered Velocity Model (See Text for Explanation)

StartingDepth

MaximumDistance (km)

V c1 Range(km=s)

V c2 Range(km=s)

V m Range(km=s)

Z Range(km)

1 km 250 5.45–5.65 5.95–6.20 7.55–7.85 39–41350 5.45–5.60 5.95–6.25 7.55–7.95 39–41450 5.45–5.60 5.85–6.15 7.85–7.95 38–41

15 km 250 5.45–5.60 5.75–6.15 7.55–7.75 39–42350 5.45–5.65 5.75–6.15 7.55–7.95 39–41450 5.45–5.65 5.95–6.15 7.75–7.95 38–42

29 km 250 5.45–5.65 5.60–6.25 7.50–7.85 40–42350 5.45–5.65 5.60–6.25 7.50–7.85 40–42450 5.45–5.65 5.60–6.25 7.75–7.95 38–41

Range of velocities for the upper (V c1 ) and lower (V c2 ) crust, mantle (Vm) and Moho depth (Z ) obtained with grid search for3-layer velocity model, for 1, 15, and 29 km starting earthquake depths and 250, 350, and 450 km maximum station distances.

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shows that the Kraljevo earthquake occurred at 15.4 km depthand was followed by mostly shallower aftershocks distributedwest of the mainshock along a subvertical plane striking east–west, as evidenced by cross sections AA0 and BB0 in Figure 6b.The focal depths obtained range from about 3 to 16 km depth,with most of the aftershocks occurring between 14 and16 km depth.

Figure 7 presents a closer view of the Kraljevo earthquakeepicentral area, showing the temporal evolution of the se-quence. The 57 earthquakes that occurred within the first24 hours (black dots) extend about 8 km laterally and about10 km vertically west of the mainshock, with six events havingmagnitude ranging from 3 to 3.4. Dark gray dots represent 84aftershocks that occurred during the first week after the main-shock (from 4 to 9 November 2010). Again, the majority ofthe aftershocks, among them the largest aftershock withML 4.4, occurred west of the mainshock. During this period,besides the largest aftershock, four events had a magnituderanging from 3 to 3.9.

Light gray dots represent 49 events that occurred betweenone week and one month after the mainshock (10–30 Novem-ber 2010). In this period only one event with ML > 3 oc-curred. The magnitudes of these events range from 1.5 and

3.1 with depths between 10 and 17 km. White dots representevents that occurred in December 2010. These earthquakes areassociated with magnitudes less than 2.0. They are distributedmore diffusely than the rest of the aftershocks and occur botheast and west of the mainshock. Finally, the entire earthquakesequence has spread about 15 km laterally and 13 km vertically.A closer analysis of the aftershock distribution, furthermore,reveals two main directions. Events with ML ≥ 3 (Fig. 8)clearly delineate an east–west trend along a 7–8 km longsegment located between the mainshock and the largest after-shock, with depths ranging from 13.6 to 15.4 km. In addition,a subcluster of later events (14–27 November) extending10–11 km and aligned in the north-northwest–south-southeast direction can also be observed in the western partof the sequence (Fig. 9).

FOCAL MECHANISM SOLUTIONS

Focal mechanism solutions were determined from first polar-ities using programs FOCMEC (Snoke et al., 1984) andFPFIT (Reasenberg and Oppenheimer, 1985). Figure 8 showsfocal mechanism solutions for all earthquakes with ML ≥ 3.On average, 22 readings for these events were used having 10%of misfit and station distribution ratio of 0.62, indicating arobust solution (Reasenberg and Oppenheimer, 1985). Stationdistribution ratio shows the goodness of spatial distribution ofstations with respect to the event, and also indicates if stationsare located on nodal planes. For robust solutions, station dis-tribution ratio should be above 0.5.

Focal mechanisms for the majority of the events withML ≥ 3, including the mainshock, show strike-slip solutions.The mainshock focal mechanism solution is 274° strike,66° dip, 6° rake and shows consistency with the earthquakesequence distribution (almost vertical fault striking east–west).It is also consistent with focal mechanism solutions publishedby USGS, INGV, the University of Athens (http://www.emsc‑csem.org, last accessed April 2013), the Global CMTProject (www.globalcmt.org/CMTsearch.html, last accessed April2013), and the Montenegro Seismological Observatory, whichalso proposed a strike-slip solution for that event. The majorityof other solutions (Fig. 8) also reveal a strike-slip motion. Theaftershock alignment along an east–west direction gives rea-sons to consider the east–west nodal plane as the most likely,implying a sinistral motion on an east–west-trending fault. Anexception is the event with ID 36, which shows thrust faulting,and events with IDs 37 and 149, which indicate normal fault-ing. These events possibly occurred on secondary faults.

Focal mechanism solutions have also been determined forevents that correspond to the secondary subcluster aligned inthe north-northwest–south-southeast direction (Fig. 9). Theseevents occurred between 14 and 27 November and have mag-nitudes ranging from 1.5 to 3.1. The obtained focal mechanismsolutions vary from thrust to oblique strike-slip, with theexception of one event, which shows a normal fault solution.Because of almost perfect alignment of these earthquakes andfocal mechanism solutions that differ from the ones of the

Table 3Velocity Models Used as a Priori Starting Models in VELEST

Parameter Min MaxUpper crust velocity 5:45 km=s 5:65 km=sLower crust velocity 5:60 km=s 6:25 km=sMantle velocity 7:50 km=s 7:95 km=sMohorovičić discontinuity depth 33–42 km 33–42 km

▴ Figure 4. Velocity models for Pwaves obtained with VELEST forMoho depths ranging from 33 to 42 km. Two different startingvelocities in the upper and lower crust and mantle were usedand marked as MIN and MAX (see Table 3). Obtained rms valuesare classified by colors, ranging from light gray, which indicatesthe highest rms, to black, which indicates the lowest rms. For sim-plicity, selected models are shown.

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east–west cluster, these earthquakes may reveal a secondaryfault with preferred nodal plane being the plane in thenorth-northwest–south-southeast direction. The focal mecha-nism solution of the largest aftershock, at the intersectionbetween the 7–8 km long east–west segment and this north-northwest–south-southeast trending 10–11 km long subclus-ter, is compatible with both directions.

INTERPRETATION AND DISCUSSION

Data from 100 local and regional stations have been used todevelop a 1D velocity model for P- and S-waves for the studyarea. Even though we tested models with up to 15 homo-geneous layers, results show that the significant increase inP-wave velocity occurs only at three interfaces: between theupper and middle crust at 12 km depth, between middleand lower crust at 20 km depth, and at the Mohorovičić dis-continuity, which is at 39 km depth according to this model(Fig. 5). In the S-wave velocity model, on the other hand, veloc-ity increases more gradually with depth, with the largest in-crease at about 30 km depth (Fig. 5). The depths of theupper to middle crust interface and Mohorovičić discontinuityin our Kraljevo 1D model are consistent with results of deep-sounding profiling done by Dragašević and Andrić (1973/1974). Their profile extends from the Adriatic Sea over theDinarides to the Carpathian Arc in eastern Serbia. Alongthe profile, they identified the upper to middle crust boundaryat 10–12 km depth, and the Moho at about 40 km depth. Ob-tained average velocities for upper (VP ! 5:79 km=s, V S !3:35 km=s), middle (VP ! 5:87 km=s, VS ! 3:4 km=s), lowercrust (VP ! 6:24 km=s, VS ! 3:84 km=s), and upper mantle(VP ! 7:9 km=s, VS ! 4:29 km=s) for the Kraljevo 1Dmodel

are very similar to the results of Herak and Herak (1995) forthe intra-area velocity model of the Dinarides. It is worth tonotice that the average VP=V S ratio decreases from 1.73 in theupper crust to 1.63 in the lower crust, and further increases atthe Moho discontinuity to 1.84, possibly indicating a change inmineralogy. Although the high VP=V S ratio in the upper man-tle probably reflects the increase in mafic mineralogy, the lowVP=V S ratio in the lower crust could be related to the exist-ence of more felsic mineralogies (granite intrusions) in thick-ened crust (Sobolev and Babeyko, 1994), or could result fromseismic anisotropy which may cause S waves to show up earlieron the seismograms.

According to our results, the mainshock occurred at15.4 km depth. Such a depth is in good agreement with thesolutions proposed by the Global CMT Catalog (13.8 km), theUSGS (15.6 km), and Saint Louis University EarthquakeCenter (16 km). It may indicate a slightly deeper brittle–plastictransition in this region compared with typical continentalsettings. This earthquake was followed by mostly shalloweraftershocks for which the spatio-temporal distribution, com-bined with the obtained focal mechanism solutions, suggeststhe activation of an east–west-trending fault, followed at a laterstage by the activation of a secondary, north-northeast–south-southwest-trending fault. The majority of events, among themthe mainshock (with magnitude 5.4) and larger aftershocks(with magnitudes larger than 3), delineate a subvertical faultstriking east–west, and their focal mechanism solutions advo-cate for a sinistral strike-slip motion. Information about thefaulting type associated with the north-northwest–south-southeast subcluster is more elusive, but would be compatiblewith thrust to oblique strike-slip motion. The focal mechanismsolutions obtained in this article, along with those determined

▴ Figure 5. P- and S-wave velocity model obtained with VELEST for the study area.

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in the past ten years for other moderate-size events (SSS,EMSC), consistently show a southwest–northeast maximumcompressive stress orientation throughout the region, usuallyinterpreted as the result of the collision between the Eurasianplate and the Adriatic microplate that has been the cause ofuplift in the Dinarides since the Neogene.

Theoretical dimensions for the rupture zone can be esti-mated for the mainshock using the relationships of Wells andCoppersmith (1994). Considering a magnitude Mw 5.3–5.4and assuming a strike-slip faulting style, calculation gives arupture length of 5–6 km and a downdip rupture width of3.3–3.4 km. Such values show that the total area covered by

▴ Figure 6. (a) Kraljevo earthquake sequence located by the Serbian Seismological Survey. A total of 195 events have been located. Starindicates hypocenter of the main 3 November 2010 earthquake. (b) Kraljevo earthquake sequence located using HypoDD and the new 1DKraljevo velocity model obtained in this work. 195 events have been relocated. Star indicates hypocenter of the main 3 November 2010earthquake; diamond indicates the largest aftershock (ML 4.4).

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the aftershocks (15 by 13 km; see Fig. 6) certainly overestimatesthe rupture area. These values are more compatible, on theother hand, with the distribution of the ML ≥ 3 aftershocksthat extend west of the mainshock, 7–8 km horizontally andabout 2 km vertically. Following this hypothesis, the largestaftershock would be located at the western edge of the main-shock rupture zone or slightly beyond, and could either be dueto remaining strain energy or have been triggered by staticstress transfer caused by the mainshock. Static stress transfercould also be invoked to explain the later activity observedalong the north-northwest–south-southeast direction.

Several earthquakes with magnitude greater than 5.0 andup to 6.0 (Fig. 2) have occurred in the broader vicinity of

Čačak–Kraljevo basin in the past century. Hence, it should notbe forgotten that strain rates in this region are sufficient togenerate relatively large, though infrequent, earthquakes onavailable fault segments. Two cities with population greaterthan 100,000 people are located within this basin and lie onvery thick and young alluvial sediments (Basic Geologic Mapof SFRY, Marković et al., 1963). Because unreinforced ma-sonry is a typical building style in the area, there is a possibilityfor high vulnerability of the buildings. The identification of themajor east–west-trending fault with a moderate earthquake-generating potential and the secondary north-northwest–south-southeast-trending fault are important for accurateassessment of the seismogenic potential of the area and should

▴ Figure 7. Spatial and temporal distribution of the Kraljevo earthquake sequence. (a) Index map showing the locations of the two crosssections AA0 and BB0. (b) White star, mainshock; diamond, ML 4.4 aftershock; (c) black dots, aftershocks that occurred within 24 hoursafter the mainshock; (d) gray dots represent aftershocks that occurred during the first week (from days 2 to 7); light gray and white dotspresent aftershocks that occurred from day 7 until 31 December 2010.

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be used when advocating for new seismic building standardsand retrofitting of existing buildings. Future studies of the areashould involve development of 2D or 3D velocity models,which should give better constrained crustal structure. LowVP=V S ratio detected in the lower crust should also be exam-ined further. In addition, future efforts should be focused onimprovement of existing fault maps in order to improve geo-hazard prevention.

CONCLUSION

We present relocated a Kraljevo (Mw 5.4) earthquake sequencethat started on 3 November 2010. We first developed a new1D velocity model based on a dataset that we used to relocateearthquakes and to obtain focal mechanism solutions. Distri-butions of the earthquake sequence along with the focal mech-anism solutions indicate activation of one major subvertical

▴ Figure 8. Focal mechanism solutions and assigned ID numbers for events with ML ≥ 3 (see Ⓔ Table S1, available in the electronicsupplement to this article, for ID numbers, locations, time, and magnitude of the events with ML ≥ 3 from the Kraljevo earthquakesequence).

▴ Figure 9. Subcluster with events aligned in north-northwest–south-southeast direction. Dots represent all events that occurredbetween 14 and 27 November 2010; star indicates the mainshockand diamond ML 4.4 aftershock.

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east–west-trending fault and one secondary north-northwest–south-southeast-trending fault. The southwest—northeast ori-entation of maximum compressive stress derived from the focalmechanism solutions of the mainshock and largest aftershocksis consistent with a compressive regime in the region thatresults from collision between the Euroasian plate and theAdriatic microplate.

ACKNOWLEDGMENTS

We thank the following researchers for providing data andsuggestions: Slavica Radovanović, the Serbian SeismologicalNetwork; Branislav Glavatović, Montenegro SeismologicalObservatory; Iva Dasović, University of Zagreb, Croatia; Vas-silis Karastithis, Institute of Geodynamics; Christos Evangeli-dis, National Observatory of Athens, Greece; Manolis Scordilisand Ioanna Karagianni, Aristotle University, Greece; Constan-tin Ionescu and Bogdan Grecu, National Institute of EarthPhysics, Bucharest, Romania; and Slobodan Knežević, Facultyof Geology and Mining, University of Belgrade, Serbia. Thiswork was supported by National Science Foundation CRESTaward HRD-0833184. Any opinions, findings, and conclu-sions or recommendations expressed in this study are those ofthe authors and do not necessarily reflect the views of theNational Science Foundation.

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Sanja Knezevic AntonijevicUniversity of North Carolina at Chapel Hill

Department of Geological SciencesChapel Hill, North Carolina 27599 U.S.A.

sanjaknezevicantonijevic@gmail.com

Pierre ArroucauNorth Carolina Central University

Center for Research Excellence in Science and Technology1801 Fayetteville Street

Durham, North Carolina 27707 U.S.A.

Gordana VlahovicNorth Carolina Central University

Department of Environmental, Earth and Geospatial Sciences1801 Fayetteville Street

Durham, North Carolina 27707 U.S.A.

610 Seismological Research Letters Volume 84, Number 4 July/August 2013