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  • Bala (Ankara) Earthquakes: Implications for ShallowCrustal Deformation in Central Anatolian Section of the

    Anatolian Platelet (Turkey)

    ONUR TAN1, M. CENGİZ TAPIRDAMAZ1, SEMİH ERGİNTAV1, SEDAT İNAN1, YILDIZ İRAVUL2,RUHİ SAATÇILAR3, BEKİR TÜZEL2, ADİL TARANCIOĞLU1, SALİH KARAKISA2,

    RECAİ F. KARTAL2, SAMİ ZÜNBÜL2, KENAN YANIK2, MEHMET KAPLAN2, FUAT ŞAROĞLU1, ALİ KOÇYİĞİT4, ERHAN ALTUNEL5 & NURCAN MERAL ÖZEL6

    1 TÜBİTAK Marmara Research Center, Earth and Marine Sciences Institute, Gebze, TR−41470 Kocaeli, Turkey (E-mail: [email protected])

    2 MRWS General Directorate of Disaster Affairs, Earthquake Research Department, Lodumlu, TR−06530 Ankara, Turkey

    3 Sakarya University, Department of Geophysics, Esentepe Campus, TR−54187 Sakarya, Turkey4 Middle East Technical University, Department of Geological Engineering, TR−06531 Ankara, Turkey

    5 Eskişehir Osmangazi University, Department of Geological Engineering, Meşelik Campus, TR−26480 Eskişehir, Turkey

    6 Boğaziçi University, Kandilli Observatory and Earthquake Research Institute, Çengelköy, TR−34684 İstanbul, Turkey

    Received 06 July 2009; revised typescript receipt 15 December 2009; accepted 03 December 2009

    Abstract: Central Anatolia is quiet in terms of seismic activity, and rarely earthquakes up to magnitude 5.6 occur in theinner part of the Anatolian block or Anatolian platelet. Southeast of Ankara, the capital city of Turkey, two earthquakesequences with maximum magnitude of 5.6 occurred in 2005 and 2007. We discuss these shallow crustal deformationin the Anatolian platelet, in the light of seismological data from these earthquakes (ML= 5.6) and their aftershocks.Following the earthquake of December 20, 2007 near Bala town, Ankara, we installed seven temporary stations in thefirst 24 hours to observe the aftershock activity and these operated for more than 2 months. Approximately 920aftershocks with magnitudes 5.5>ML>0.8 were located precisely. This is the first well-observed earthquake activity inthe Central Anatolian section of the Anatolian platelet. We also re-analyzed the 2005 Bala earthquake sequence. Thedistribution of the well-located aftershocks and the focal mechanism solutions of the December 20, 2007 event defineNW−SE-oriented right-lateral strike-slip faulting on a possible weak zone, namely the Afşar fault zone, as a result of theinternal deformation in the Anatolian platelet. Our analyses seem to indicate that the Bala earthquake sequences areprobably related to increasing seismic activity, following devastating 1999 earthquakes in the Marmara region, to thewest.

    Key Words: Afşar fault zone, aftershock, Coulomb, Central Anatolia, crustal deformation, earthquake

    Bala (Ankara) Depremleri: Anadolu Levhasının Orta AnadoluKesiminde Sığ Kabuk Deformasyonuna Katkılar

    Özet: İç Anadolu depremsellik açısından sessizdir ve Anadolu bloğu içinde az da olsa 5.6 büyüklüğüne kadar depremlermeydana gelmektedir. Türkiye’nin başkenti Ankara’nın güneydoğusunda 2005 ve 2007 yıllarında maksimumbüyüklükleri 5.6 olan iki deprem dizisi meydana gelmiştir. Bu çalışmada, bu depremler ve artçı sarsıntılarından eldeedilen sismolojik veriler ışığında Anadolu levhasının sığ kabuk deformasyonu tartışılmıştır. Ankara’nın Bala ilçesinde20 Aralık 2007 tarihinde meydana gelen depremden sonraki ilk 24 saat içinde bölgeye yedi geçici deprem istasyonukurulmuş ve yaklaşık 2 ay çalıştırılmıştır. Büyüklükleri 5.5>ML>0.8 arasında olan yaklaşık 920 arçı sarsıntının hassaslokasyonu yapılmıştır. Bu, Anadolu levhasının Orta Anadolu bölümündeki en iyi gözlemlenebilen deprem aktivitesidir.

    449

    Turkish Journal of Earth Sciences (Turkish J. Earth Sci.), Vol. 19, 2010, pp. 449–471. Copyright ©TÜBİTAKdoi:10.3906/yer-0907-1 First published online 3 December 2009

  • IntroductionIn line with increased funding for earthquakeresearch in Turkey (İnan et al. 2007), the TÜBİTAKMarmara Research Center (MRC) Earth and MarineSciences Institute (EMSI) and the GeneralDirectorate of Disasters Affairs (GDDA) EarthquakeResearch Department (ERD) initiated, with financialsupport from the State Planning Organization(SPO), a new project to establish the necessaryhuman and equipment infrastructure for rapidaftershock studies in Turkey. The aim is to determinethe characteristics and behaviour of destructiveearthquakes (Mw ≥ 6.0) by obtaining detailedaftershock records and GPS measurements. The firstreal experiment under the scope of this project wasdone following the December 20, 2007 Bala(Ankara) Earthquake (09:48 UTC, ML= 5.6). One ofthe main objectives of this project is immediatedeployment of seismology stations after themainshock in order to observe the earliest aftershockactivity. Hence, the first station was deployed 9 hoursafter the mainshock. Although the earthquake isrelatively weak (ML= 5.6), the team decided tomonitor aftershock activities for two reasons: firstlythat the earthquake was felt strongly in the CapitalCity, Ankara which is about 50 km northwest of theepicenter; and secondly that the epicentre area isquite close to the Tuz Gölü (Salt Lake) Fault Zone(TGFZ) which is a major fault zone in the region thathas been inactive for a long time (Figure 1).

    In this study, we present detailed aftershockanalyses of the December 20, 2007 Bala earthquake,and also we re-analyse the moderate size earthquake(ML= 5.3) that occurred on July 30, 2005 in the sameregion and its large aftershocks.

    Geological SettingAs shown in Figure 1A, the Anatolian platelet (AP) isbounded to the north by the giant North Anatolian

    Fault System (NAFS) and on the south-southeast bythe East Anatolian Fault System (EAFS) (Şengör1979). The NAFS and the EAFS facilitate the tectonicescape of the Anatolian Block to the west (Şengör &Yılmaz 1981). The western part of the AP shows atransition to the Aegean extensional system (AES).The central area does not host major faults andseems to achieve its tectonic escape by movingwestward along the NAFS and EAFS without muchinternal deformation (Şengör & Yılmaz 1981;Reilinger et al. 1997; McClusky et al. 2000). The APcontains palaeotectonic structures such as the İzmir-Ankara-Erzincan Suture Zone (İAESZ), the SakaryaContinent (SC) and the Kırşehir Block (KB).Palaeomagnetic studies show that, while anti-clockwise rotation (~25° ccw) is observed east ofKırşehir Block (Figure 1A), neotectonic units in thewestern part of the Anatolian platelet show ~18°clockwise rotation (i.e. Tatar et al. 1996; Platzman etal. 1998; Gürsoy et al. 1998; Piper et al. 2002).However, minor internal deformation includesneotectonic secondary strike-slip faults andextensional basins (Bozkurt 2001). Koçyiğit &Deveci (2008) and Koçyiğit (2009) reported that thedirection of the compression in the region was NW–SE until late Pliocene, when a neotectonic regimewas initiated controlled by active strike-slip faultingcaused by approximately N–S compression. Theright- and left-lateral faults trend NW–SE and NE–SW, respectively (Figure 1B). The most importantstructure is the Tuz Gölü Fault Zone (TGFZ, firstnamed by Beekman 1966) with a mapped length ofabout 200 km (Koçyiğit & Beyhan 1988; Çemen et al.1999). Görür et al. (1984) point out that the TGFZhas been active since the Oligocene, and Gürsoy et al.(1998) mention that the TGFZ is a boundary zonebetween blocks with contrasting deformation. Tataret al. (1996) reported that Central Anatolia showscounterclockwise rotation since the late Eocene andÇemen et al. (1999) interpreted that this rotation wasprobably responsible for the Neogene movement

    BALA (ANKARA) EARTHQUAKES

    450

    Ayrıca 2005 Bala depremleri de tekrar analiz edilmiştir. Çok iyi konumlandırılmış 20 Aralık 2007 depremi artçı sarsıntıdağılımı ve fay düzlemi çözümleri, Anadolu levhasının iç deformasyonu nedeniyle olası bir zayıflık zonunda (Afşar fayzonu) KB−GD yönelimli sağ-yanal doğrultu atımlı faylanmanın meydana geldiğini göstermektedir. Yapılan analizlerde,Bala depremlerinin Marmara Bölgesi’nde meydana gelen 1999 depremleri sonrasında daha doğudaki sismik aktiviteartışıyla ilişkili olabileceğini göstermektedir.

    Anahtar Sözcükler: Afşar fay zonu, artçı sarsıntı, Coulomb, Orta Anadolu, kabuk deformasyonu, deprem

  • O. TAN ET AL.

    451

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  • BALA (ANKARA) EARTHQUAKES

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    30.07.2005M =5.0L

    15.03.2008M =5.0L

    20.12.2007M =5.6L

    27.12.2007M =5.0L

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    Figure 1. Continued.

    B

  • along the TGFZ and other northwest-trending faultsof the region. One of the main questions is themechanism of the TGFZ. Şaroğlu et al. (1987)observed that the TGFZ is a reverse fault with right-lateral strike-slip component. Beekman (1966) andKoçyiğit & Beyhan (1998) reported that the TGFZ isa right-lateral strike-slip fault zone with a normal slipcomponent. Dirik and Göncüoğlu (1996) remarkedthat the fault zone consists of parallel to subparallel,normal and oblique right-lateral strike-slip faultsdisplaying a step-like half-graben and horst-grabenpattern. On the other hand, Çemen et al. (1999)mentioned that the fault may have been formed as anormal fault, suggesting extension or strike-slipfaulting with a normal component of movementindicating major transtension at the time of itsinitiation. However, as all the faults have noimportant seismological activity (M>4.0–5.0) atpresent, there is no information about their deepstructure in the region. Aydemir (2009) usednational earthquake catalogues and interpreted thatthe area to the south-southeast of the TGZF iscompletely (seismically) inactive because of theabsence of small earthquakes (M

  • Keskin, ~50 km NE of Bala. The model has a toplayer with Vp= 5.0 km/s P wave velocity and containsgradually increasing velocities. The other modelpresented by Ergin et al. (2003) after a study in a sodamine area near the town of Kazan (~90 km N ofBala). They used the velest algorithm of Kissling et al.(1994) and described four crustal layers. The toplayer of the model has a slow P-wave velocity,generally representing the sediments in the area.There are two other velocity boundaries at 6 and 20km depth. The model given by Ergin et al. (2003)provided better time residuals at the stations andgenerated lesser uncertainties in the locationparameters. The average seismic velocity of theuppermost two layers (sediments) in this modelagrees well with 2D seismic prospecting data(Aydemir & Ateş 2006). The shear-wave velocities(Vs) are calculated by the ratio Vs= Vp/1.73. Averagetime residuals for aftershocks (RMS) should be lessthen 0.3 s in the inversion location.

    Calculation of local Richter magnitude (ML) isone of the important points that must be mentionedhere. Although 4.5 Hz geophones are easy to installin the rupture zone quickly, they are difficult formagnitude calculations because of their narrowfrequency response and high damping ratio.Amplitudes of earthquake waveforms sensed bygeophones decrease rapidly and waveform durationsbecome extremely short compared to broad-bandseismometer records. So, we did not find anyaftershock coda-duration magnitudes exceeding 3.0.In order to calculate local magnitudes (ML), wearranged a methodology by using the SeismicAnalysis Code (Goldstein et al. 1998). First, the

    sensor and digitizer responses are separated from thevelocity records. Then, each waveform is convolvedwith the Wood-Anderson seismometer response togenerate displacement record. The maximum zero-to-pick horizontal amplitude was selected and MLwas calculated for each station. The minimum andmaximum extreme values (larger than standarddeviation) were removed and the remainingmagnitudes were averaged for that event. In fact, themaximum amplitude at a station does not projectreal value for ML because the recorded waveforms donot contain low frequencies (i.e. 1 Hz). Nevertheless,this is the only way to approximate the magnitudes ofthe aftershocks. The magnitudes of the selected largeaftershocks which occurred during the survey werecompared with values in the national networks(Table 3). Although the coda-duration magnitudesfrom geophone records (this study-md) were too

    BALA (ANKARA) EARTHQUAKES

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    Table 1. The locations of seismic stations deployed immediately after the December 20, 2007 Bala mainshock. One Güralp 3TDbroad-band seismometer is used in Bala town. The other stations have Reftek-130 (R130) recorder with OYO Geospace GS-11D geophone.

    Code Latitude (°N) Longitude (°E) Elevation (m) Town/Village Instrument

    BLA 39.5427 33.1230 1312 Bala Güralp 3TD DRP 39.3416 32.7423 1120 Durupınar R130, GS-11D BKS 39.1981 33.2628 973 Büyükkışla R130, GS-11D CEV 39.4518 33.5685 1020 Üçev R130, GS-11D SOF 39.2883 33.0964 958 Sofular R130, GS-11D OGB 39.6850 32.8303 1080 Oğulbey R130, GS-11D YAR 39.1703 32.9270 1170 Yaraşlı R130, GS-11D

    Table 2. The crustal seismic velocity models for the region.Models A and B were reported by Toksöz et al. (2003)and Ergin et al. (2003) respectively. In this study,model B was accepted. h and Vp indicates layerthickness and P-wave velocity, respectively. The shear-wave (Vs) velocities were calculated by Vs = Vp / 1.73ratio.

    Model - A Model - B

    h (km) Vp (km/s) h (km) Vp (km/s)

    0 – 5 5.0 0 – 1 2.55 – 10 5.5 1 – 6 5.7

    10 – 20 6.1 6 – 20 6.120 – 36 6.4 20 – 33 6.8

    36– 7.8 33 – 8.0

  • O. TAN ET AL.

    455

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  • small, as expected, there was no significantdifference between the local magnitudes (ML). Basedon our approach, the local magnitudes of theaftershocks were calculated to be between 0.8 and5.5.

    Although we tried to minimize the errors oflocation parameters, several factors such as networkgeometry, phase reading quality and crustalstructure uncertainties limited our endeavours.Relative earthquake location methods can improveabsolute hypocentre locations. For this purpose, weused a double-difference algorithm (hypoDD)developed by Waldhauser & Ellsworth (2000). ThehypoDD algorithm assumes that the hypocentralseparation between two earthquakes is smallcompared to the event-station distance and the scalelength of velocity heterogeneity, so that the ray pathsbetween the source region and a common station aresimilar along almost the entire ray path. If so, thedifference in travel times for two events observed atone station can be accurately attributed to the spatialoffset between the events (Fréchet 1985; Got et al.1994; Waldhauser & Ellsworth 2000). By linkinghundreds or thousands of earthquakes togetherthrough a chain of nearby shocks, it is possible toobtain high-resolution hypocentre locations overlarge distances without the use of station corrections.Two inversion approaches are used in a standardhypoDD analysis. The singular value decomposition(SVD) method is very efficient for the well-conditioned systems which have small earthquakeclusters (~100 events). However, because of the largesize of our data and unknown parameters for thelarge cluster (more than 200 events), SVD cannot beused effectively and the conjugate gradient algorithm

    (LSQR), which solves the damped least-squaresproblem, was selected to save computer memoryusage, computation time and efficiency of thealgorithm (see Waldhauser & Ellsworth 2000 fordetails). After relocating the hypocentres, horizontaland vertical error assessment must be done carefully.Unfortunately SVD gives proper least square errors,LSQR reports underestimated errors and these errorsmust be reviewed by statistical resampling methodsand by relocating small subsets of events using SVDmode.

    We also read P-wave first motion polarities tofind out focal mechanism solutions of the mainshockand the large aftershocks (ML≥4.0). All availablepolarities from national seismic stations and theaftershock network were read carefully andambiguous polarities were never added to thesolutions. The takeoff angles were calculatedaccording to the same velocity structure used for thelocation determination. The possible nodal planeswhich agree with the first motion polarities weresearched, running the focmec program (Snoke et al.1984). No polarity error is allowed in the solutions.Events with multiple acceptable solutions indicatingdifferent mechanism or with faulting parametersuncertainties exceeding ±20° were not reported inthis study.

    Because of the good data set for the 2007 Balaearthquake and its aftershocks, we tried to applyCoulomb failure stress change analyses tounderstand the stress change in the region caused bythe earthquakes. We followed similar methods tothose described in King et al. (1994) and Stein et al.(1997) and used the program Coulomb 3.0 (Lin &Stein 2004; Toda et al. 2005). We assumed an elastic

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    Table 3. Comparison of the magnitudes reported by different agencies for selected aftershocks. The ERD and KOERI calculate themagnitudes from national broad-band seismology stations. The values in this study are from the temporary aftershocksobservation stations which were equipped with geophones.

    This Study ERD KOERIDate Time

    (dd.mm.yyyy) (hh:mm) md ML md ML md ML

    26.12.2007 23:47 3.8 5.5 5.3 5.5 – 5.527.12.2007a 13:47 2.9 5.0 4.9 5.0 – 4.827.12.2007b 17:56 3.3 4.3 4.2 4.3 – 4.001.02.2008 09:11 2.5 4.4 - 4.4 – 4.1

  • half-space with a shear modulus of 3.2×1011dyne·cm2 and Poisson's ratio of 0.25 in calculations.In this exercise a fault friction of 0.4 was assumed.

    Earthquake SequenceThe mainshock of December 20, 2007 was recordedby the ERD national broad-band seismic networkand the preliminary location was reported as39.417°N 33.045°E. We collected waveforms frommore than one hundred national stations from theERD and KOERI, and re-read the P and S phases tore-locate the mainshock more precisely. Wecalculated the Bala main shock coordinates as39.431°N 33.088°E with minimum horizontal error(±2 km). The new location is about 4 km east of thepreliminary reported location. The hypocentre depthis 4.4±2 km. The main shock was preceded by sixevents in the two hours before the main shock (Table4, Figure 2). Although the magnitudes of theforeshocks (2.8≤md≤3.6) are not large enough andthe station distribution is sparse, their locationsagree well with the aftershock distribution describedin the next section. The first event (A) occurred atthe north end of the region and the next four events(B, C, D and F) were in the south. Only one event (E)was far from the activity area.

    We located 923 aftershocks occurring in 71 dayswith the Hypocenter algorithm. The horizontal andvertical location errors are 1–2 and 1–3 kmrespectively and the average station time residuals(RMS) are 0.15 seconds. The aftershocks occurred inan approximately 20×5 km narrow band trendingNW–SE. To improve the hypocentre locations with

    hypoDD analysis, we preprocess in order to selectdata from connected earthquakes to build a networkof links between event pairs. The maximumhypocentral separation was chosen as 2 km. Theminimum number of phases for an event-pair to berecorded at a common station is defined as 8, whichis the minimum value to solve unknown parametersof pairs (6 for space and 2 for time). Severalinversions are executed with different modelparameters to find a stable solution. We couldrelocate 706 aftershocks precisely. 217 events wereexcluded in inversion. 68 of them are very shallowevents (~1 km) with poor vertical control. The other149 events have also poor links with neighbouringevents and cannot be used in the iterations. Therelocated events are shown in Figure 3. Theuncertainties of the hypocentre locations afterhypoDD (LSQR) analyses are tested with twodifferent methods. First we select highly correlatedevent-pairs which represent three different parts inthe aftershock area. These sub-clusters are shown inFigure 3 with the letters N, C and S which refer to thenorth, centre and south sub-clusters respectively.Each selected event-pair has P and S phase data fromat least 5 common stations and has 20 neighbouringevent-pairs to form a good continuous chain. Thenumber of events in each sub-cluster and theirhorizontal and vertical errors are given in Table 5.SVD analysis of hypoDD shows the maximumhorizontal uncertainty is about 400 m and thevertical errors are little more than 700 m. Our secondtest is based on a statistical approach. We use thesame initial catalogue data and inversion parametersin the final LSQR solution. We add random numbers

    O. TAN ET AL.

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    Table 4. The foreshock parameters of the December 20, 2007 Bala Earthquake reported by the ERD.

    Event Date Time Latitude Longtitude Depth Magnitude(dd.mm.yyyy) (hh:mm) (°N) (°E) (km) (md)

    Foreshock A 20.12.2007 07:36 39.4698 33.0732 7 3.6Foreshock B 20.12.2007 07:51 39.3747 33.1142 6 3.2Foreshock C 20.12.2007 08:12 39.4043 33.1433 7 2.8Foreshock D 20.12.2007 08:18 39.3763 33.1180 7 3.1Foreshock E 20.12.2007 08:27 39.4398 32.9462 7 2.8Foreshock F 20.12.2007 08:57 39.3542 33.1430 6 3.3

  • BALA (ANKARA) EARTHQUAKES

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  • between ±1 km to the initial absolute locations in X,Y and Z directions. This allows us to shift thehypocentre location in space and re-link eventsrandomly. After inversion, the location shift of eachevent is calculated. The process is repeated severaltimes and 1000 well-conditioned inversion solutionsare collected. Approximately 175000 samples areacquired to see the statistical distribution. Theoutliers in the data set are removed using theinterquartile range (IQR) method. More than 95% ofthe total samples remain after the IQR analysis. Thedataset represents a normal distribution and the 95%confidence interval (±2σ) shows location variationinterval according to the different initial models(Figure 4). This test shows latitudinal, longitudinaland vertical location changes are not more than ±230m, ±260 m and ±550 m respectively.

    The aftershock zone developed quickly and 45%of the total 923 recorded events occurred in the first10 days. Another 45% of them occurred in Januaryand then the number decreased dramatically in thelast month of the observation. Although theaftershocks align along a NW–SE narrow zone (i.e.A-A’ profile), the events shift eastwards in thesouthern (S) segment. Moreover, another smallcluster with a few events occurring on the 54th and55th days of the sequence was seen in the NE of thearea (near Çiğdemli village). The aftershocktemporal behaviour is given in Figure 5, according tothe locations on the A-A’ profile in Figure 3. Themainshock occurred in the northern part (~6 km) ofthe cluster and the first sub-cluster activity occurredin the SE (~13–16 km) for a few days. After

    December 26, further activity began in the centralsegment (~8–10 km) and also in the northern part.However, the large aftershocks occurred close to themainshock and north of it (around Afşar village).These events, especially occurring after January 1,were not followed by smaller events. However, a fewtremors form small groups in the southern part(arrows in Figure 5). This can be interpreted as theasperities on that segment being unable to release itsenergy with a single (relatively large) event, and sogenerating several micro-earthquakes in a shorttime. The spatiotemporal distribution of the Balaaftershocks indicates that the northern and southernparts of the deformation area may have differentasperity properties. Although the northern part hasstrong asperities which release its energy inmoderate and small events, the southern partcontains several small and weak asperities whichgenerate micro-earthquakes only.

    The depth section of the sequence shows that theearthquakes occurred between 3 and 9 km deep(Figures 3 & 6A). No event is deeper than 14 km.Those events that especially occurred near themainshock and north of it align within a very narrowband. The depth sections of these parts show thatlarge aftershocks occurred on a vertical plane whichmay indicate strike-slip fault segments (Figure 6B,C). However, there are many more events in centralpart and they scatter across a wider area within thecentral part (Figure 6D). The southernmost sub-cluster is elongated in an approximately E–Wdirection and events concentrate at 4–6 km depth(Figure 6F). Deeper aftershocks (8–10 km) were alsoobserved in this segment.

    Fault Mechanisms and Stress Changes We collected all available digital records from thenational seismograph networks of ERD and KOERIto read the P-wave first motion polarities of the 2005and 2007 Bala earthquakes (ML≥4.0). Theparameters of the earthquakes are summarized inTable 6 as location, local Richter magnitude andfaulting parameters. The strike, dip and rake anglesof assumed fault plane and the number of P-wavefirst motion polarities are shown in the followingcolumns. The rupture area is necessary to calculate

    O. TAN ET AL.

    459

    Table 5. The horizontal (Exy) and vertical (Ez) errors of thehighest correlated events before and after SVDanalyses in hypoDD. The earthquakes were selectedfrom the three sub-clusters of the Bala aftershocks(Figure 3A). N– North, C– Centre, S– South.

    Before hypoDD After hypoDD(m) (m)

    Region Numberof Events Exy Ez Exy Ez

    N 90 ±1500 ±2800 < ±400 < ±700

    C 130 ±1600 ±2200 < ±300 < ±700

    S 134 ±1500 ±2000 < ±200 < ±500

  • BALA (ANKARA) EARTHQUAKES

    460

    Figure 4. Statistical distribution of event shifts after hypoDD-LSQR inversions with different initial locations. Eachhistogram contains about 95% of total samples (~175000). The ±2σ (two times of standard deviation) linesshows 95% confidence interval.

  • the Coulomb failure stress change. Therefore, wetried to determine the size of the rupture area (L×W)and average displacement over the fault surface (Dav)using the generalized assumptions mentioned inseveral studies (i.e. Wells & Coppersmith 1994; Mai& Beroza 2000; Tan & Taymaz 2005, 2006). The faultplane solutions of 14 events with P-wave first motionpolarities are shown in Figure 7. The focal spheresare plotted in lower hemisphere projection andcompressional quadrants are shaded. The availablepolarities constrain the nodal planes very well andthe uncertainties are less than ±5° for most of thesolutions.

    The 2005 Bala earthquake has no surface ruptureand has no reliable aftershock record. The onlyavailable data set was from the national

    seismological networks in Turkey. The large range ofthe location uncertainties of the small events (md=3.0–4.0) in the national catalogues (> 5 km) posedifficulties in interpretation of the aftershock datafor such a small area. So, we tried to relocate the 2005mainshock and its eight aftershocks to clarify thedeformation in the region (Table 6, Figure 8), basedon the raw waveforms. The waveforms from the twonational networks were collected and the phaseswere re-read to discover a better location than thatreported in the catalogues. The phase readings fromthe special broadband networks of KOERI in Gölbaşı(50 km NW) and Keskin (50 km NE) were also used.The average location errors of these relocated nineearthquakes barely exceed ±2 km. The mainshock(#1) and its first large aftershock (#2) occurred in the

    O. TAN ET AL.

    461

    Figure 5. Temporal behaviour of the aftershocks in 2007 and 2008. The locations of the events are plotted according to theA–A’ profile (NW–SE) in Figure 3. The large aftershocks occurred close to the mainshock and north of it whilethe micro-earthquakes form small groups in the southern part. The star is the mainshock (20 December 2007).The arrows are examples for the small micro-earthquake groups in the southern segment.

  • BALA (ANKARA) EARTHQUAKES

    462

    Figure 6. Depth sections of the aftershocks of the December 20, 2007 Bala earthquake. The locations of the profiles areshown in Figure 3B. The star is the mainshock hypocentre. The main activity seems to be confined to 3 to 9km depth interval.

  • O. TAN ET AL.

    463

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  • BALA (ANKARA) EARTHQUAKES

    464

    Up

    Down

    P axis

    T axis

    30.07.2005-21:45 31.07.2005-00:45 31.07.2005-15:18 31.07.2005-23:41

    01.08.2005-00:45 01.08.2005-02:02 01.08.2005-13:22

    20.12.2007-09:48 26.12.2007-23:47 27.12.2007-13:47 01.02.2008-09:11

    15.03.2008-10:15 11.09.2008-08:33 23.09.2008-09:09

    Figure 7. P-wave first motion polarities of the 2005 and 2007 Bala mainshocks and their large (ML ≥ 4.0) aftershocks. The event datesand origin times are given above the focal spheres (see Table 6). The compressional quadrants are shaded in grey. The blackand white circles refer to up and down P-wave first motion polarities respectively. P and T axes are also represented by thesolid and open diamonds respectively.

  • O. TAN ET AL.

    465

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    solu

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  • northeastern end of the sequence. The other threeevents (#3, #4 and #5) are located between themainshock and 2007 activity. If the direction of thelocations is considered, the focal mechanismsolutions indicate NE–SW sinistral strike-slipfaulting in 2005 (Koçyiğit 2009). However, there isno more data to confirm this. The other aftershocksof August 1, 2005 (#6, #7 and #8) are located in thesame cluster as the 2007 sequence, and might haveoccurred on a NW–SE dextral strike-slip fault.

    The fault plane solutions of the December 20,2007 Bala earthquake (#10) and its large aftershocksalign NW–SE and suggest right-lateral strike-slipfaulting. One of the interesting points is that thelatest large aftershocks (#12, #15, #16 and #18) arelocated in the northern segment of the cluster.However, the largest aftershock (#11:26.12.2007,ML= 5.5) occurred at the junction of the 2005 and2007 activities. The relatively scattered aftershockdistribution in this area may be caused by theseconjugate earthquake activities, sourced from theconjugate faulting mentioned by Koçyiğit (2009).Large aftershock activities did not occur south of thispoint, except for event #14 which has no reliablesolution. We also found two normal faultmechanisms with strike-slip component. If right-lateral motion is considered, the December 27, 2007(#13, ML= 5.0) and September 11, 2008 (#17, ML=4.0) events show SW and NE-dipping planesrespectively. The events may be caused by localextensional forces between strike-slip fault segments.Unfortunately, there are no sufficiently large eventsto allow faulting mechanism in the southernmostsegment to be understood. Therefore, we selectedcloser micro-earthquakes to try to find joint faultplane solutions. The four small groups of events havesimilar P-wave polarities and give reliable solutions(Table 6, Figure 8). While Group A has strike-slipmechanism, groups B, C and D show normal faultingwhich indicates extension in this part of the region.

    The length of the TGFZ and its unknown historicseismic activity raises the question of futureearthquake hazard along the fault, because of thenearby Bala earthquakes. These earthquakes may beconsidered too small to trigger activity on theneighbouring fault segments. However, King et al.(1994) pointed out that increases of Coulomb stress

    of less than 1 bar sometimes appear to be sufficientto trigger events, depending on the stress level of thefault segments to be triggered. Thus, we tried tocalculate the static Coulomb stress changes tounderstand the relation between the mainshock andthe aftershocks. Because of the uncertainties of the2005 earthquake sequence mentioned in theprevious sections, we only analyzed the December20, 2007 earthquake and its large aftershocks. Thestress changes were calculated on specific faultsbecause there was no reliable information for thedirection and magnitude of the regional stress field.The average fault plane parameters (strike 130°, dip80°, rake 180°) from the solutions given in Table 6was used as the orientation of the specific faults atthe calculation grid points and the locking depth waschosen as 5 km. The Coulomb stress change of theDecember 20, 2007 mainshock is shown in Figure9A. The four lobes of increased stress rise wereobserved at the ends of the ruptured segment. Thedistribution of the aftershocks agrees well with thispattern. Most of them occur on the rupturedsegment because unknown details of the faultgeometry and slip distribution affect stress change inareas closer to the fault, as mentioned by King et al.(1994). The southernmost cluster, which shifts to theeast according to the general distribution, occurredcompletely in the Coulomb stress rise lobe. Thesmall cluster observed near Çiğdemli village mayalso be explained by the stress rise after themainshock.

    Due to the similar faulting mechanisms andorientations, the cumulative Coulomb stress changeof the all aftershocks in 2007 and 2008 do not greatlydiffer from that of the mainshock (Figure 9B); onlythe magnitudes of stress rise/fall change. The rate ofthis magnitude change (< 0.01 bar) is dominant inthe NW and the direction of the stress rise lobe in theSE probably indicates that the Bala earthquakescannot lead to stress increase on the Tuz Gölü FaultZone (TGFZ), because the Bala fault zone is not thenorthwestern continuation of the Tuz Gölü FaultZone.

    Discussion The earthquake area is located near the boundarybetween the Kırşehir block and the Sakarya

    BALA (ANKARA) EARTHQUAKES

    466

  • O. TAN ET AL.

    467

    Tu zGöl ü

    F.

    A

    B

    SaltLakeSaltLake

    Figure 9. The Coulomb stress change analyzes for the Bala earthquakes on specificfaults (strike 130°, dip 80°, rake 180°) at a locking depth of 5 km. (A) Therelationship between the Coulomb stress change of the December 20, 2007mainshock and its aftershocks (white circles). (B) The cumulative effect of theall large events (ML ≥ 4.0) in the Bala region in 2007 and 2008. The white linesare ruptured segments used in the calculations and their lengths areproportional to L in Table 6. The stress rise and drop areas are in red and blue,respectively. The contours of stress change values are labelled in bars.

  • Continent which palaeomagnetic data indicaterotates anticlockwise. Geological field observationsshow that the N–S contractional neotectonic regimeinitiated in the Late Pliocene (Koçyiğit & Deveci2008; Koçyiğit 2009) causes several conjugate faultswhich have surface rupture tens to hundreds of kmlong. Following the 1999 İzmit (Mw= 7.4) and Düzce(Mw= 7.2) earthquakes, postseismic motions broadlydistributed around coseismic ruptures and increasedthe stress transfer to within Anatolia. One of theattributes of the stress transfer is seen in the GPSdata. Resolvable postseismic changes, by GPS timeseries, to the velocity field extend at least as far eastas the location of the continuous GPS station inAnkara, 200 km SE of the rupture of the devastingİzmit 1999 earthquakes in the Marmara region ofTurkey. Seven years after the earthquake sequence,deviations from the interseismic velocity fielddecreased to ~3 mm/yr (~15% of pre-earthquakemotion rate) at Ankara (Ergintav et al. 2009).Another attribute comes from the increasingseismological activity at the same time. Especially,NW–SE-trending earthquake activity was observedbetween the NAFS and Ankara after the 1999earthquakes. Figure 10 shows the seismic activity inthe study area before and after the devastating 1999earthquakes of İzmit (17.08.1999, M= 7.4) andDüzce (12.11.1999, M= 7.2); the black and yellowdots depict the earthquakes with M ≥ 3.0 occurringbefore and after the 1999 earthquakes, respectively.The increase in the number of M ≥ 3.0 earthquakesin the study area following the 1999 earthquakesequence is interpreted to be the result of stresstransfer to the east with time. The largest event is theJune 6, 2000 Orta-Çankırı earthquake (Mw= 6.0),which shows N–S-trending left-lateral strike-slipfaulting at shallower depths due to the NW–SE-directed operation of the principal compressivestress around the NAFS (Koçyiğit et al. 2001), butbecomes N–S normal faulting at greater depthsowing to the variation in fault geometry with depth,noted by both seismological and InSAR observations(Taymaz et al. 2007; Çakır & Akoğlu 2008). A fewevents (M ≥ 4.0) have also occurred in Ankara andÇankırı provinces since 2000. These datasets indicatethat the deformation of the Anatolian block in thisarea causes moderate size earthquakes in weakdeformation areas such as conjugate fault zones

    (Afşar and Balaban-Küredağ fault zones in Figure1B), especially since the stress changes following thetwo large earthquakes in 1999.

    The location and faulting properties of both the2005 and 2007 earthquake activities near the town ofBala (Ankara, Turkey) seem to hold a key inunderstanding the regional deformation. Based onboth aftershock distribution pattern and fieldgeological mapping data, the fault plane of the July30, 2005 (ML= 5.3) earthquake seems most probablyNE–SW (NE-trending Balaban-Küredağ left-lateralstrike-slip fault zone). However, with betterseismological data for the December 20, 2007 (ML=5.6) earthquake we are certain that that earthquakeoccurred on a NW–SE fault segment (NW-trendingAfşar Fault Zone). However, the shifting pattern ofaftershocks shows approximately E–W extension inthe southernmost end of the activity due to the N–Soblique-slip normal faulting component of the majorstrike-slip faulting. The fault plane solutions agreewell with the conjugate faulting pattern, includingthe NW-trending dextral strike-slip faulting, NE-trending sinistral strike-slip faulting and N–S-trending oblique-slip normal faulting. This also fitswell with the strike-slip fault pattern obtained fromgeological mapping in and around Bala (Figure 1B).The cumulative stress change indicates that there is arisk of the next destructive earthquake occurring inthe north, where most of the large aftershocks alsooccurred. In contrast, the stress increase is small onthe Tuz Gölü Fault Zone. Because the nationalseismology networks cannot observe the micro-earthquake activity (M

  • O. TAN ET AL.

    469

    Figure 10. Seismicity in and around the study area before and after the devastating 17 August 1999 (M= 7.4) and 12 November 1999(M= 7.2) earthquakes (USGS-NEIC, M ≥ 3.0). (A) Seismicity in the region covering the epicentral areas of these bigearthquakes (red stars) and the nearby areas including the study area (Bala). The circles depict the circular distances of 100,200, 300, 400 km from the deformation zone related to these big earthquakes. (B) Seismicity in the study area, (C) Seismicityin the southern part of the epicentral areas of the 1999 earthquakes. Black dots are earthquakes before 17 August 1999earthquake. White dots are earthquakes occurring between 17 August and 12 November 1999 earthquakes. Yellow dotsdepict earthquakes occurring after 12 November 1999. The large events (M ≥ 5.0) are shown with red dots. Note thenoticeable increase in yellow dots in the study area; suggesting increase of deformation (e.g., transfer of stress from the westto the east by years) in the study area.

    (A)

    (B)

    (C)

    Black Sea

    SeaMarmara

    The fault plane solutions, the aftershockdistributions and the fault pattern show that themain deformation zones trend in NW–SE and NE–SW directions, with many different moderate sizefault concentrations causing a series of ML ≥ 5earthquakes.

    Based on the Coulomb models and also theepicentral distribution of aftershocks, the main stressincrease is northwestwards (towards Ankara; thecapital city of Turkey). So, continuous monitoring ofthis area plays an important role in understandingthe behaviour of stress changes in the area.

  • AcknowledgementThis project (DEPAR – Urgent Monitoring StudiesAfter Earthquake) was supported by the StatePanning Organization (DPT) of Turkey and

    TÜBİTAK Marmara Research Center. We thankOrhan Tatar for his suggestions. The maps andgraphs were drawn using the Generic Mapping Tool(GMT) (Wessel & Smith 1991).

    BALA (ANKARA) EARTHQUAKES

    470

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