Journal of Geodynamics 47 (2009) 280287

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Journal of Geodynamics

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A. GholaInternational In

a r t i c l

Article history:Received 24 AReceived in reAccepted 16 Ja

Keywords:SeismicitySeismotectoniIranZagrosAftershocks

e (Mwcollisocal sle alirily cry lowh cenThe frthqu

aftershocks are dominantly strike-slip.Wepropose that the strike-slipmechanismsbelong to right-lateralfault systems that accommodate differential motion at the transition between the Zagros collision zoneand the Makran subduction zone. If so, this suggests that the convergence between Arabia and centralIran is at present accommodated along the transition zone by a partitioning process.

2009 Elsevier Ltd. All rights reserved.

1. Introdu

An earth28, 2006 (0(Fig. 1). TheSWNE treof the ZagrFault (MZRFcentral Iran

Geologicgested thatand theMakting the whdiffuse, rathdifferentialactive subd2005; Yami

An intershock and

Correspoand SeismologFarmanieh, 19

E-mail add

0264-3707/$ doi:10.1016/j.jction

quake with magnitude Mw 6.0 occurred on February731 GMT) close to the village of Tiab, in southern Iranepicentral location of the Tiab earthquake lies along a

nd of intense seismicity at the southeastern most endos, a few kilometers north of the Main Zagros Reverse), the geological suture between the Arabian plate and(Fig. 1).al studies and seismological investigations have sug-the transition between the Zagros continental collisionran subduction zone is notmarked by amajor fault cut-ole lithosphere. Instead, it is generally believed that aer than localized, transition zone accommodates themotion between the Zagros region to the west anduction in the Makran to the east (Regard et al., 2004,ni-Fard et al., 2007).esting feature of this earthquake is that the mainits aftershock sequence are located at the transition

nding author at: International Institute of Earthquake Engineeringy (IIEES), P.O. Box 19395-3913, No. 26, Arghavan St., North Dibajie,537-14476 Tehran, Iran.resses: faryam@iiees.ac.ir, fyaminif@yahoo.fr (F. Yamini-Fard).

between Zagros continental collision and the Makran subductionzone, where focal mechanism solutions and the deeper extent ofteleseismically located earthquakes imply underthrusting of theArabian plate beneath central Iran (Engdahl et al., 2006; Talebianand Jackson, 2004; Maggi et al., 2000). Furthermore, a microseis-micity study 50km southeast of the epicentral area of the Tiabearthquakeshedsnewlighton the low-anglenortheastward thrust-ing of the Arabian plate beneath central Iran (Yamini-Fard et al.,2007).

To study the aftershocks of the Tiab earthquake we deployed atemporary seismic network around the epicentral area for a periodof 1 month. One of the main purposes of this deployment was toinvestigate the possibility of underthrusting of the Arabian platebeneath the central Iranian blocks in this region.

We are also interested in deciphering possible northward con-tinuation of the MinabZendanPalami (MZP) fault system whichbounds the western limit of the ZagrosMakran transition zone.It is thought that the MZP fault, along which no seismic activityhas been recorded, terminates at the southeastern tip of the MZRFand transfers the Zagros deformation to the Makran prism (Regardet al., 2005). Moreover, it has been suggested that a transpres-sional regime characterized by a NE trending maximum principalstress direction homogeneously affects the region at the presenttime (Regard et al., 2005). However, the present study providesevidence for a partitioning process along the western limit of the

see front matter 2009 Elsevier Ltd. All rights reserved.og.2009.01.005ruary 28, 2006 Tiab earthquake, Mw 6.ion between the Zagros continental colltion zone

mzadeh, F. Yamini-Fard , K. Hessami, M. Tatarstitute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran

e i n f o

ugust 2008vised form 16 January 2009nuary 2009

cs

a b s t r a c t

The 28 February, 2006 Tiab earthquakzone between the Zagros continentalsequence is recorded by a temporary lis diffuse and we cannot dene a simpgently northward and they are primaseismogenic layer than the Zagros. Vethrusting of the Arabian plate beneatpreviously based on teleseismic data.nism similar to that of other strong ear .com/ locate / jog

mplications for tectonics of then and the Makran

6.0), is the rst earthquake to have occurred in the transitionion and the Makran subduction zone for which the aftershockeismic network. The epicentral distribution of the aftershocksgnment at the surface. The depth of the aftershocks increasesoncentrated between 15 and 21km depth, implying a deeper-angle thrust faulting deduced from this local study supportstral Iran at the southeastern end of the Zagros as suggestedocal mechanism of the main shock indicates a thrust mecha-akes in this region, while most of the focal mechanisms of the

A. Gholamzadeh et al. / Journal of Geodynamics 47 (2009) 280287 281

Fig. 1. Seismwhite circlesBerberian, 19ary 28, 2006(http://www.sUniversity cata

transition ztime.

2. Geologic

Iran liessia, which c(Vernant etfolding andsubductionmountainsstrike-slip f

The Makby the Jazmbase of theand west, ttal margin othe east antem and thepresent studMakran regduring theprisms, witin the Makrexposure.

The NWabout 1800Anatolian Fwhere theZagros belt

The defoto the relatMiddle-LateStoneley, 19during therecent time

1981; McQuarrie et al., 2003; Masson et al., 2005). Current seismic-ity and folding indicate that this deformation is still active.

The thickness of cover sediments deposited on the shelf alongthe northern margin of the Arabian plate ranges between 5 and

butal IraTheled fz Salogicngi,alo

n anfoldechae prillar(81

uppealt Fony wJacksngi, 1nortRevemelavi

ch mto beidth

l toter olex ss (Almeatheaaj-Sto th

uakeicity of the eastern end of the Zagros from 1964 to 2004,(Engdahl et al., 2006) and major faults (Regard et al., 2004;95; NIOC, 1977a,b). The star is the epicenter of the Febru-

Tiab earthquake (IIEES catalogue). CMTsolutions, beach ballseismology.harvard.edu/CMTsearch.html) presented from Harvardlogue.

one rather than a transpressional regime at the present

al setting

between the lithospheric plates of Arabia and Eura-onverge at a rate of 2125mm/year at 26.5N, 56.5Eal., 2004). The resulting shortening is accommodatedbythrust faulting in the Zagros mountain belt and Makranzone in the south and in the Alborz and Kopeh Dagh

13kmNation1984).decoupHormu

GeoBarazalocated(JacksoZagrosfocalmcate th1984; Gdepthsin themuz Sled mament (Baraza

TheZagrosever, so1974; Ain whiMZRF,has a wparallecharacite dupsystem

GPSthe souSanandcloselyearthqin the north, and by slip on several major NS trendingaults in central and eastern Iran.ran accretionary prism in Iran is bounded to the northurian depression and to the southeast is marked by thecontinental slope, some 150km offshore. To the southhe area is bounded by the narrow and steep continen-f Oman. The Makran accretionary prism is bounded to

d west by large transform faults of the MZP fault sys-Oranch Fault Zone, respectively. Based on previous andies (e.g., Farhoudi andKarig, 1977; Platt et al., 1985), the

ion is composed of a large sedimentary prism accretedCenozoic. All the typical characteristics of accretionaryh the exception of a trench, can be identied or inferredan, which, however, is unique in its degree of surface

SE trending Zagros fold and thrust belt extends forkm from a location some 300km southeast of the Eastault in northeastern Turkey to the Strait of Hormuznorth-south trending MZP fault system separates thefrom the Makran accretionary prism.rmation within the Zagros fold and thrust belt is dueive convergence between Arabia and Eurasia since the

Cretaceous (Falcon, 1974; Stcklin, 1974; Koop and82). However, the Zagros fold and thrust belt formedmain phase of the Zagros orogeny in Late Miocene tos (Stcklin, 1968;Haynes andMcQuillan, 1974; Stoneley,

3. Data an

In orderportable thcentral areaafter the maof logistical2. We recortimewas caextracted fr

After intial velocity50km soutimprove threcorded bythan 180,epicenter aoff betweenis small. Plotively for sand j, respewe comput(Fig. 3).

We invea 1D velocitis not known in detail (Stcklin, 1968; Falcon, 1974;nian Oil Company, 1975, 1976; Jackson and McKenzie,sedimentary cover of the Zagros fold and thrust belt isrom its underlying basement along the Lower Cambrianlt Formation (OBrien, 1957; Kent, 1979).al and geophysical features (Lees, 1952; Snyder and1986), and earthquake hypocenters interpreted to beng thrusts in the basement beneath the Hormuz Saltd Fitch, 1981; Ni and Barazangi, 1986) imply that theand thrust belt involved thick-skinned shortening.Mostnismsolutions of earthquakes in the Zagros region indi-esence of active reverse faults (Jackson and McKenzie,d and Wyss, 1995). The most recently determined focal5 km) imply that moderate to large earthquakes occurrmost part of the Arabian basement, beneath the Hor-rmation (Tatar et al., 2004). These observations haveorkers to infer NE-dipping reverse faults in the base-on and Fitch, 1981; Jackson and McKenzie, 1984; Ni and986; Berberian, 1995; Tatar et al., 2004).heastern limit of the Zagros belt is marked by the Mainrse Fault (MZRF) (Stcklin, 1974;Berberian, 1995).How-researchers (Falcon, 1969, 1974; Haynes and McQuillan,, 1994) consider the Sanandaj-Sirjan Zone, a thrust beltetamorphic rocks are exposed, located to the NE of thea segment of the Zagros belt. The Sanandaj-Sirjan Zoneof 150250km, and has structural trends which are

the rest of the Zagros orogenic elements. The tectonicf this zone is dominated by either large-scale compos-tructures or low- and high-angle NE-dipping imbricateavi, 1994).surements indicate that there is clear shortening acrossstern segment of the Main Zagros Reverse Fault and theirjan Zone (Bayer et al., 2006). This zone correspondse area where the aftershock sequence of the 2006 Tiabis concentrated.

alysis

to studyaftershocksof theTiabearthquake, anetworkofree-component stations was deployed around the epi-of the mainshock starting on February 28, a few hoursinshock, and lasting for about 1 month (Fig. 2). Becauseproblems, the networkwas completed only afterMarchded continuously, with a sampling rate of 100Hz andlibrated everyhourwith aGPS receiver. 508 eventswereom the continuous data and analyzed.itial processing, all events were located using an ini-model obtained for the Minab region which is located

hwest of the study area (Yamini-Fard et al., 2007). Toe velocity model, we selected a subset of 279 events,a minimum of 5 stations, with an azimuthal gap less

residual RMS less than 0.3 s and uncertainties both innd depth less than 3km. With these criteria the trade-

the velocity structure and the location of the eventstting TsjTsi (S arrival time to stations i and j, respec-

ame event) versus TpjTpi (P arrival time to stations ictively for same event) for all events and all stations,e a Vp/Vs ratio of 1.78920.004 with 8777 arrival times

rted the arrival times of the selected set of events fory structure using the program VELEST (Kissling, 1988).

282 A. Gholamzadeh et al. / Journal of Geodynamics 47 (2009) 280287

Fig. 2. Conguration of the temporary seismic network deployed from February 28,2006 to April 1, 2006. Triangles are seismic stations.

Because the inversion result is strongly dependent on the start-ing velocityrandomly deach layer)which thesignicantlposed of a sThis multilavelocity disvelocity moThe result otwo layers aaveraging tTable 1). Th

Fig. 3. Vp/Vs ratio computed from 8777 arrival times.

Table 1Final velocity structure calculated by a 1D inversion method for the Tiab region.

Top of the layer (km) P velocity (kms1)

0 6.02 0.04

ini-.calcolaripola, weategod 20

d within 20, and we can use these solutions only to give anion of the type of faulting.

Fig. 4. Determthe right handmodel, we explored a solution space of 100 modelsistributed (with differences as large as 0.5 kms1 inaround our initial model. We kept only the models for1D inversion converged well (e.g., the RMS decreasesy to values less than 0.1 s). Our initial model is com-tack of layers 2km thick, of uniform velocity 6.0 kms1.yered model helps in locating the depths of the largestcontinuities but we do not use it to nd the naldel because the number of unknowns is too large.f these inversions (Fig. 4) suggests that no more thanre resolvable. We compute the nal velocity model byhe results of all inversions that converge (Fig. 4 ande corresponding residual RMS reduces from 0.19 s for

16

the Yammodel

Wewavepof thesphereC. In c10 anstraineindicatination of the velocity structure by 1D inversion of travel times. We use 100 randomly disside. (a) Model with 15 layers, 2 km thick. (b) Simplied starting model based on rst ste6.54 0.08

Fard et al. (2007) model to 0.14 s for the nal two-layer

ulated 41 focal mechanisms using a minimum of 8 P-tieson the focal sphere (Table2). Considering thequalityrity reading and the azimuthal coverage on the focalseparated the solutions into three categories A, B andries A and B, two nodal planes are constrained within, respectively. In category C, none of the planes is con-tributed starting models (left) that converge to the models plotted inp.

A. Gholamzadeh et al. / Journal of Geodynamics 47 (2009) 280287 283

Table 2Parameters of the focal mechanisms: Lat, Lon are the coordinates of the earthquake.

Nb. Date Time Lat Lon Depth Mag de1 Az1 R1 de2 Az2 R2 AzP deP AzT deT Q

1 2006/03/05 532 27.895 56.471 11.5 3.6 49.97 192.31 68.9 44.41 43.27 113.2 297.08 2.88 37.02 73.7 A2 2006/0 30 358.1 180 204.66 37.76 331.53 37.8 B3 2006/0 58.42 335.63 122.2 193.12 61.65 88 8.01 C4 2006/0 20 14.17 180 213.05 41.64 355.28 41.6 C5 2006/0 27.99 306.55 136.78 182.39 23.4 317.07 58.4 C6 2006/0 75.23 76.19 100.35 157.8 29.5 0.26 59 C7 2006/0 46.92 314.54 165.49 177.5 20.7 284.53 37.8 C8 2006/0 80.15 274.17 151.52 139.97 26.95 43.66 12.2 C9 2006/0 54.07 26.92 142.55 262.68 3.83 357.22 49.7 B

10 2006/0 56.17 127.92 127 192.65 4.33 95.23 59.6 C11 2006/0 43.96 41.19 157.82 267.4 18.89 16.67 44 C12 2006/0 6 282.28 90 192.28 39 12.28 51 A13 2006/0 67.48 260.28 117.23 132.16 58.39 10.17 18.1 B14 2006/0 71.25 314.95 143.99 9.49 9.58 271.81 38.38 B15 2006/0 67.48 287.43 159.64 147.56 29.87 239 2.5 C16 2006/0 20 278.68 90 188.68 25 8.68 65 C17 2006/0 24.81 284.86 144.58 154.11 28.02 292.3 54.5 A18 2006/0 71.25 14.37 111.17 255.21 58.39 120.53 23.4 B19 2006/0 30 340.48 180 187.04 37.76 313.91 37.8 B20 2006/0 48.44 324.85 131.93 206.57 4.33 303.99 59.6 C21 2006/0 71.25 99.34 156.14 149 2.5 57.56 29.9 A22 2006/0 61.12 170.31 171.5 34.07 14.48 131.19 25.7 C23 2006/0 75.52 329.9 153.43 20.27 7.44 286.14 28.9 C24 2006/0 72.77 270.16 121.57 336.72 21.47 216.84 51.7 A25 2006/0 77.11 273.17 127.08 335.56 23.09 220.28 45 A26 2006/0 74.52 150.59 161.43 13.43 23.94 282.7 1.63 A27 2006/0 75.28 2006/0 82.29 2006/0 76.30 2006/0 64.331 2006/0 69.32 2006/033 2006/034 2006/035 2006/036 2006/037 2006/038 2006/039 2006/040 2006/041 2006/0

Mag is the ma

4. Aftersho

We reloc(Table 1). Thdene simptation of thwe also plolocated: ERa minimumsystematic bcentral areaThis imagefrom all eveepicenters.

In ordererogeneitywe relocateHypoDD (Wtance betwstations, theby HypoDDclusters. Thquakes andevent pairsand distanc3/05 1922 28.178 56.92 19.6 3.9 90 88.1 603/06 309 28.145 56.903 18.7 4.4 43.88 205.89 49.083/07 1537 28.113 56.868 18.9 3.4 90 104.17 703/07 1822 28.124 57.005 18.1 3.1 71.25 76.24 68.833/08 1230 28.157 56.925 19 4.5 17.96 220.58 55.733/09 1329 28.089 56.906 16.8 3.4 79.45 54.57 44.013/09 2234 28.092 56.945 18.1 2.4 61.98 178.87 11.173/10 2340 28.094 56.911 18.6 2.9 60.5 141.12 42.393/10 2346 28.048 56.822 14.7 2.9 48.44 254.37 48.073/11 1107 28.098 56.943 17 2.7 74.81 147.55 48.243/11 2133 28.089 56.931 21.6 4.7 84 102.28 903/14 1811 27.891 56.517 18.3 2.7 34.78 133.61 42.193/17 1147 27.936 57.105 16.8 3.3 56.17 58.09 22.763/18 1549 28.1 56.945 18 3.0 71.25 189.34 23.863/20 1701 28.077 56.884 16.1 2.6 70 98.68 903/23 1651 27.954 56.736 23 3.6 75.92 47.7 69.353/24 0905 27.957 56.741 21.5 3.3 27.99 244.69 43.223/24 1614 28.164 56.967 20.2 4.2 90 70.48 603/25 1306 28.087 56.966 21.7 2.6 56.17 91.3 533/25 1615 27.947 56.746 19 2.7 67.48 197.43 20.363/25 1824 28.095 56.963 20.4 2.6 82.56 264.44 29.153/25 1908 28.095 56.968 23.4 2.9 64.34 67.02 16.13/25 1912 28.1 56.961 20.8 2.9 35.53 25.9 30.643/25 1912 58.6 56.96 20.2 2.9 38.95 19.61 20.783/25 1914 28.094 56.96 23.1 4.0 72.12 55.47 16.283/25 1916 28.103 56.961 21.6 3.9 39.7 19.97 23.043/25 1920 28.088 56.96 21.2 2.9 61.12 77.81 8.53/25 2004 28.094 56.957 21.1 2.9 83.72 260.75 13.653/25 2031 28.102 56.963 21.3 2.3 75.52 265.1 26.573/25 2040 28.087 56.965 22.5 3.4 69.3 76.73 22.21

3/25 2051 28.09 56.96 21.6 3.1 84.52 85.84 17.17 72.3/25 2157 28.092 56.958 21.5 3.4 83.72 260.75 13.65 76.3/25 2345 28.105 56.965 21.3 3.5 83.59 251.17 39.57 50.3/26 2337 28.101 56.96 21.4 3.1 90 276 25 653/27 2302 28.102 56.917 18.7 3.9 86.47 52.93 44.89 45.3/28 1728 28.088 56.96 21.9 3.2 71.25 64.34 23.86 67.3/29 1431 28.092 56.961 21.6 3.2 64.34 72.02 16.1 75.3/29 1741 27.958 56.871 25.2 3.6 80.15 239.16 28.48 61.93/29 1809 28.097 56.96 20.8 3.2 86.16 81.18 14.51 75.3/30 2106 28.099 56.959 21.6 4.1 74.81 258.65 13.17 77.gnitude, Az, R, de are Azimuth, rake and dip of fault plane 1 and 2. AzP, deP, AzT, deT are a

ck seismicity

ated all 508 events with our calculated velocity modele aftershock distribution is diffuse and it is difcult tole alignments (Fig. 5). In order to ensure our interpre-e aftershock seismicity and of the related active faults,t the epicenters of 354 events that are more preciselyZ and ERH

284 A. Gholamzadeh et al. / Journal of Geodynamics 47 (2009) 280287

Fig. 6. Epicenseismic netwonumber of P pseismicity.

Fig. 7. Seismi(Waldhauser ado not show a

A histogity of them17km (Figin depth, deMZRF, MZPimages at dsections fortrend of thedicular to threlocated alow-angle aBB depth pshows verti

5. Focal m

Focal meless than 18basedonpoa thrust me(mechanismtral distribution of 354 selected aftershocks recorded by temporaryrk located by HYPO71 program: ERZ and ERH

A. Gholamzadeh et al. / Journal of Geodynamics 47 (2009) 280287 285

Fig. 9. Depth distribution of aftershocks in AA in NS direction (left), BB in N45 direction (middle) and CC in EW direction (right) cross-sections (see Fig. 7). From top tobottom, the cross-sections show 354 selected events located by HYPO71, 234 events relocated by the double difference method, and focal mechanisms. The mechanism ofthe largest event is from the main shock CMT solution. A gentle increase of focal depths toward the NE can be observed in the AA and BB cross-sections.

Fig. 10. Calculated focalmechanismswith class A andB: black quality A, gray qualityB. CMT is the CMT solution of mainshock. The majority of aftershock focal mecha-nisms are strike-slip, different from the mainshock reverse mechanism.

Fig. 11. Rose diagramof the azimuth of the P-axes extracted from focalmechanisms.The N3040 orientation from strike-slip mechanisms is dominant (See Table 2).

286 A. Gholamzadeh et al. / Journal of Geodynamics 47 (2009) 280287

2006) and that they occur in the basement. An increase of focaldepths toward the north in this region can be interpretted as avery low-angle thrust dipping north, consistent with the mecha-nism of the main shock and other CMT solutions in the adjacentregions.

Anotherwithin thesion and thmotion betin a northconrm theright-lateraimportantto the Makmotion norone of the scollisionsubetween th

The averslipmechanN45E on thetonic) obseto imply ththe Sanadajthe ZagrosP-axis direc(i.e. N10E)GPS observ2004; Bayeand reversetral Iran anthe transitio

7. Conclus

An after(Mw 6.0) conortheastwZagros Reveof the aftermicroseismas focal deplocated eastconrm repinferred frothe Arabianthe Zagros.

Furthermmechanismtransition frzone. Theresional tectoand strike-between stexplained bness that hdomains.

Acknowled

We wourections, codraft of thianonymoustially impro

team in the eld for installation, maintenance and data collectionin very hard conditions.

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shock study of the February 28, 2006 Tiab earthquakenrms the existence of very low-angle thrusting in aarddirection, to thenorthof the intersectionof theMainrse Fault and MinabZendanPalami fault. The depthsshocks in this region are deeper than well determinedicity focal depths in the Zagros but are almost the sameths of recorded microseimicity in the transition zoneof the MinabZendanPalami fault system. Our resultsorts of deeper seismicity in the region that have beenm teleseismic studies and indicates underthrusting ofplate beneath central Iran at the southeastern end of

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The February 28, 2006 Tiab earthquake, Mw 6.0: Implications for tectonics of the transition between the Zagros continental collision and the Makran subduction zoneIntroductionGeological settingData analysisAftershock seismicityFocal mechanismsDiscussionConclusionsAcknowledgementsReferences