Southern Strand Research Articles

Spatiotemporal Clustering of Large Earthquakes Along the Central‐Eastern Sections of the Altyn Tagh Fault, China

AbstractThe understanding of the spatial‐temporal distribution of past earthquakes is essential to assess the event recurrence behavior and to estimate the size of potential earthquakes along active strike‐slip fault systems. However, the scarcity of paleoseismic data remains a major hurdle in this endeavor. This is the case of the longest strike‐slip fault in Asia, the Altyn Tagh Fault (ATF). We documented six very likely large earthquakes that potentially ruptured the Aksay section of the ATF. Employing a Bayesian approach, we present modeled date ranges of 6339–5220 BC, 5296–4563 BC, 3026–2677 BC, 1324–808 BC, 314–632 AD, and 915–1300 AD. The mean recurrence time is 1,329 ± 588 years with a coefficient of variation (COV) of ∼0.44. In the same fault section, 90 horizontal offsets record an average coseismic slip of 5.1 ± 1.4 m for the last event and suggest four older earthquakes plausibly with a similar slip distribution. Although at the local‐scale the COV indicates quasi‐periodic rupture behavior, the individual interevent times exhibit significant irregularity, a pattern also observed in adjacent fault sections (Xorxoli, Annanba and Tashi sections). We found that such irregularities are a natural consequence of long‐term fault interactions, which allow for synchronized ruptures along the northern and southern strands of the central‐eastern ATF. Our rupture model highlights bursty periods of seismic activity with mean interevent times of 475 ± 108 years separated by long‐lull periods of 1.1–1.6 kyr. Based on this temporal organization and considering the 401‐year elapsed time since the most recent event on the Xorxoli section, there exists a possibility of a forthcoming large earthquake occurring within the next century.

Insights on the 2023 Kahramanmaraş Earthquake, Turkey, from InSAR: fault locations, rupture styles and induced deformation

SUMMARY We successfully detected widely distributed ground displacements for the 2023 Kahramanmaraş, Turkey, earthquakes by conducting interferometric synthetic aperture radar (InSAR) analyses using a ScanSAR observation mode. Major deformation extended approximately 350 and 150 km along the southern and northern strands bifurcating in the west of the East Anatolian Fault, produced by the main shock and the largest aftershock. The deformation map reveals that the ruptures propagated on the Erkenek, Pazarcık and Amanos segments on the southern strand and the Çardak segment on the northern strand. The fault plane of the northern strand bends approximately 45° at both edges with Z-shaped crank geometry. The bending fault at the western edge runs just along the Çardak segment but does not reach the Savrun segment, while at the eastern edge it deviates from known active faults such as Sürgü, Malatya faults and Doğansehır fault zone. A 3-D displacement map demonstrates that almost pure left-lateral fault motions were distributed along the two strands, with little vertical deformation. The moment magnitudes estimated from the slip distribution model were 7.82 and 7.66 for the southern and northern strands, respectively, with the Erkenek and Çardak segments having the largest released seismic moments on each strand, corresponding to approximately 31 and 57 per cent of the total, respectively. The Coulomb Failure Function change values indicate that the main shock can promote the largest aftershock with a standard value of the effective friction coefficient. Additionally, the unclamping effect controlled by the frictional property of the rock was a key factor in pulling the trigger of the seismic event on the northern strand. The historically accumulated and released seismic energies were imbalanced for the Pazarcık and Erkenek segments, suggesting that the 2023 event does not support a simple characteristic earthquake model; rather, it may be consistent with a supercycle model, in which the slip remnants from the characteristic earthquakes have been historically accumulated as coupling on a fault and released as huge earthquakes at longer intervals.

Slip Partitioning Across Subparallel Strands of the North Anatolian Fault in the Marmara Region

Summary Marmara Region hosts a substantial part of the inhabitants in Turkey, more than 30% of the total population in the cities of Istanbul, Bursa, and Kocaeli. This region has experienced several large earthquakes in the past and is still under threat of destructive earthquakes in the future. There, subparallel strands of the North Anatolian Fault (NAF) expand into the region, distributing the earthquake hazards across the whole region. Thus, it is a key issue to investigate how the tectonic process is distributed between these subparallel strands to discriminate their individual earthquake hazards. In this context, we used GPS slip rates to quantify the slip partitioning of the subparallel strands of the fault system. In addition to available slip rates compiled by Bulut et al. (2019) and Kreemer et al. (2014), we analyzed 71 GPS sites (51 continuous and 20 campaign-based) to intensify the GPS network in the region. GPS observations reveal 81.7%, 7.7%, and 10.7% slip ratios for northern, middle, and southern strands. Time series analysis for four continuous GPS stations in the western Marmara Region also confirms these ratios.

Neotectonics of the Sarıköy-Đnova and Çan-Bayramiç-Ezine fault zones: basin formation, age and Slip Rates, NW Anatolia-Turkey

he Sarıköy-Đnova and the Çan-Bayramiç-Ezine fault zones (SIFZ, ÇBEFZ) comprise the southern strand of the North Anatolian Fault System in the Biga Peninsula. They are located in the area between Sarıköy to northeast and the Dalyan settlement around Bozcaada in the North Aegean Sea to southwest. Both of the fault zones are active. This is evidenced by the 6 March 1737 (Ms =7.0) Đnova, 1st February 1809 Hurma (Ms = 6.1), and the 8 February 1826 Güllüce (Ms = 6.2) historical earthquakes resulted from these fault zones. Maximum lengths of fault segments comprising the SIFZ and the ÇBEFZ are 14 km and 15 km, respectively. Based on the maximum lengths of fault segments, the magnitude of the peak earthquakes to be originated from these faults are Mw = 6.3 and 6.6, respectively. Based on both the geological and geographic markers, the total right lateral offsets accumulated on the SIFZ and ÇBEFZ are 12 km and 20 km, respectively. These offset values imply to the slip rates of 4.6 mm/yr and 7.7 mm/yr, respectively. Five pull-apart basins were developed on both fault zones. These are the Sarıköy, Đnova, Kazabat, Çan and Ezine-Bayramiç basins. The first three of them are pure strike-slip pull-apart basins, while the type of the rest basins is superimposed. The angular unconformity between the nondeformed basin fill of Quaternary age and the folded to thrust-faulted basement rocks of pre-Quaternary age reveals strongly that the pull-apart basins have formed during the Quaternary time. This relationship also reveals that the commencement age of the strike-slip neotectonic regime and formation of associated fault zones are the Early Quaternary.

Morphotectonic Structures along the Southwestern Margin of Lesvos Island, and Their Interrelation with the Southern Strand of the North Anatolian Fault, Aegean Sea, Greece

A hydrographic survey of the southwestern coastal margin of Lesvos Island (Greece) was conducted by the Naftilos vessel of the Hellenic Hydrographic Service. The results have been included in a bathymetric map and morphological slope map of the area. Based on the neotectonic and seismotectonic data of the broader area, a morphotectonic map of Lesvos Island has been compiled. The main feature is the basin sub-parallel to the coast elongated Lesvos Basin, 45 km long, 10–35 km wide, and 700 m deep. The northern margin of the basin is abrupt, with morphological slopes towards the south between 35° and 45° corresponding to a WNW-ESE normal fault, in contrast with the southern margin that shows a gradual slope increase from 1° to 5° towards the north. Thus, the main Lesvos Basin represents a half-graben structure. The geometry of the main basin is interrupted at its eastern segment by an oblique NW-SE narrow channel of 650 m depth and 8 km length. East of the channel, the main basin continues as a shallow Eastern Basin. At the western part of the Lesvos margin, the shallow Western Basin forms an asymmetric tectonic graben. Thus, the Lesvos southern margin is segmented in three basins with different morphotectonic characteristics. At the northwestern margin of Lesvos, three shallow basins of 300–400 m depth are observed with WNW-ESE trending high slope margins, probably controlled by normal faults. Shallow water marine terraces representing the last low stands of the glacial periods are observed at 140 m and 200 m depth at the two edges of the Lesvos margin. A secondary E-W fault disrupts the two terraces at the eastern part of the southern Lesvos margin. The NE-SW strike-slip fault zone of Kalloni-Aghia Paraskevi, activated in 1867, borders the west of the Lesvos Basin from the shallow Western Basin. The Lesvos bathymetric data were combined with those of the eastern Skyros Basin, representing the southern strand of the North Anatolian Fault in the North Aegean Sea, and the resulted tectonic map indicates that the three Lesvos western basins are pull-aparts of the strike-slip fault zone between the Skyros Fault and the Adramytion (Edremit) Fault. The seismic activity since 2017 has shown the co-existence of normal faulting and strike-slip faulting throughout the 90 km long Lesvos southern margin.

Active tectonics of Gülpınar-Tuzla area (Biga Peninsula, NW Turkey): The source of 6 February-24 March 2017 earthquake cluster

The variation in the motion sense of Anatolian platelet in Aegean Sea resulted in a strike-slip tectonic regime and related neotectonic domain, the central to northern Aegean neotectonic province, which also includes both western Marmara Sea and Biga Peninsula. Our study focuses mostly on the Gülpınar-Tuzla earthquake area located at the southwestern tip of Biga Peninsula, which is controlled by the southern major strand of the North Anatolian Fault System (NAFS). The strand consists of two sections, the onshore Biga and offshore Babakale-Skyros sections. Both sections are seismically very active. The Gülpınar-Tuzla earthquake area is composed of the Paleozoic metamorphic rocks, the Oligo-Miocene granitoid pluton, Lower-Middle Miocene calc-alkalic volcanic rocks and the Upper Miocene-Pliocene sedimentary sequence. All of these rocks, which formed and deformed (folded to tilted) in palaeotectonic period, are overlain with an angular unconformity by the Quaternary neotectonic basin fill, that is nearly flat-lying except for the faulted basin margins. Both the onshore and offshore sections of the southern strand are linked to each other in terms of the structures characterizing the Babakale pull-apart basin and the Gülpınar-Tuzla earthquake area. The latter is shaped by the NE-trending Gülpınar (GFZ) and Yenice-Gönen Fault Zone (YGFZ), the ENE-trending Edremit fault zone (EFZ), the WNW-trending Tuzla Fault Zone (TFZ) and three strike-slip basins (Ayvacık, Behramkale and Tuzla basins) developed along them. Some segments of both the TFZ and GFZ were reactivated by the occurrences of seven moderate-to small-sized independent earthquakes and related aftershocks over 2760. Five of the independent earthquakes have an origin of oblique-slip normal faulting, while the rest two seismic events are strike-slip faulting in origin. Focal mechanism solution diagrams of these two groups of earthquakes reveal that the Gülpınar-Tuzla area is under the control of a strike-slip neotectonic regime, which commenced in Early Quaternary time owing to the major inversion in extensional palaeotectonic regime. This is also supported by the palaeostress analysis of slip-plane data measured on fault slickensides. The uniform slip rates on both the YGFZ and EFZ are 10.8 mm/yr and 7.3 mm/yr, respectively.

The June 12, 2017 M6.3 Karaburun-Lesvos earthquake of the Northern Aegean Sea: Aftershock forecasting and stress transfer

The June 12, 2017 Karaburun-Lesvos (North Aegean Sea) earthquake occurred along the NW-SE trending Lesvos fault, along the southern strand of the North Anatolian Fault Zone. In the present study seismotectonic aspects of the 2017 Karaburun-Lesvos earthquake and its aftershock sequence are studied. A rupture model based on finite source analysis of the teleseismic waveforms has shown that the earthquake was associated with a failure of single asperity. About 5 days after the mainshock a temporary seismic network of 8 broadband stations (Real-time Aftershock Forecasting in Turkey, RAFT) had been deployed along the Turkish Aegean coast to enhance the existing regional seismic monitoring and the acquired data have been used to relocate the aftershocks. The temporary deployment significantly improved the aftershock detection capacity and resulted in more precise locations. Prior to the monitoring enhancement a single widespread aftershock cluster was observed; however, the relocated aftershocks, augmented by the RAFT stations, identified two distinct spatially isolated clusters. The first day of the aftershock sequence has been used for retrospective real-time aftershock forecasting up to 7 days following the mainshock. Our results indicate that with a method developed by Omi et al. (2013) can be used forecasting aftershocks over a week period successfully employing incompletely detected aftershocks occurred in the first day following the mainshock. Stress tensor analysis of the 33 aftershock source mechanisms has shown local dominance of the extensional tectonics with azimuth and plunge pairs for the three principal stress axes as σ 1 , σ 2 and σ 3 are (255°; 76°), (131°; 8°) and (39°; 11°), respectively. Coulomb stress changes imparted by the mainshock onto the nodal planes of the aftershocks show that ~67% of the 33 aftershocks have been exposed to positive stress change at least on one of the nodal planes. • Relocated aftershocks identified two distinct spatially isolated aftershock clusters. • Incompletely detected aftershocks can be used for operational aftershock forecasting. • ~67% of the aftershocks were exposed to positive mainshock stress changes.

Spatio-temporal behaviour of continental transform faults: implications from the late Quaternary slip history of the North Anatolian Fault, Turkey

The slip history of the North Anatolian Fault (NAF) is constrained by displacement and age data for the last 550 ka. First, I classified all available geological estimates as members of three groups: Model I for the eastern, Model II for the central, and Model III for the western segments where the North Anatolian Shear Zone gradually widens from east to west. The short-term uniform slip solutions yield similar results, 17.5 +4/–3.5 mm/a, 18.9 +3.7/–3.3 mm/a, and 16.9 +1.2/–1.1 mm/a from east to the west. Although these model rates do not show any significant spatial variations among themselves, the correlation with geodetic estimates, ranging between 15 mm/a and 28 mm/a for different sections of the NAF, displays significant discrepancies especially for the central and western segments of the fault. Discrepancies suggest that most strain is accumulated along the NAF, but some portion of it is distributed along secondary structures of the North Anatolian Shear Zone. The deformation rate is constant at least for the last 195 ka, whereas the limited number of data show strain transfer from northern to the southern strand between 195 and 320 ka BP in the Marmara Region when the incremental slip rate decreases to 13.2 +3.1/–2.9 mm/a for the northern strand of the NAF. Considering the possible uncertainties of incremental displacements and their timings, more studies on slip rate are needed at different sites, including major structural elements of the North Anatolian Shear Zone. Although most of the strain is localized along the main displacement zone, the NAF, secondary structures are still capable of generating earthquakes that can hardly reach Mw7.

Quaternary Activity of the Monastir and Grombalia Fault Systems in the North‒Eastern Tunisia (Seismotectonic Implication)

The Monastir and Grombalia fault systems consist of three strands that the northern segment corresponds to Hammamet and Grombalia faults. The southern strand represents Monastir Fault also referred to as the Skanes-Khnis Fault. These NW-trends are observed continuously in the major outcropping features of north-eastern Tunisia including both the Cap Bon peninsula and the Sahel domain. Along the Hammamet Fault, the north-eastern strand of Grombalia fault system, left lateral drainage offset of amount 220 m is found in Fawara valley. To the South, the left lateral movement is occurred along the Monastir Fault based on 180 m of Tyrrhenian terrace displacement. Field observations supported by satellite images suggest that the Monastir and Grombalia fault systems appear to slip mostly laterally with components of normal dip slip. Assuming the development of the stream networks during the Riss-Wurm interglacial (115000–125000 years) and the age of the Tyrrhenian terrace (121 ± 10 ka), the strike slip rates of the Hammamet and Monastir faults are calculated in the range of 1.5–1.8 mm/yr. There vertical slip rates are estimated to be 0.06 and 0.26 mm/yr, respectively. These data are consistent with the displacement rate in the Pelagian shelf (1–2 mm/yr) but they are below the convergence rate of African-Eurasian plates (8 mm/yr). Our seismotectonics study reveals that a maximum earthquake of Mw = 6.5 could occur every 470 years in the Hammamet fault zone and Mw = 6–every 263 years in the Monastir fault zone.

Stress modeling to determine the through-going active fault geometry of the Western North Anatolian Fault, Turkey

The North Anatolian Fault (NAF) is a 1200 km long dextral strike-slip fault which is part of an east-west trending dextral shear zone (NAF system) between the Anatolian and Eurasian plates. The North Anatolian shear zone widens to the west, complicating potential earthquake rupture paths and highlighting the importance of understanding the geometry of active fault systems. In the central portion of the NAF system, just west of the town of Bolu, the NAF bifurcates into the northern and southern strands, which converge, then diverge to border the Marmara Sea. At their convergence east of the Marmara Sea, these two faults are linked through the Mudurnu Valley. The westward continuation of these two fault traces is marked by further complexities in potential active fault geometry, particularly in the Marmara Sea for the northern strand, and towards the Biga Peninsula for the southern strand. Potential active fault geometries for both strands of the NAF are evaluated by comparing stress models of various fault geometries in these regions to a record of focal mechanisms and inferred paleostress from a lineament analysis. For the Marmara region, the best-fit active fault geometry consists of the northern and southern bounding faults of the Marmara basin, as the model representing this geometry better replicated primary stress orientations seen in focal mechanism data and stress field interpretations. In the Biga Peninsula region, the active geometry of the southern strand has the southern fault merging with the northern fault through a linking fault in a narrow topographic valley. This geometry was selected over the other two as it best replicated the maximum horizontal stresses determined from focal mechanism data and a lineament analysis.

Estimation of the long-term slip, surface uplift and block rotation along the northern strand of the North Anatolian Fault Zone: Inferences from geomorphology of the Almacık Block

Abstract The Almacik Block is one of the key morphotectonic units in the eastern Marmara Region associated with the long-term slip partitioning within the North Anatolian Fault Zone (NAFZ). In this study, we provide new geomorphic reconstructions of offset drainage basins, morphometric analysis of topography, and longitudinal profiles of the rivers crossing different flanks of the Almacik Block. Our geomorphic reconstructions of offset drainage basins along the Hendek and Karadere faults imply mean offsets of 2.3 ± 0.4 km and 8.4 ± 0.7 km, respectively, during the Quaternary. Our dataset also imply that slip partitioning occurs in a broader zone than previously proposed, and that the total 10.7 ± 0.6 km offset along the Hendek and Karadere faults of the northern strand must be taken into account for long-term slip partitioning in the Eastern Marmara Region. Together with previously suggested 10 km offset along the southern strand (Yaltirak, 2002), 16 ± 1.0 km offset along the middle strand (Ozalp et al., 2013) and the 52 ± 1.0 km offset along the Mudurnu Segment of the northern strand (Akbayram et al., 2016) our newly proposed geomorphic markers raise the cumulative offset in the eastern Marmara region associated with the NAF to 89 ± 1.0 km since the Latest Pliocene – Quaternary. In addition to these lateral displacements, our morphometric analysis and longitudinal profiles of the rivers imply up to 1130 ± 130 m surface uplift of the Almacik Block as a combined result of vertical displacement within the deformation zone of the northern strand of the NAFZ. Finally, by assuming that river basins act as passive deformation markers, our basin azimuth analyses imply 20° ± 2° clockwise rotation of the Almacik Block associated with the NAFZ.

29 NOVEMBER 1795 KAHRAMANMARAŞ EARTHQUAKE, SOUTHERN TURKEY

Major tectonics on the Kahramanmaraş region are the northern strand of EAFZ (Sürgü fault zone, Çardak segment, Savrun segment and Toprakkale segment) and the southern strand of EAFZ (Gölbaşı segment, Amanos sgment), Engizek Fault Zone, Kahramanmaraş Fault Zone and The Narlı segment of DSFZ. An earthquake, occurred in Kahramanmaraş, 1795, is mentioned a manuscript named Divan-ı Hasmi which is found at Koyunoğlu Library in Konya. According to manuscript the Mercalli Intensity of the earthquake has calculated as eight and the magnitude is seven. The calculations made have strengthened the possibility of earthquake occurncy on the Gölbaşı segment of EAFZ.

M ≥ 7 earthquake rupture forecast and time‐dependent probability for the sea of Marmara region, Turkey

AbstractWe forecast time‐independent and time‐dependent earthquake ruptures in the Marmara region of Turkey for the next 30 years using a new fault segmentation model. We also augment time‐dependent Brownian passage time (BPT) probability with static Coulomb stress changes (ΔCFF) from interacting faults. We calculateMw > 6.5 probability from 26 individual fault sources in the Marmara region. We also consider a multisegment rupture model that allows higher‐magnitude ruptures over some segments of the northern branch of the North Anatolian Fault Zone beneath the Marmara Sea. A total of 10 differentMw = 7.0 toMw = 8.0 multisegment ruptures are combined with the other regional faults at rates that balance the overall moment accumulation. We use Gaussian random distributions to treat parameter uncertainties (e.g., aperiodicity, maximum expected magnitude, slip rate, and consequently mean recurrence time) of the statistical distributions associated with each fault source. We then estimate uncertainties of the 30 year probability values for the next characteristic event obtained from three different models (Poisson, BPT, and BPT + ΔCFF) using a Monte Carlo procedure. The Gerede fault segment located at the eastern end of the Marmara region shows the highest 30 year probability, with a Poisson value of 29% and a time‐dependent interaction probability of 48%. We find an aggregated 30 year Poisson probability ofM > 7.3 earthquakes at Istanbul of 35%, which increases to 47% if time dependence and stress transfer are considered. We calculate a twofold probability gain (ratio time dependent to time independent) on the southern strands of the North Anatolian Fault Zone.

Crustal-scale shear zones and heterogeneous structure beneath the North Anatolian Fault Zone, Turkey, revealed by a high-density seismometer array

Continental scale deformation is often localised along strike-slip faults constituting considerable seismic hazard in many locations. Nonetheless, the depth extent and precise geometry of such faults, key factors in how strain is accumulated in the earthquake cycle and the assessment of seismic hazard, are poorly constrained in the mid to lower crust. Using a dense broadband network of 71 seismic stations with a nominal station spacing of 7 km in the vicinity of the 1999 Izmit earthquake we map previously unknown small-scale structure in the crust and upper mantle along this part of the North Anatolian Fault Zone (NAFZ). We show that lithological and structural variations exist in the upper, mid and lower crust on length scales of less than 10 km and less than 20 km in the upper mantle. The surface expression of the NAFZ in this region comprises two major branches; both are shown to continue at depth with differences in dip, depth extent and (possibly) width. We interpret a 10 km) zone in the crust with a similar variation of dip but there is no clear evidence that this shear zone penetrates the Moho. Layers of anomalously low velocity in the mid crust (20–25 km depth) and high velocity in the lower crust (extending from depths of 28–30 km to the Moho) are best developed in the Armutlu–Almacik block between the two shear zones. A mafic lower crust, possibly resulting from ophiolitic obduction or magmatic intrusion, can best explain the coherent lower crustal structure of this block. Our images show that strain has developed in the lower crust beneath both northern and southern strands of the North Anatolian Fault. Our new high resolution images provide new insights into the structure and evolution of the NAFZ and show that a small and dense passive seismic network is able to image previously undetectable crust and upper mantle heterogeneity on lateral length scales of less than 10 km.

Analysis of the spatial viscosity variation in the crust beneath the western North Anatolian Fault

Abstract Profiles of the relative geodetic velocity variation across the western North Anatolian Fault (NAF) near the location of the 1999 Izmit earthquake show two important characteristics: (i) before the earthquake, strain is localized in a region

Determining the geometry of the North Anatolian Fault East of the Marmara Sea through integrated stress modeling and remote sensing techniques

Abstract The 1200 km long North Anatolian Fault (NAF) is part of an east–west trending dextral shear zone (NAF system), along the boundary between the Anatolian and Eurasian plates, that widens to the west. This widening zone of deformation complicates potential earthquake rupture paths and highlights the importance of understanding the geometry of active fault systems. In the central portion of the NAF system – just west of the town of Bolu – the NAF splits into two major faults: the northern and southern strands. These two faults diverge, almost converge, and then diverge again to border the Marmara Sea. Earthquake data from the region where the two faults converge indicate that they may be linked by an active fault. We model the active fault geometries with and without the linking fault to explore its impact on the output regional stress field (from a finite element model). These results are compared to focal mechanism records and lineament analyses to determine which geometry best simulate the stress field in a regional model. Our results show that a linking fault between the northern and southern strands of the NAF system is necessary to best match the primary stress orientations of the model with the maximum paleostress orientations inferred from deformation patterns, and observed in earthquake focal mechanisms. Furthermore, the linking fault should be a significant component in future models of the NAF system within the region.

The East Anatolian Fault: geometry, segmentation and jog characteristics

Abstract A detailed account is given of the fault geometry and segment structure of the East Anatolian Fault Zone as a whole based on mapping of active faults, supported by available seismological and palaeoseismological data. We divide the East Anatolian Fault into two main strands: southern and northern. The main southern strand is c. 580 km long between Karlıova and Antakya, and connects with the Dead Sea Fault Zone and the Cyprus Arc via the Amik triple junction. The northern strand, termed the Sürgü–Misis Fault system, is c. 350 km long and connects with the Kyrenia–Misis Fault Zone beneath the Gulf of İskenderun. We infer that slip partitioning between the main and northern strands of the East Anatolian Fault accommodates 2/3 and 1/3 of the slip rate of the lateral motion between the Arabian and Anatolian plates, respectively in the Çelikhan–Adana–Antakya region. Taking account of the time elapsed from the latest events on the East Anatolian Fault, we suggest that the Pazarcık and Amanos segments have the potential to produce destructive earthquakes in the near future. Supplementary material: The data and interpretations given here are supported by five additional annotated field photographs and two tables of factual data, these are available at www.geolsoc.org.uk/SUP18568

Evolution of the Bababurnu Basin and shelf of the Biga Peninsula: Western extension of the middle strand of the North Anatolian Fault Zone, Northeast Aegean Sea, Turkey

Abstract The generally transpressional character of the North Anatolian Fault Zone in western Turkey implies a transtensional character in the Northern Aegean Sea. The geological evolution of the basins lying above and immediately adjacent to this fault zone is still poorly understood. Similarly little information exists regarding the tectonic configurations created by the westerly and southerly extension of the middle and southern strands of the North Anatolian Fault Zone into the Northern Aegean Sea, other than what has been inferred using somewhat speculative maps onland. This study combines ∼1600 km of single-channel seismic reflection profiles and piston-core data with detailed land mapping to develop a comprehensive evolutionary model. We suggest that the tectonic system of the Biga Peninsula and the northeastern Aegean Sea are characterized by transpressional broken-slat geometry onland and transtensional broken-slat geometry at sea. The system includes a set of prominent NE–SW-striking faults with considerable strike-slip component in the east (northwestern Turkey and eastern Aegean Sea) and a second set of major NW–SE-striking extensional faults in the west (eastern Greece). We further suggest that rotational wedges form the link between the NE–SW- and NW–SE-striking faults. Detailed mapping of the seismic data and fault linkages with the onland geology demonstrate that the middle strand of the North Anatolian Fault Zone exits into the Aegean Sea near the town of Behramkale and that it extends into the Bababurnu pull-apart basin in the northeastern Aegean Sea as a right-releasing step-over active during the Quaternary.

Geological slip rates along the North Anatolian Fault in the Marmara region

Reliable piercing points on both sides of the Sea of Marmara enabled us to obtain an estimate of the slip‐rate over time scales of 10–15 ka on different fault strands of the North Anatolian Fault (NAF) system. We analyzed geomorphic features in the gulfs of Izmit, Gemlik (Sea of Marmara) and Saros (NE Aegean Sea), which were passively displaced by the NAF strands after their abandonment related to the post‐glacial sea level rise. Results for the main northern strand, consistently similar on both sides of the Marmara pull‐apart, are in the order of 10 mm/yr, about one half of that expected from geodetic measurements and accepted plate‐tectonic models. In the southern branch of the NAF, the estimated rate of ∼4 mm/yr is only slightly higher than that given by geodetic models. Our findings have implications for both neo‐tectonic reconstructions of the submerged portion of the NAF system, and fault interactions and seismic hazard estimates in the Marmara region. They suggest that, either the total Anatolia/Eurasia plate motion is more diffuse than previously reported, or geodetic data are not representative of the geological time‐scale deformations. Moreover, they suggest that a significant amount of stress is accommodated along the southern strand of the NAF system, on which the last large (M ≈ 7) earthquakes dates back to 1419, 1855 and 1863 AD.

An Assessment of the Seismicity of the Bursa Region from a Temporary Seismic Network

A temporary earthquake station network of 11 seismological recorders was operated in the Bursa region, south of the Marmara Sea in the northwest of Turkey, which is located at the southern strand of the North Anatolian Fault Zone (NAFZ). We located 384 earthquakes out of a total of 582 recorded events that span the study area between 28.50–30.00°E longitudes and 39.75–40.75°N latitudes. The depth of most events was found to be less than 29 km, and the magnitude interval ranges were between 0.3 ≤ ML ≤ 5.4, with RMS less than or equal to 0.2. Seismic activities were concentrated southeast of Uludag Mountain (UM), in the Kestel-Igdir area and along the Gemlik Fault (GF). In the study, we computed 10 focal mechanisms from temporary and permanents networks. The predominant feature of the computed focal mechanisms is the relatively widespread near horizontal northwest-southeast (NW–SE) T-axis orientation. These fault planes have been used to obtain the orientation and shape factor (R, magnitude stress ratio) of the principal stress tensors (σ1, σ2, σ3). The resulting stress tensors reveal σ1 closer to the vertical (oriented NE–SW) and σ2, σ3 horizontal with R = 0.5. These results confirm that Bursa and its vicinity could be defined by an extensional regime showing a primarily normal to oblique-slip motion character. It differs from what might be expected from the stress tensor inversion for the NAFZ. Different fault patterns related to structural heterogeneity from the north to the south in the study area caused a change in the stress regime from strike-slip to normal faulting.