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Holocene slip rate of the North Anatolian Fault beneath the Sea of Marmara
Earth and Planetary Science Letters 227 (2004) 411 – 426 www.elsevier.com/locate/epsl
Holocene slip rate of the North Anatolian Fault beneath the Sea of Marmara A. Poloniaa,*, L. Gasperinia, A. Amorosib, E. Bonattia,c, G. Bortoluzzia, N. C ¸ agatayd, L. Capotondia, M.-H. Cormierc, N. Gorurd,e, C. McHughc, L. Seeberc a
CNR-ISMAR, Sezione Istituto Geologia Marina, Bologna Via Gobetti, 101, 40129 Bologna, Italy b Universita’ degli Studi di Bologna, Dip. Scienze della Terra c Lamont Doherty Earth Observatory, Columbia University, New York, USA d Istanbul Technical University, Istanbul, Turkey e TUBITAK Marmara Research Center Received 5 January 2004; received in revised form 20 July 2004; accepted 21 July 2004 Available online 18 October 2004 Editor: V. Courtillot
Abstract The North Anatolian Fault (NAF) is a major continental transform system that extends E–W across Turkey for over 1600 km, separating the Anatolian and Eurasian plates. A portion of its northern branch runs below the Sea of Marmara. This portion constitutes a bseismic gapQ because the last destructive earthquakes occurred at the western (1912 Ganos earthquake, M 7.4) and eastern (1999 I˙zmit and Dqzce earthquakes, M w 7.4, 7.2) edges of the Marmara basin. It is likely that fault ruptures will fill this gap in the next decades. This region of the North Anatolian Fault is critical to our understanding of fault interactions, stress buildup during seismic cycle and seismic hazard in the Istanbul area. We obtained high-resolution acoustic images of the NAF in the floor of the eastern Marmara Sea (Gulf of I˙zmit), and measured fault-related offsets of 14C-dated subseafloor channels and paleoshorelines. The resulting average slip rate on the fault is ~10 mm/year for the last 10 kyr. This is less than half the total Anatolia–Eurasia relative motion, estimated at 24 mm/year for the last ~10 years from satellite geodetic measurements. We conclude that either much of the strike-slip motion along this branch of the NAF did not occur on the main fault segment or the slip rate increased recently, or both. These results affect kinematic models of the NAF and assessments of seismic hazard for the city of Istanbul and the surrounding region. D 2004 Elsevier B.V. All rights reserved. Keywords: North Anatolian Fault; Marmara Sea; Gulf of Izmit; submarine earthquake geology; slip rate; seismic hazard
* Corresponding author. Tel.: +39 51 6398888; fax: +39 51 6398940. E-mail address: [email protected] (A. Polonia). 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.07.042
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1. Introduction The westward progression of major ruptures along the North Anatolian Fault (NAF) [1], culminating with the 1999 ˙Izmit and Dqzce destructive events, leaves a 170-km-long seismic gap along the Sea of Marmara capable of generating large earthquakes [2,3]. If this progression will continue, a major rupture is expected below the Sea of Marmara [4–6]; the probability of such an event within the next decades is very high [3,7,8]. The hazard facing Istanbul and adjacent areas varies widely depending on where and how the predicted Marmara seismic rupture will take place. The right-lateral NAF splays in the Sea of Marmara region into two major fault branches about 100 km apart. According to geological and geodetic data [9,10] most of the lateral motion is transferred obliquely from the southern to the northern branch across the Marmara basin [11]. Different structural models have been put forward for the NAF in the Marmara Sea region and the tectonic regime is described as resulting from the interaction of Anatolia strike-slip with Aegean extension [12,13], as a single through-going master strikeslip fault [14–22] or as a pull-apart system produced by fault segmentation, oversteps and slip partitioning [9,11,23–28]. Estimates of the slip rate along the NAF over geological times and distribution of motion along the various strands of the NAF, can help reach a realistic assessment of seismic hazards for this populous area of Turkey. Geodetic measurements [10] suggest a 24 mm/year right-lateral motion between Anatolia and Eurasia in this region, with more than 80% of the total motion, i.e., more than 20 mm/year, along the northern branch [11,29]. GPS geodetic measurements cover a period of 10 years [10], a short interval of time compared to recurrence period of major earthquakes in the region [30]. On the other hand, the slip rate over a very large time scale (5 Ma) has been reconstructed through the analysis of displaced geological features in the Ganos region [9]. We present an estimate of the Holocene slip rate along individual fault segments identified on the floor of the Gulf of ˙Izmit (Fig. 1), where highresolution bathymetry and seismic reflection profiles detected displacement of sedimentary features along the NAF main strand. Displacement of geomorphic
or cultural features across seismogenic faults are a common tool on land to determine slip distribution after large earthquakes or to estimate average rates of deformation cumulated after several events. The advantage of attempting this on the sea floor is that, in general, stable and continuous sedimentary sequences are preserved better below than above water; however, submarine paleoseismology requires indirect acoustic techniques as well as high-precision sediment sampling.
2. Methods The shelves and adjacent slopes of the northeastern Sea of Marmara, including the Gulf of I˙zmit, were mapped by high-resolution multibeam bathymetry and side-scan sonar during two cruises [31] with the R/V Odin Finder (October 2000) and R/V Urania (Spring 2001). We also carried out high-resolution, shallow penetration seismic reflection profiles (subbottom CHIRP and multichannel seismic profiles) and obtained ROV images along ruptures of the seafloor. bGround-truthingQ of the high-resolution geophysical imagery was obtained by high-precision sediment coring, particularly in the vicinity of faults, in order to date and correlate seismic events [31]. CHIRP profiles and cores were acquired on tight grids across specific fault strands.
3. Structural setting and active faults The northern branch of the NAF enters the Gulf of I˙zmit from the east, where it ruptured during the 1999 earthquakes. The Gulf of I˙zmit is at the transition between the linear, strike-slip regime of the NAF system to the east and the transtensional (pull-apart) basins of the Marmara Sea to the west. Three interconnected basins (Western, Karamqrsel and Eastern) mark extensional deformation within a dominant strike-slip regime [32]. Oversteps, splays and bending of the fault-track are visible at different scales, giving rise to second-order features, as pressure ridges and small troughs (Figs. 1 and 2) which probably play secondary roles in the plate motion but may affect the seismic behavior of the major faults.
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Fig. 1. (a) Bathymetric map of the Gulf of I˙zmit which combines the Odin Finder and Urania multibeam data, with on-land European Research Satellite SAR interferometric Digital Elevation Model. Simplified structural interpretation is derived from integrated analysis of geophysical data, including morphobathymetry, CHIRP-sonar and multichannel seismics. The three boxes (AREA-1, -2, -3) represent the regions where tight grids of geophysical data have been collected across piercing points.
Although the physiography of the two main basins, Karamqrsel and Western, appears to be different in morphobathymetric maps, their deep structure, imaged by our MCS profiles [33], is very similar. They are depressions, bounded by short, en-enchelon extensional and strike-slip segments, forming a sequence of pull-apart basins (Fig. 1). The direction of maximum extension is ENE–WSW, while strikeslip segments trend E–W. The Karamqrsel basin shows two pull-apart basins separated by a topographic relief. This relief is due to a symmetric fold (axis oriented ENE–WSW) that bounds the northwestern edge of the deeper basin. The most prominent tectonic features are WNW–ESE striking normal fault segments, dipping toward the center of the basin. The inner and the outer normal fault systems are parallel and have counterparts in the basin’s edges. This is indicative of a bsteady-stateQ regime, with a rather constant position and geometry of the principal displacement zone. The main fault track intersects the Hersek Promontory at 29831VE. About 4 km east of this feature, at 29834VE, the fault splays in two branches that cross the promontory at two different locations (Fig. 1). West of the Hersek Promontory, the deformation zone becomes broader and it includes two different fault strands. To the north, the main NAF corresponds to several en-enchelon faults, each consisting of west– northwest, left-stepping fault segments (Fig. 2). Each
segment, around 1-km long, produces 1-m high topographic scarps and displaces Holocene sediments [31]. About 1 km to the south, the secondary NAF is marked by two E–W strike-slip faults that die out at a NE–SW topographic high (Fig. 2). West of the topographic high, the southernmost strike-slip fault merges to the main NAF at about 29824VE, through a series of NW–SE en-echelon faults. West of this area, a single major fault was found and traced with a resolution of a few meters. Recent deformation appears to be localized within a relatively narrow (few tens of meters) zone and to be mainly strike-slip. Westward, toward the C¸inarcik Basin, the NAF follows the south wall of a deeply eroded canyon (Fig. 2).
4. Three key areas We collected tight grids (60–80 m line spacing) of multibeam and CHIRP profiles and core transects in three key areas (Fig. 3) in the western basin, where we recognized sedimentary features displaced along the NAF trace (i.e., piercing points). The late Quaternary paleoceanographic history of Sea of Marmara is controlled by intermittent connections with the Mediterranean Sea on one side and the Black Sea on the other. Several lines of evidence indicate that Marmara was a fresh water basin during
414 A. Polonia et al. / Earth and Planetary Science Letters 227 (2004) 411–426 Fig. 2. Shaded relief map of the western basin with structural interpretation derived from the analysis of multibeam and seismic data. (A–AV) E/W oriented CHIRP profile across the southern strand of the NAF. A fluvial channel is well imaged below 15 m of Holocene sediments. (B–BV) N/S oriented CHIRP profile across the wide and diffuse NAF displacement zone to the east. Deformation in this area is accommodated by short, en-echelon and south-dipping fault to the north, and vertical strike-slip segments to the south.
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Fig. 3. Location of the CHIRP profiles collected in the western basin. In the three key areas, the average spacing between lines is about 80 m.
the late last Glacial and early deglaciation, up to ~12 kyr BP Subsequently, inundation by Mediterranean salty waters gradually converted it into a marine basin [34]. This major event is best recorded on the shelves, which were exposed during the Last Glacial Maximum (LGM), when sea level reached 120 m below the present level, and the connection with the Mediterranean through the Dardanelles (sill depth at 85 m) was interrupted. Correlations of onshore and offshore stratigraphic units indicate that the coarse late Glacial sediments of the western Gulf of I˙zmit above 85 m overly an erosional surface formed during the last Glacial sea-level low-stand. These deposits are overlain by a thick (up to 20–25 m) transgressive/ high-stand sequence, largely constituted by pelagic mud. This upper unit fills and drapes topographic– sedimentary features at its base, such as channels and scarps, formed during the last low-stand and early transgression, preserving their original geometries and morphology. Close-spaced grids of subbottom seismic reflection profiles reveal that these features, presently inactive, are displaced by tectonic deformation. We focused on
high-resolution images of the seafloor and of the shallow subsurface obtained in three key areas (Figs. 1 and 3), where a submerged canyon, a paleoshoreline, a fluvial channel and a paleoisland were identified. We used these features as bpiercing pointsQ to determine amounts and rates of deformation. 4.1. AREA1: displaced submarine canyon A submarine canyon connects the western Gulf of I˙zmit continental shelf to the C ¸ inarcik, 1200-m-deep basin (Figs. 1 and 2). The east–west-trending canyon bends sharply to the south of 908 at 29823V; the E–Wtrending NAF (Fig. 4a) offsets right-laterally the N–S canyon axis (Figs. 2 and 4b). We used multibeam bathymetry and subbottom CHIRP profiles to image morphology and identify, acoustically, different seabottom domains (Fig. 5). Based on acoustic reflectivity, i.e., on physical properties of the sediments, we matched similar patterns on opposite sides of the faults. Comparing reflectivity profiles collected N and S of the main fault track, as well as reflectivity patterns superimposed on bathymetry (Fig. 5), we
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Fig. 4. CHIRP profiles collected in AREA-1. (a) N/S-oriented profile b2-32 across the NAF, east of the submarine canyon, showing the deformation style in the narrow displacement zone. (b) E/W-oriented profile b2-20 across the offset canyon. The canyon floor is flat and filled by high reflectivity sediments that hamper seismic penetration below few msec.
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Fig. 5. Key AREA-1: shaded relief multibeam bathymetric map (contour each 5 m, illumination from NE) draped by sea-floor reflectivity from CHIRP sonar data (red to blue colour scale). Red colour indicates high reflectivity while blue colour is related to low-reflectivity. The flatbottomed canyon is displaced by the main NAF strand. The offset canyon is at depth of 190 m. This implies that it was carved under water (at approximately 100-m water depth or even shallower) and the change from erosion to deposition would have been associated with the Holocene marine transgression with water depth increase of 85 m. (a) Two reflectivity profiles outside the displacement zone have been chosen in order to correlate zones of similar reflectivity pattern north and south of the fault zone. This allows us to estimate a horizontal slip of about 100 m because the canyon became inactive after the last episode of sea-level rise.
estimated a ~100-m dextral strike-slip displacement of the canyon head along the fault track. Information on kinematics of the fault requires an estimate of the interval of time, during which the ~100-m strike-slip motion took place. The flat bottom of the canyon suggests that it is presently inactive, i.e., that its regime changed from bincisionQ to bdepositionQ sometime in the past. Since then, the canyon has been offset bpassivelyQ by the fault. We assume that the change from berosionQ to bdepositionQ took place in the canyon during the last episode of transgression dated 11 000–10 000 years BP.
4.2. AREA2: channel
85 m paleoshoreline and fluvial
High-resolution bathymetry and subbottom CHIRP profiles revealed a prominent contour-parallel scarp at a depth of approximately 85 m. The scarp outlines a brough–smoothQ transition across the 85-m isobath and subhorizontal sedimentary sequences on-lapping near the isobath [31]; it trends ~N–S and intersects the fault trace at 29824V45UE, immediately S of a smooth bend towards E (Fig. 2). Subbottom profiles show subsurface strata outcropping at the scarp that may be
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the result of beach erosion and low-stand deposits (Fig. 6a). The integrated analyses of sediment cores, including facies analysis and quantitative assessment of foraminiferal and ostracod distribution [35], provides invaluable paleodepth information, leading to refined identification of a paleoshoreline at ~85 m below the present day sea level. The sill depth in the Dardanelles Strait during the Last Glacial Maximum, estimated at 85 m below present sea level [36,37], may have controlled the formation of the 85-m paleoshoreline in the fresh water Marmara Lake [34]. Immediately E of the paleoshoreline, our CHIRP records show the presence of a V-shaped erosional channel (Figs. 2A and 6a). This channel is now inactive, having been partially buried by a thick (up to 20 msec TWT) transparent unit that hampers its recognition from morphobathymetric data alone. A close-spaced grid of CHIRP profiles (Figs. 1 and 3) was obtained where the buried channel intersects the NAF displacement zone (AREA2) that in this area is constituted by the main strand to the north, and the secondary splay to the south (Fig. 6b). We were able to reconstruct and map the subbottom topography of this area (Fig. 7b) before the last episode of sea level rise by picking the erosional base of the Holocene mud deposits within each single CHIRP profile. The map shows a channel meandering landward of the 85 m paleoshoreline within a coastal plain: as it approaches the NAF displacement zone, it is displaced by the main fault track and by the secondary splay to the S. A close-up view indicates right-lateral displacements of the channel of about 80 m along the main fault (Fig. 7b, inset 1), and of about 10 m along the secondary splay (Fig. 7b, inset 2). Five cores were recovered along a N–S transect across the channel. Core IZ 20 is 264-cm long and was recovered at 83 m water depth north of the major fault (Figs. 7b and 8). The lower part of the core, between 264 and 224 cm, includes laminated clays with freshwater molluscs. These are sharply overlain by a few centimeterthick organic layer, black in colour, with upward transition to a distinctive alternation of organic-rich clays and very thin sand layers. This succession is capped by about 15 cm of homogeneous clay, with rare marine fossils (194–180 cm). A sharp discontinuity at 180 cm separates this part of the core from an
overlying 3-cm-thick sand layer, containing abundant mollusk shells of infralittoral conditions, with pebbles and wood fragments. This layer is sharply overlain by 177 cm of homogeneous mud, with local flat lamination. A relatively darker colour is recorded between 160 and 100 cm. The lower part of the core is interpreted to reflect lake conditions, with upward transition to swamp- and channel-related (levee or crevasse splay) deposits. Channel abandonment due to rising sea level is documented by the onset of a marine fauna and slightly predates shoreline transgression. Radiocarbon dates obtained from organic material sampled at 175– 176 cm suggest a 10.200F50 year BP age for the paleoshoreline. The sharp boundary with the overlying clay suggests rapid deepening and transition to present conditions. The darker interval observed between 160 and 100 cm could be related to slight oxygen deficiency conditions (lower sapropelic interval of C ¸ agatay et al. [38]). 4.3. AREA3: paleoshoreline and paleoisland This area is located where the deformation zone across the main fault segment is wider. The main fault track is characterized here by series of short left stepping en-enchelon segments outcropping on the sea bottom. These segments are paralleled 1 km to the S by the track of the secondary splay that merges toward the main fault in AREA-1, at 29824V20UE (Fig. 2). The distance between the main fault and the secondary splay is maximum at 29826V30UE, where we observed a prominent topographic high (minimum depth of 35 m) bounded to the N by the track of the splay (Fig. 2). Subbottom CHIRP profiles show that this feature is formed by a transpressional fold with its axis oriented N45. We interpret this topographic high as a bpaleoislandQ because it shows wave-cut terraces at its top ( 45 m of depth) and because cores collected at different stations sampled coarse-grained sediments with pebbles and gravel. Core IZ 25 is 350-cm long and was collected at 42.9 m water depth (Fig. 2 and 9). The lower part of the core, between 350 and 135 cm, is dominated by homogeneous clay with abundant fossils diagnostic of freshwater conditions (Dreissena). Between 135 and 117 cm, marine molluscs are
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Fig. 6. CHIRP profiles collected in AREA-2. (a) E/W-oriented profile b1-50 across the 85 m paleoshoreline and the buried river channel. Location of core IZ-20 is indicated. (b) N/S-oriented profile b1-3 across the NAF displacement zone in the region where the southern splay merges towards the main strand.
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Fig. 7. Key area 2. Left: shaded relief multibeam bathymetric map (contour each 1 m, illumination from NE). Right: gradient map of the base of the Holocene sediments obtained through a semiautomatic picking of CHIRP-subbottom profiles. It images sedimentary and tectonic features below the holocenic drape. (1) Laplacian (gray scale) and contour lines of the Holocene base topography in the area where the river channel intersects the main NAF strand. (2) Laplacian (gray scale) and contour lines of the Holocene base topography in the area where the river channel intersects the southern strand. The gradient map of the Holocene base topography shows the meandering river channel which is displaced along the NAF fault strands. We calculate an offset of ~80 and ~10 m in the main and secondary strand respectively. Location of CHIRP profile b1-29 shown in Fig. 2 is indicated.
associated with this freshwater fauna. A sandy succession becoming finer upwards was recorded between 117 and 92 cm; its erosional base, the presence of pebbles and the faunal assemblage are characteristic of a littoral environment. This interval shows upward transition to a thin layer made up of homogeneous mud. The uppermost 69 cm consist of fine, medium and coarse sand, with a sharp lower boundary and very abundant infralittoral shells. The lower part of the core documents lake conditions, with upward transition to a mixed layer, showing a progressive increase in marine influence. The sand layer and its gradual upward transition to a mud-dominated interval are inferred to represent a transgressive succession from nearshore to offshore deposits. Vertical changes in pollen assemblages suggest that the 45 m paleoshoreline identified on top of the paleoisland, could be regarded as coeval with the 85 m paleoshoreline observed within AREA1. However, due to the pollen stratigraphy resolution limits, we cannot exclude that the palaeoshoreline may be correlated to different sea level still stand events in
the Marmara Sea. Paleoshorelines have been observed at different depths in the Marmara Sea, including the 65 m scarp described by C ¸ agatay et al. [38] in other areas of the Western and Karamqrsel Basins.
5. Slip rate estimates The offsets measured at the canyon and at the fluvial channel are similar in length, within an error of 10%. This error is due to a combination of vertical and horizontal resolution of the multibeam and CHIRP sonars, of the positioning (differential GPS) and of the discrete sampling of the feature at the grid knots that we interpolated in order to obtain the topographic images of the subsurface. The errors in the age estimates are related to 14C determinations, if we assume our reconstruction of paleooceanographic events is correct. We assume an error well within 5% for the 14C dating. We stress that 14 C ages are supported by other proxies (pollen and microfossils).
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Fig. 8. Lithology, facies association and photograph of core IZ-20 collected close to the fluvial channel in AREA-2. To the right, the CHIRP subbottom profile collected during coring operations.
Taking into account all possible errors we estimate an average rate of 10F1.5 mm/year for the rightlateral motion along this major strand of the NAF during the Holocene (i.e., the last 10.000 years).
The offset channel provides not only an estimate of horizontal slip rate, but also of vertical slip rate. If we assume that the channel probably was not forming a bwater fallQ across the fault strand but had a relatively
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Fig. 9. Lithology, facies association and photograph of core IZ-25 collected on the paleo-island in AREA-3. To the right, the CHIRP subbottom profile collected during coring operations.
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smooth profile (i.e., the basement was eroded subhorizontally during the LGM), then we can directly estimate the Holocene vertical subsidence of the southern block relative to the northern one. The vertical slip rate thus calculated is in the order of 2– 2.4 mm/year, equally distributed between the northern and southern strand of the fault in this region. This is a maximum value because the fault scarp may have existed also during the LGM, and the assumption of a smooth profile across the principal displacement zone can overestimate the vertical slip rate. On the other hand, core IZ-25, collected on the paleoisland, has an important bearing for the kinematics of the fault. The formation of the paleoisland may be due to localized transpressive deformation related to a left-step of the secondary splay (pressure ridge). If the paleoshoreline sampled on the pressure ridge is 10.000 years old and formed at 85 m, then the paleoisland must have undergone a vertical uplift in the range of 40 m (vertical slip rate of 4 mm/year). This is a minimum value because the 85m paleoshoreline is deeper in the block south of the fault. Unfortunately, we were not able to detect the paleoshoreline south of the fault, but we can argue that the vertical throw is similar to that described for the LGM erosional surface (i.e., about 10 m). Thus, the local vertical uplift rate of the fault along the pressure ridge would be about 5 mm/year. W of the pressure ridge, deformation during the same period is negligible. This observation reinforces the assumption of a narrow deformation zone in AREA1 and AREA2.
date the strike-slip deformation, while NW–SE oriented structures are mainly transtensive. In fact, slip rate estimates in AREA 2 show how the NW–SEoriented fault strand has similar horizontal and vertical slip rates (about 1 mm/year), while the main E–W fault segment has horizontal slip rate eight to nine times larger than vertical slip rate. A relatively wide deformation zone (2–3 km) is shown by multi- and single-channel reflection profiles in some areas of the Gulf of I˙zmit [19,31,33]. On the other hand, other regions, such as those of boxes 1 and 2, are characterised by a relatively narrow displacement zone where the deformation is localised near the fault trace [31]. Multichannel and subbottom seismic profiles obtained close to AREAS 1 and 2 [33] show several fault segments on both side of the principal displacement zone that do not affect the Holocene sediments.
6. Width of strike-slip deformation along the NAF
Although minor secondary displacements (secondary splays, en-enchelon normal faults, etc.) are common within the NAF in the Gulf of I˙zmit, they show negligible offsets during the Holocene within the studied areas. The same is true for the northweststriking normal fault observed on the southern shore in front of AREAS 1 and 2, where the faults are draped by younger sediments; thus, they probably ceased their activity sometime within the Quaternary [20,39]. Therefore, we assume these faults did not rupture during the last 10.000 years (at least not in the areas where we measure the fault displacement) and did not contribute to the overall Anatolia–Eurasia relative plate motion during the last 10.000 years. We
Most oceanic transform faults have a relatively narrow strike-slip deformation zone because relative homogeneity of the upper crust favours a focusing of the strain. In contrast, continental transform faults, such as the NAF, are generally emplaced on variety of different terrains in terms of age, lithology and mechanical properties, and show a broad deformation zone. The NAF in the Gulf of I˙zmit shows fault stepovers, segmentation and strain partitioning between strike-slip and extension, with the effect of further widening the deformation zone. Our data in AREA 2 show that the E–W oriented fault strands accommo-
7. Geological versus geodetic slip rate estimate Our 10 mm/year estimate for the Holocene slip rate along the main segment of the NAF in the Gulf of I˙zmit is less than half the geodetic estimate for the relative motion between the Anatolian and Eurasian plates during the period spanning from 1988 to 1997 [10]. Two different scenarios may explain the contrast between the two different estimates of slip rate. Hypothesis (1). The Holocene plate motion along this major branch of the NAF has not been focused along a single fault but it has been diffuse and distributed among different fault strands.
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cannot exclude that the numerous faults bounding onland the Gulf of ˙Izmit were active during the Holocene, contributing to increase the slip rate along the plate boundary. They have been described, however, as bnorth-dipping normal faultsQ [21]. Hypothesis (2). The GPS geodetic measurements reflect a recent increase in the rate of strike-slip motion and are not representative of the long-term behavior of the fault at a time scale close to the recurrence interval of the major earthquakes. Despite the dense coverage of the GPS stations, the critical factor remains the period of observation compared to the seismic cycle. According to the historical record, the recurrence interval of major events in this area is about 200–250 years [30,40], i.e., one order of magnitude larger than the time interval of the GPS measurements. Several decades of GPS data would be necessary to obtain a statistically meaningful sampling of the total long term-slip (coseismic+postseismic), representative of permanent deformation and plate tectonic motion. Long-term slip rate estimates from geological reconstructions [9,41] are systematically smaller than 24 mm/year and are in the range of 14–20 mm/year during the last 3–4 Ma. Recently, Provost et al. [42], through a 3-D mechanical modeling of the GPS velocity field, provided an estimate of the NAF slip rate in the Marmara Sea in the order of 17.5 mm/year, which is a value significantly smaller than the 24 mm/year rate of McClusky et al. [10]. Moreover, they suggest that the Anatolia motion is not constant with distance along the fault (as pointed out by Mart and Ryan [43]). Discrepancies between Provost et al. [42] and previous studies are particularly evident in the Marmara region where additional complexity in the fault geometry and/or rheology of the crust has to be invoked. Very recently, a simple analytic viscoelastic model of the surface velocity field through a clustered earthquake cycle has been proposed to explain apparent differences between short- and long-term fault slip rates observed in many active regions of the world [44]. The model suggests that discrepancies between geologic and geodetic estimates of slip rate may be related to the clustering of earthquakes in time on the same fault that affects the rate and pattern of interseismic deformation.
Although a combination of these effects (i.e., plate motion distributed among different fault strands, recent increase in plate motion and/or interseismic deformation related to earthquake clustering) is most likely, available geophysical data favor a narrow deformation zone across the main NAF strand for the last 10.000 years within our study area. Consequently, we assume that the 10 mm/year slip rate is representative of the long-term behavior of the fault. This finding has important implications on kinematic models of the NAF and on assessment of seismic hazard for the city of Istanbul and the surrounding region.
8. Assessment of hazards Many authors [3,5,7] pointed out that, after the 1999 earthquake, the Coulomb stress onto faults west of the rupture rose, increasing the probability of an earthquake closer to Istanbul. The Marmara Sea is considered prone to rupture because it represents a seismic gap where 5.5 m of slip has accumulated since 1766 [3]. All these authors assumed an average slip rate in the range of 2–3 cm/year neglecting high creep rates for faults in the Sea of Marmara. Reducing the slip rate would not alter the spatial pattern of estimated historical slip as pointed out by Hubert-Ferrari et al. [3], but could reduce the amount of slip accumulated on single-fault branches of the NAF since the last seismic events. This possibility has to be taken into account for the assessment of seismic hazard that would consequently decrease. However, we cannot exclude the possibility that other secondary fault strands played an important role in absorbing the plate motion during the last 10.000 years. In fact, we lack multibeam coverage in water depth shallower than 30 m and we cannot exclude the presence of strike-slip faults parallel to the main displacement zone described at sea. Acquisition of further data in shallow water and sea–onland correlations are thus essential to understand fault behaviour in the Sea of Marmara.
Acknowledgements This research is supported by a Progetto Strategico of the Italian National Research Council (CNR), the Turkish Council for Scientific and Technical Research,
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grant OCE-0096668 from the National Science Foundation and Collaborative Linkage Grant #976826 from the North Atlantic Treaty Organization. We are grateful to the scientific party, captains and the ship crews onboard the R/V Odin Finder and Urania parties. Most figures were generated using the GMT software [45]. We are greatly indebited to Giovanni Bortoluzzi for his help in processing multibeam data and for fruitful discussions. We thank R. Armijo and Y. Mart for thoughtful reviews. We also thank X. LePichon for providing preliminary bathymetric data for cruise planning purposes and A.M.C. Sengor for fruitfull discussions during the Marmara-2000 cruise.
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Holocene slip rate of the North Anatolian Fault beneath the Sea of Marmara A. Poloniaa,*, L. Gasperinia, A. Amorosib, E. Bonattia,c, G. Bortoluzzia, N. C ¸ agatayd, L. Capotondia, M.-H. Cormierc, N. Gorurd,e, C. McHughc, L. Seeberc a
CNR-ISMAR, Sezione Istituto Geologia Marina, Bologna Via Gobetti, 101, 40129 Bologna, Italy b Universita’ degli Studi di Bologna, Dip. Scienze della Terra c Lamont Doherty Earth Observatory, Columbia University, New York, USA d Istanbul Technical University, Istanbul, Turkey e TUBITAK Marmara Research Center Received 5 January 2004; received in revised form 20 July 2004; accepted 21 July 2004 Available online 18 October 2004 Editor: V. Courtillot
Abstract The North Anatolian Fault (NAF) is a major continental transform system that extends E–W across Turkey for over 1600 km, separating the Anatolian and Eurasian plates. A portion of its northern branch runs below the Sea of Marmara. This portion constitutes a bseismic gapQ because the last destructive earthquakes occurred at the western (1912 Ganos earthquake, M 7.4) and eastern (1999 I˙zmit and Dqzce earthquakes, M w 7.4, 7.2) edges of the Marmara basin. It is likely that fault ruptures will fill this gap in the next decades. This region of the North Anatolian Fault is critical to our understanding of fault interactions, stress buildup during seismic cycle and seismic hazard in the Istanbul area. We obtained high-resolution acoustic images of the NAF in the floor of the eastern Marmara Sea (Gulf of I˙zmit), and measured fault-related offsets of 14C-dated subseafloor channels and paleoshorelines. The resulting average slip rate on the fault is ~10 mm/year for the last 10 kyr. This is less than half the total Anatolia–Eurasia relative motion, estimated at 24 mm/year for the last ~10 years from satellite geodetic measurements. We conclude that either much of the strike-slip motion along this branch of the NAF did not occur on the main fault segment or the slip rate increased recently, or both. These results affect kinematic models of the NAF and assessments of seismic hazard for the city of Istanbul and the surrounding region. D 2004 Elsevier B.V. All rights reserved. Keywords: North Anatolian Fault; Marmara Sea; Gulf of Izmit; submarine earthquake geology; slip rate; seismic hazard
* Corresponding author. Tel.: +39 51 6398888; fax: +39 51 6398940. E-mail address: [email protected] (A. Polonia). 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.07.042
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1. Introduction The westward progression of major ruptures along the North Anatolian Fault (NAF) [1], culminating with the 1999 ˙Izmit and Dqzce destructive events, leaves a 170-km-long seismic gap along the Sea of Marmara capable of generating large earthquakes [2,3]. If this progression will continue, a major rupture is expected below the Sea of Marmara [4–6]; the probability of such an event within the next decades is very high [3,7,8]. The hazard facing Istanbul and adjacent areas varies widely depending on where and how the predicted Marmara seismic rupture will take place. The right-lateral NAF splays in the Sea of Marmara region into two major fault branches about 100 km apart. According to geological and geodetic data [9,10] most of the lateral motion is transferred obliquely from the southern to the northern branch across the Marmara basin [11]. Different structural models have been put forward for the NAF in the Marmara Sea region and the tectonic regime is described as resulting from the interaction of Anatolia strike-slip with Aegean extension [12,13], as a single through-going master strikeslip fault [14–22] or as a pull-apart system produced by fault segmentation, oversteps and slip partitioning [9,11,23–28]. Estimates of the slip rate along the NAF over geological times and distribution of motion along the various strands of the NAF, can help reach a realistic assessment of seismic hazards for this populous area of Turkey. Geodetic measurements [10] suggest a 24 mm/year right-lateral motion between Anatolia and Eurasia in this region, with more than 80% of the total motion, i.e., more than 20 mm/year, along the northern branch [11,29]. GPS geodetic measurements cover a period of 10 years [10], a short interval of time compared to recurrence period of major earthquakes in the region [30]. On the other hand, the slip rate over a very large time scale (5 Ma) has been reconstructed through the analysis of displaced geological features in the Ganos region [9]. We present an estimate of the Holocene slip rate along individual fault segments identified on the floor of the Gulf of ˙Izmit (Fig. 1), where highresolution bathymetry and seismic reflection profiles detected displacement of sedimentary features along the NAF main strand. Displacement of geomorphic
or cultural features across seismogenic faults are a common tool on land to determine slip distribution after large earthquakes or to estimate average rates of deformation cumulated after several events. The advantage of attempting this on the sea floor is that, in general, stable and continuous sedimentary sequences are preserved better below than above water; however, submarine paleoseismology requires indirect acoustic techniques as well as high-precision sediment sampling.
2. Methods The shelves and adjacent slopes of the northeastern Sea of Marmara, including the Gulf of I˙zmit, were mapped by high-resolution multibeam bathymetry and side-scan sonar during two cruises [31] with the R/V Odin Finder (October 2000) and R/V Urania (Spring 2001). We also carried out high-resolution, shallow penetration seismic reflection profiles (subbottom CHIRP and multichannel seismic profiles) and obtained ROV images along ruptures of the seafloor. bGround-truthingQ of the high-resolution geophysical imagery was obtained by high-precision sediment coring, particularly in the vicinity of faults, in order to date and correlate seismic events [31]. CHIRP profiles and cores were acquired on tight grids across specific fault strands.
3. Structural setting and active faults The northern branch of the NAF enters the Gulf of I˙zmit from the east, where it ruptured during the 1999 earthquakes. The Gulf of I˙zmit is at the transition between the linear, strike-slip regime of the NAF system to the east and the transtensional (pull-apart) basins of the Marmara Sea to the west. Three interconnected basins (Western, Karamqrsel and Eastern) mark extensional deformation within a dominant strike-slip regime [32]. Oversteps, splays and bending of the fault-track are visible at different scales, giving rise to second-order features, as pressure ridges and small troughs (Figs. 1 and 2) which probably play secondary roles in the plate motion but may affect the seismic behavior of the major faults.
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Fig. 1. (a) Bathymetric map of the Gulf of I˙zmit which combines the Odin Finder and Urania multibeam data, with on-land European Research Satellite SAR interferometric Digital Elevation Model. Simplified structural interpretation is derived from integrated analysis of geophysical data, including morphobathymetry, CHIRP-sonar and multichannel seismics. The three boxes (AREA-1, -2, -3) represent the regions where tight grids of geophysical data have been collected across piercing points.
Although the physiography of the two main basins, Karamqrsel and Western, appears to be different in morphobathymetric maps, their deep structure, imaged by our MCS profiles [33], is very similar. They are depressions, bounded by short, en-enchelon extensional and strike-slip segments, forming a sequence of pull-apart basins (Fig. 1). The direction of maximum extension is ENE–WSW, while strikeslip segments trend E–W. The Karamqrsel basin shows two pull-apart basins separated by a topographic relief. This relief is due to a symmetric fold (axis oriented ENE–WSW) that bounds the northwestern edge of the deeper basin. The most prominent tectonic features are WNW–ESE striking normal fault segments, dipping toward the center of the basin. The inner and the outer normal fault systems are parallel and have counterparts in the basin’s edges. This is indicative of a bsteady-stateQ regime, with a rather constant position and geometry of the principal displacement zone. The main fault track intersects the Hersek Promontory at 29831VE. About 4 km east of this feature, at 29834VE, the fault splays in two branches that cross the promontory at two different locations (Fig. 1). West of the Hersek Promontory, the deformation zone becomes broader and it includes two different fault strands. To the north, the main NAF corresponds to several en-enchelon faults, each consisting of west– northwest, left-stepping fault segments (Fig. 2). Each
segment, around 1-km long, produces 1-m high topographic scarps and displaces Holocene sediments [31]. About 1 km to the south, the secondary NAF is marked by two E–W strike-slip faults that die out at a NE–SW topographic high (Fig. 2). West of the topographic high, the southernmost strike-slip fault merges to the main NAF at about 29824VE, through a series of NW–SE en-echelon faults. West of this area, a single major fault was found and traced with a resolution of a few meters. Recent deformation appears to be localized within a relatively narrow (few tens of meters) zone and to be mainly strike-slip. Westward, toward the C¸inarcik Basin, the NAF follows the south wall of a deeply eroded canyon (Fig. 2).
4. Three key areas We collected tight grids (60–80 m line spacing) of multibeam and CHIRP profiles and core transects in three key areas (Fig. 3) in the western basin, where we recognized sedimentary features displaced along the NAF trace (i.e., piercing points). The late Quaternary paleoceanographic history of Sea of Marmara is controlled by intermittent connections with the Mediterranean Sea on one side and the Black Sea on the other. Several lines of evidence indicate that Marmara was a fresh water basin during
414 A. Polonia et al. / Earth and Planetary Science Letters 227 (2004) 411–426 Fig. 2. Shaded relief map of the western basin with structural interpretation derived from the analysis of multibeam and seismic data. (A–AV) E/W oriented CHIRP profile across the southern strand of the NAF. A fluvial channel is well imaged below 15 m of Holocene sediments. (B–BV) N/S oriented CHIRP profile across the wide and diffuse NAF displacement zone to the east. Deformation in this area is accommodated by short, en-echelon and south-dipping fault to the north, and vertical strike-slip segments to the south.
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Fig. 3. Location of the CHIRP profiles collected in the western basin. In the three key areas, the average spacing between lines is about 80 m.
the late last Glacial and early deglaciation, up to ~12 kyr BP Subsequently, inundation by Mediterranean salty waters gradually converted it into a marine basin [34]. This major event is best recorded on the shelves, which were exposed during the Last Glacial Maximum (LGM), when sea level reached 120 m below the present level, and the connection with the Mediterranean through the Dardanelles (sill depth at 85 m) was interrupted. Correlations of onshore and offshore stratigraphic units indicate that the coarse late Glacial sediments of the western Gulf of I˙zmit above 85 m overly an erosional surface formed during the last Glacial sea-level low-stand. These deposits are overlain by a thick (up to 20–25 m) transgressive/ high-stand sequence, largely constituted by pelagic mud. This upper unit fills and drapes topographic– sedimentary features at its base, such as channels and scarps, formed during the last low-stand and early transgression, preserving their original geometries and morphology. Close-spaced grids of subbottom seismic reflection profiles reveal that these features, presently inactive, are displaced by tectonic deformation. We focused on
high-resolution images of the seafloor and of the shallow subsurface obtained in three key areas (Figs. 1 and 3), where a submerged canyon, a paleoshoreline, a fluvial channel and a paleoisland were identified. We used these features as bpiercing pointsQ to determine amounts and rates of deformation. 4.1. AREA1: displaced submarine canyon A submarine canyon connects the western Gulf of I˙zmit continental shelf to the C ¸ inarcik, 1200-m-deep basin (Figs. 1 and 2). The east–west-trending canyon bends sharply to the south of 908 at 29823V; the E–Wtrending NAF (Fig. 4a) offsets right-laterally the N–S canyon axis (Figs. 2 and 4b). We used multibeam bathymetry and subbottom CHIRP profiles to image morphology and identify, acoustically, different seabottom domains (Fig. 5). Based on acoustic reflectivity, i.e., on physical properties of the sediments, we matched similar patterns on opposite sides of the faults. Comparing reflectivity profiles collected N and S of the main fault track, as well as reflectivity patterns superimposed on bathymetry (Fig. 5), we
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Fig. 4. CHIRP profiles collected in AREA-1. (a) N/S-oriented profile b2-32 across the NAF, east of the submarine canyon, showing the deformation style in the narrow displacement zone. (b) E/W-oriented profile b2-20 across the offset canyon. The canyon floor is flat and filled by high reflectivity sediments that hamper seismic penetration below few msec.
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Fig. 5. Key AREA-1: shaded relief multibeam bathymetric map (contour each 5 m, illumination from NE) draped by sea-floor reflectivity from CHIRP sonar data (red to blue colour scale). Red colour indicates high reflectivity while blue colour is related to low-reflectivity. The flatbottomed canyon is displaced by the main NAF strand. The offset canyon is at depth of 190 m. This implies that it was carved under water (at approximately 100-m water depth or even shallower) and the change from erosion to deposition would have been associated with the Holocene marine transgression with water depth increase of 85 m. (a) Two reflectivity profiles outside the displacement zone have been chosen in order to correlate zones of similar reflectivity pattern north and south of the fault zone. This allows us to estimate a horizontal slip of about 100 m because the canyon became inactive after the last episode of sea-level rise.
estimated a ~100-m dextral strike-slip displacement of the canyon head along the fault track. Information on kinematics of the fault requires an estimate of the interval of time, during which the ~100-m strike-slip motion took place. The flat bottom of the canyon suggests that it is presently inactive, i.e., that its regime changed from bincisionQ to bdepositionQ sometime in the past. Since then, the canyon has been offset bpassivelyQ by the fault. We assume that the change from berosionQ to bdepositionQ took place in the canyon during the last episode of transgression dated 11 000–10 000 years BP.
4.2. AREA2: channel
85 m paleoshoreline and fluvial
High-resolution bathymetry and subbottom CHIRP profiles revealed a prominent contour-parallel scarp at a depth of approximately 85 m. The scarp outlines a brough–smoothQ transition across the 85-m isobath and subhorizontal sedimentary sequences on-lapping near the isobath [31]; it trends ~N–S and intersects the fault trace at 29824V45UE, immediately S of a smooth bend towards E (Fig. 2). Subbottom profiles show subsurface strata outcropping at the scarp that may be
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the result of beach erosion and low-stand deposits (Fig. 6a). The integrated analyses of sediment cores, including facies analysis and quantitative assessment of foraminiferal and ostracod distribution [35], provides invaluable paleodepth information, leading to refined identification of a paleoshoreline at ~85 m below the present day sea level. The sill depth in the Dardanelles Strait during the Last Glacial Maximum, estimated at 85 m below present sea level [36,37], may have controlled the formation of the 85-m paleoshoreline in the fresh water Marmara Lake [34]. Immediately E of the paleoshoreline, our CHIRP records show the presence of a V-shaped erosional channel (Figs. 2A and 6a). This channel is now inactive, having been partially buried by a thick (up to 20 msec TWT) transparent unit that hampers its recognition from morphobathymetric data alone. A close-spaced grid of CHIRP profiles (Figs. 1 and 3) was obtained where the buried channel intersects the NAF displacement zone (AREA2) that in this area is constituted by the main strand to the north, and the secondary splay to the south (Fig. 6b). We were able to reconstruct and map the subbottom topography of this area (Fig. 7b) before the last episode of sea level rise by picking the erosional base of the Holocene mud deposits within each single CHIRP profile. The map shows a channel meandering landward of the 85 m paleoshoreline within a coastal plain: as it approaches the NAF displacement zone, it is displaced by the main fault track and by the secondary splay to the S. A close-up view indicates right-lateral displacements of the channel of about 80 m along the main fault (Fig. 7b, inset 1), and of about 10 m along the secondary splay (Fig. 7b, inset 2). Five cores were recovered along a N–S transect across the channel. Core IZ 20 is 264-cm long and was recovered at 83 m water depth north of the major fault (Figs. 7b and 8). The lower part of the core, between 264 and 224 cm, includes laminated clays with freshwater molluscs. These are sharply overlain by a few centimeterthick organic layer, black in colour, with upward transition to a distinctive alternation of organic-rich clays and very thin sand layers. This succession is capped by about 15 cm of homogeneous clay, with rare marine fossils (194–180 cm). A sharp discontinuity at 180 cm separates this part of the core from an
overlying 3-cm-thick sand layer, containing abundant mollusk shells of infralittoral conditions, with pebbles and wood fragments. This layer is sharply overlain by 177 cm of homogeneous mud, with local flat lamination. A relatively darker colour is recorded between 160 and 100 cm. The lower part of the core is interpreted to reflect lake conditions, with upward transition to swamp- and channel-related (levee or crevasse splay) deposits. Channel abandonment due to rising sea level is documented by the onset of a marine fauna and slightly predates shoreline transgression. Radiocarbon dates obtained from organic material sampled at 175– 176 cm suggest a 10.200F50 year BP age for the paleoshoreline. The sharp boundary with the overlying clay suggests rapid deepening and transition to present conditions. The darker interval observed between 160 and 100 cm could be related to slight oxygen deficiency conditions (lower sapropelic interval of C ¸ agatay et al. [38]). 4.3. AREA3: paleoshoreline and paleoisland This area is located where the deformation zone across the main fault segment is wider. The main fault track is characterized here by series of short left stepping en-enchelon segments outcropping on the sea bottom. These segments are paralleled 1 km to the S by the track of the secondary splay that merges toward the main fault in AREA-1, at 29824V20UE (Fig. 2). The distance between the main fault and the secondary splay is maximum at 29826V30UE, where we observed a prominent topographic high (minimum depth of 35 m) bounded to the N by the track of the splay (Fig. 2). Subbottom CHIRP profiles show that this feature is formed by a transpressional fold with its axis oriented N45. We interpret this topographic high as a bpaleoislandQ because it shows wave-cut terraces at its top ( 45 m of depth) and because cores collected at different stations sampled coarse-grained sediments with pebbles and gravel. Core IZ 25 is 350-cm long and was collected at 42.9 m water depth (Fig. 2 and 9). The lower part of the core, between 350 and 135 cm, is dominated by homogeneous clay with abundant fossils diagnostic of freshwater conditions (Dreissena). Between 135 and 117 cm, marine molluscs are
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Fig. 6. CHIRP profiles collected in AREA-2. (a) E/W-oriented profile b1-50 across the 85 m paleoshoreline and the buried river channel. Location of core IZ-20 is indicated. (b) N/S-oriented profile b1-3 across the NAF displacement zone in the region where the southern splay merges towards the main strand.
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Fig. 7. Key area 2. Left: shaded relief multibeam bathymetric map (contour each 1 m, illumination from NE). Right: gradient map of the base of the Holocene sediments obtained through a semiautomatic picking of CHIRP-subbottom profiles. It images sedimentary and tectonic features below the holocenic drape. (1) Laplacian (gray scale) and contour lines of the Holocene base topography in the area where the river channel intersects the main NAF strand. (2) Laplacian (gray scale) and contour lines of the Holocene base topography in the area where the river channel intersects the southern strand. The gradient map of the Holocene base topography shows the meandering river channel which is displaced along the NAF fault strands. We calculate an offset of ~80 and ~10 m in the main and secondary strand respectively. Location of CHIRP profile b1-29 shown in Fig. 2 is indicated.
associated with this freshwater fauna. A sandy succession becoming finer upwards was recorded between 117 and 92 cm; its erosional base, the presence of pebbles and the faunal assemblage are characteristic of a littoral environment. This interval shows upward transition to a thin layer made up of homogeneous mud. The uppermost 69 cm consist of fine, medium and coarse sand, with a sharp lower boundary and very abundant infralittoral shells. The lower part of the core documents lake conditions, with upward transition to a mixed layer, showing a progressive increase in marine influence. The sand layer and its gradual upward transition to a mud-dominated interval are inferred to represent a transgressive succession from nearshore to offshore deposits. Vertical changes in pollen assemblages suggest that the 45 m paleoshoreline identified on top of the paleoisland, could be regarded as coeval with the 85 m paleoshoreline observed within AREA1. However, due to the pollen stratigraphy resolution limits, we cannot exclude that the palaeoshoreline may be correlated to different sea level still stand events in
the Marmara Sea. Paleoshorelines have been observed at different depths in the Marmara Sea, including the 65 m scarp described by C ¸ agatay et al. [38] in other areas of the Western and Karamqrsel Basins.
5. Slip rate estimates The offsets measured at the canyon and at the fluvial channel are similar in length, within an error of 10%. This error is due to a combination of vertical and horizontal resolution of the multibeam and CHIRP sonars, of the positioning (differential GPS) and of the discrete sampling of the feature at the grid knots that we interpolated in order to obtain the topographic images of the subsurface. The errors in the age estimates are related to 14C determinations, if we assume our reconstruction of paleooceanographic events is correct. We assume an error well within 5% for the 14C dating. We stress that 14 C ages are supported by other proxies (pollen and microfossils).
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Fig. 8. Lithology, facies association and photograph of core IZ-20 collected close to the fluvial channel in AREA-2. To the right, the CHIRP subbottom profile collected during coring operations.
Taking into account all possible errors we estimate an average rate of 10F1.5 mm/year for the rightlateral motion along this major strand of the NAF during the Holocene (i.e., the last 10.000 years).
The offset channel provides not only an estimate of horizontal slip rate, but also of vertical slip rate. If we assume that the channel probably was not forming a bwater fallQ across the fault strand but had a relatively
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Fig. 9. Lithology, facies association and photograph of core IZ-25 collected on the paleo-island in AREA-3. To the right, the CHIRP subbottom profile collected during coring operations.
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smooth profile (i.e., the basement was eroded subhorizontally during the LGM), then we can directly estimate the Holocene vertical subsidence of the southern block relative to the northern one. The vertical slip rate thus calculated is in the order of 2– 2.4 mm/year, equally distributed between the northern and southern strand of the fault in this region. This is a maximum value because the fault scarp may have existed also during the LGM, and the assumption of a smooth profile across the principal displacement zone can overestimate the vertical slip rate. On the other hand, core IZ-25, collected on the paleoisland, has an important bearing for the kinematics of the fault. The formation of the paleoisland may be due to localized transpressive deformation related to a left-step of the secondary splay (pressure ridge). If the paleoshoreline sampled on the pressure ridge is 10.000 years old and formed at 85 m, then the paleoisland must have undergone a vertical uplift in the range of 40 m (vertical slip rate of 4 mm/year). This is a minimum value because the 85m paleoshoreline is deeper in the block south of the fault. Unfortunately, we were not able to detect the paleoshoreline south of the fault, but we can argue that the vertical throw is similar to that described for the LGM erosional surface (i.e., about 10 m). Thus, the local vertical uplift rate of the fault along the pressure ridge would be about 5 mm/year. W of the pressure ridge, deformation during the same period is negligible. This observation reinforces the assumption of a narrow deformation zone in AREA1 and AREA2.
date the strike-slip deformation, while NW–SE oriented structures are mainly transtensive. In fact, slip rate estimates in AREA 2 show how the NW–SEoriented fault strand has similar horizontal and vertical slip rates (about 1 mm/year), while the main E–W fault segment has horizontal slip rate eight to nine times larger than vertical slip rate. A relatively wide deformation zone (2–3 km) is shown by multi- and single-channel reflection profiles in some areas of the Gulf of I˙zmit [19,31,33]. On the other hand, other regions, such as those of boxes 1 and 2, are characterised by a relatively narrow displacement zone where the deformation is localised near the fault trace [31]. Multichannel and subbottom seismic profiles obtained close to AREAS 1 and 2 [33] show several fault segments on both side of the principal displacement zone that do not affect the Holocene sediments.
6. Width of strike-slip deformation along the NAF
Although minor secondary displacements (secondary splays, en-enchelon normal faults, etc.) are common within the NAF in the Gulf of I˙zmit, they show negligible offsets during the Holocene within the studied areas. The same is true for the northweststriking normal fault observed on the southern shore in front of AREAS 1 and 2, where the faults are draped by younger sediments; thus, they probably ceased their activity sometime within the Quaternary [20,39]. Therefore, we assume these faults did not rupture during the last 10.000 years (at least not in the areas where we measure the fault displacement) and did not contribute to the overall Anatolia–Eurasia relative plate motion during the last 10.000 years. We
Most oceanic transform faults have a relatively narrow strike-slip deformation zone because relative homogeneity of the upper crust favours a focusing of the strain. In contrast, continental transform faults, such as the NAF, are generally emplaced on variety of different terrains in terms of age, lithology and mechanical properties, and show a broad deformation zone. The NAF in the Gulf of I˙zmit shows fault stepovers, segmentation and strain partitioning between strike-slip and extension, with the effect of further widening the deformation zone. Our data in AREA 2 show that the E–W oriented fault strands accommo-
7. Geological versus geodetic slip rate estimate Our 10 mm/year estimate for the Holocene slip rate along the main segment of the NAF in the Gulf of I˙zmit is less than half the geodetic estimate for the relative motion between the Anatolian and Eurasian plates during the period spanning from 1988 to 1997 [10]. Two different scenarios may explain the contrast between the two different estimates of slip rate. Hypothesis (1). The Holocene plate motion along this major branch of the NAF has not been focused along a single fault but it has been diffuse and distributed among different fault strands.
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cannot exclude that the numerous faults bounding onland the Gulf of ˙Izmit were active during the Holocene, contributing to increase the slip rate along the plate boundary. They have been described, however, as bnorth-dipping normal faultsQ [21]. Hypothesis (2). The GPS geodetic measurements reflect a recent increase in the rate of strike-slip motion and are not representative of the long-term behavior of the fault at a time scale close to the recurrence interval of the major earthquakes. Despite the dense coverage of the GPS stations, the critical factor remains the period of observation compared to the seismic cycle. According to the historical record, the recurrence interval of major events in this area is about 200–250 years [30,40], i.e., one order of magnitude larger than the time interval of the GPS measurements. Several decades of GPS data would be necessary to obtain a statistically meaningful sampling of the total long term-slip (coseismic+postseismic), representative of permanent deformation and plate tectonic motion. Long-term slip rate estimates from geological reconstructions [9,41] are systematically smaller than 24 mm/year and are in the range of 14–20 mm/year during the last 3–4 Ma. Recently, Provost et al. [42], through a 3-D mechanical modeling of the GPS velocity field, provided an estimate of the NAF slip rate in the Marmara Sea in the order of 17.5 mm/year, which is a value significantly smaller than the 24 mm/year rate of McClusky et al. [10]. Moreover, they suggest that the Anatolia motion is not constant with distance along the fault (as pointed out by Mart and Ryan [43]). Discrepancies between Provost et al. [42] and previous studies are particularly evident in the Marmara region where additional complexity in the fault geometry and/or rheology of the crust has to be invoked. Very recently, a simple analytic viscoelastic model of the surface velocity field through a clustered earthquake cycle has been proposed to explain apparent differences between short- and long-term fault slip rates observed in many active regions of the world [44]. The model suggests that discrepancies between geologic and geodetic estimates of slip rate may be related to the clustering of earthquakes in time on the same fault that affects the rate and pattern of interseismic deformation.
Although a combination of these effects (i.e., plate motion distributed among different fault strands, recent increase in plate motion and/or interseismic deformation related to earthquake clustering) is most likely, available geophysical data favor a narrow deformation zone across the main NAF strand for the last 10.000 years within our study area. Consequently, we assume that the 10 mm/year slip rate is representative of the long-term behavior of the fault. This finding has important implications on kinematic models of the NAF and on assessment of seismic hazard for the city of Istanbul and the surrounding region.
8. Assessment of hazards Many authors [3,5,7] pointed out that, after the 1999 earthquake, the Coulomb stress onto faults west of the rupture rose, increasing the probability of an earthquake closer to Istanbul. The Marmara Sea is considered prone to rupture because it represents a seismic gap where 5.5 m of slip has accumulated since 1766 [3]. All these authors assumed an average slip rate in the range of 2–3 cm/year neglecting high creep rates for faults in the Sea of Marmara. Reducing the slip rate would not alter the spatial pattern of estimated historical slip as pointed out by Hubert-Ferrari et al. [3], but could reduce the amount of slip accumulated on single-fault branches of the NAF since the last seismic events. This possibility has to be taken into account for the assessment of seismic hazard that would consequently decrease. However, we cannot exclude the possibility that other secondary fault strands played an important role in absorbing the plate motion during the last 10.000 years. In fact, we lack multibeam coverage in water depth shallower than 30 m and we cannot exclude the presence of strike-slip faults parallel to the main displacement zone described at sea. Acquisition of further data in shallow water and sea–onland correlations are thus essential to understand fault behaviour in the Sea of Marmara.
Acknowledgements This research is supported by a Progetto Strategico of the Italian National Research Council (CNR), the Turkish Council for Scientific and Technical Research,
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grant OCE-0096668 from the National Science Foundation and Collaborative Linkage Grant #976826 from the North Atlantic Treaty Organization. We are grateful to the scientific party, captains and the ship crews onboard the R/V Odin Finder and Urania parties. Most figures were generated using the GMT software [45]. We are greatly indebited to Giovanni Bortoluzzi for his help in processing multibeam data and for fruitful discussions. We thank R. Armijo and Y. Mart for thoughtful reviews. We also thank X. LePichon for providing preliminary bathymetric data for cruise planning purposes and A.M.C. Sengor for fruitfull discussions during the Marmara-2000 cruise.
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