Coulomb stress interactions and the 1999 Marmara earthquakes

Bu çalışmada, Coulomb gerilme yöntemi kullanılarak, 1999 yılı öncesinde Marmara bölgesinde oluşan büyük depremlerin 17-Ağustos-1999 İzmit ve 12-Kasım-1999 Düzce depremlerini nasıl etkilediği, İzmit depreminin Düzce depremine olan etkisi ve günümüzde Marmara bölgesindeki deprem tehlikesi araştırılmıştır. InSAR ve GPS verileriyle bulunan fay parametrelerinin kullanıldığı Coulomb gerilmesi hesaplamaları İzmit depreminin, önceki depremlerden kaynaklanan statik gerilme artışının oluştuğu bir alanda meydana geldiğini göstermektedir. 1999 yılı öncesi depremlerin Düzce fayı üzerindeki gerilmeyi azaltmasına rağmen, Düzce depreminin İzmit depreminden kaynaklanan yüksek gerilme artışı nedeniyle tetiklendiği sonucu bulunmaktadır. Düzce ve öncesi depremler Adalar fayı üzerindeki gerilmeyi 5 bar’ın üzerinde yükselterek Marmara bölgesindeki deprem riskini arttırmış bulunmaktadır.Anahtar Kelimeler: 1999 Marmara depremleri, Coulomb kırılma gerilmesi, fay etkileşimi, deprem tehlikesi.In this study, the effect of the previous earthquakes on the 1999 İzmit and Düzce earthquakes, the influence of the İzmit earthquake on the Düzce earthquake, and the seismic hazard in the Marmara region are investigated using Coulomb failure stress. Calculation of the Coulomb stress changes using the fault parameters deduced from modelling of the coseismic InSAR and GPS data shows that the İzmit earthquake occurred where the Coulomb stress was increased by the previous events. Despite of the stress decrease on the Düzce fault due to the events before 1999, the Düzce earthquake appears to have been triggered by the high increase in the static Coulomb stress transferred by the İzmit earthquake. The Düzce and the previous earthquakes increased the static stress in western and eastern Marmara by over 5 bars. Calculation of secular stress loading based on the modelling of interseismic GPS measurements shows that stress accumulation along the northern branch of the NAF is 0.37 bars per year. Thus, a stress increase of 5 bars corresponds to an increase normally accumulated in about 12 years by secular loading due to the continuous plate motion. In other words, the previous earthquakes brought forward the next earthquake in the Sea of Marmara by 12 years. The faults in this region therefore pose a serious seismic hazard particularly for Istanbul where over 12 million people live.Keywords: 1999 Marmara earthquakes, Coulomb failure stress, fault interaction, earthquake hazard

Neotectonics of the South Marmara Sub-Region

Neotektonik dönemde Kuzey Anadolu Fayı ve kolları, inceleme alanı ve Marmara Bölgesi’nde en etkin tektonik yapıyı oluşturmaktadırlar. KAF, Marmara Bölgesi’nde kuzey ve güney olmak üzere iki kola ayrılır. Bunlardan Marmara Denizi’nin kuzeyinden geçen kol kuzey kol, güneyinden ve inceleme alanından geçen kol ise güney koldur. İnceleme alanında kuzeyde Edincik, Kapıdağı, Bandırma-Mudanya yükselimleri, güneyde Uludağ yükselimi ve Söğütalan platosu yer alır. Bu iki yükselim alanının ortasında doğu-batı uzanımlı bir çöküntü (depresyon) alanı gelişmiştir. Bu yükselim alanları ve çöküntü alanı, neotektonik dönemde Kuzey Anadolu Fayı’nın güney kolunun etkinliği ve kontrolü ile meydana gelmişlerdir. Çöküntü alanı içerisinde ise bir çok çek-ayır biçiminde havzalar oluşmuştur. Bölgede KAF’ın güney koluna ait faylar Yenice-Gönen, Manyas-Mustafakemalpaşa, Uluabat ve Bursa faylarıdır. Sismolojik veriler bölgedeki depremlerin  büyük ölçüde KAF’a ait güney kol üzerinde meydana geldiğini, hasar yapıcı ve yıkıcı depremlerin  hem doğrultu atımlı hem de eğim atımlı faylar üzerinde oluştuğunu göstermektedir.Anahtar Kelimeler: Güney Marmara Bölgesi, Kuzey Anadolu Fayı, çek-ayır havza, yanal atımlı fay.  In neotectonic period, North Anatolian Fault and its branches are the most active in the study area and Marmara Region. NAF is divided into two bracnhes as north and south  in the Marmara Region. The northern branch is located in the north of the Sea of Marmara. The southern branch is located in study area and south of the Sea of Marmara. The South Marmara Region has ptwo uplift areas located at southern and northern margins and a depression between these uplifts. The north uplifts are Edincik, Kapıdağı and Bandırma-Mudanya, south uplifts are Uludağ uplift and Söğütalan Plateau. The uplift and depression areas are contrelled by the southern branch of the NAF in the neotectonic period. In the depression area a lot of pull-apart style basins happened. The southern branch of the North Anatolian Fault, which consists of the Yenice-Gönen, Manyas-M.Kemalpaşa, Uluabat and Bursa faults. According to seismological data of  South Marmara Region, earthquakes have occured on the southern branch of the NAF and earthquakes show that the faults have not only strike-slip but also normal fault characteristics.Keywords: South Marmara Region, North Anatolian Fault, pull-apart basin, strike-slip faul

Estimates of Seismic Potential in the Marmara Sea Region from Block Models of Secular Deformation Constrained by Global Positioning System Measurements

We model the geodetically observed secular velocity field in northwestern Turkey with a block model that accounts for recoverable elastic-strain accumulation. The block model allows us to estimate internally consistent fault slip rates and locking depths. The northern strand of the North Anatolian fault zone (NAFZ) carries approximately four times as much right-lateral motion (∼24 mm/yr) as does the southern strand. In the Marmara Sea region, the data show strain accumulation to be highly localized. We find that a straight fault geometry with a shallow locking depth of 6-7 km fits the observed Global Positioning System velocities better than does a stepped fault geometry that follows the northern and eastern edges of the sea. This shallow locking depth suggests that the moment release associated with an earthquake on these faults should be smaller, by a factor of 2.3, than previously inferred assuming a locking depth of 15 km.Earth and Planetary Science

Coseismic and early post-seismic slip associated with the 1999 Izmit earthquake (Turkey), from SAR interferometry and tectonic field observations

International audienceWe use combined tectonic field observations and SAR data to determine an improved model of the slip associated with the 1999 Izmit earthquake, which ruptured the North Anatolian Fault at the eastern end of the Sea of Marmara. The leading goal is to understand the main features of the coseismic and post-seismic deformation, which are captured together in the SAR data. To achieve this, we make a critical analysis of the ERS1-2 SAR data, which allows atmospheric effects to be identified and removed. We also use detailed field mapping and measurements of the earthquake surface rupture. Dislocations in elastic half-space and a forward modelling strategy allow us to obtain a slip model by steps. A trial-and-error approach is combined with conventional inversion techniques to determine the slip in the different regions of the fault. The SAR data are well explained with three main zones of high slip along the fault, releasing a total moment of 2.3 × 1020 N m (Mw= 7.6), which is higher than the seismological estimates (1.7-2.0 × 1020 N m). The inhomogeneous slip distribution correlates with fault segments identified at the surface. The Izmit rupture appears to have extended 30 km west of the Hersek peninsula into the Sea of Marmara with slip tapering from 2 m to zero. The western end of the rupture is located 40 km SSE from Istanbul. We show that some features seen near to Mudurnu and Gevye and previously interpreted as slip on secondary faults are explained mostly as atmospheric effects correlated with the topography. Using our approach and the available GPS data we obtain a slip model that represents the coseismic slip alone, which suggests that the moment release during the main shock was 1.9 × 1020 N m (Mw= 7.5), consistent with the seismological estimates. We conclude that the SAR data include the effects of 2 m of fast after-slip during the month following the main shock, within a zone of the fault located 12- 24 km below the epicentral region. Near the hypocentre at a depth of 18 km, the fault appears to have experienced dynamic slip of 1 m associated with the main shock, followed by 2 m of rapidly decelerating post-seismic shear during the following month. We suggest that the distribution of heterogeneous slip and loading along the different fault segments may be important factors controlling the propagation of large earthquake ruptures along the North Anatolian Fault

Morphology, displacement and slip rates along the North Antolian Fault (Turkey)

Geological and geomorphological offsets at different scales are used to constrain the localization of deformation, total displacement, and slip rates over various timescales along the central and eastern North Anatolian Fault (NAF) in Turkey. The NAF total displacement is reevaluated using large rivers valleys (80 ± 15 km) and structural markers (Pontide Suture, 85 ± 25 km; Tosya-Vezirko ̈pru ̈ basins, 80 ± 10 km). These suggest a Neogene slip rate of 6.5 mm/yr over 13 Myr. The river network morphology shows offsets at a range of scales (20 m to 14 km) across the main fault trace and is also used to estimate the degree to which deformation is localized. At a smaller scale the morphology associated with small rivers is offset by 200 m along the NAF. The age of these features can be correlated with the Holocene deglaciation and a slip rate of 18 ± 3.5 mm/yr is determined. This is consistent with a rate of 18 ± 5 mm/yr deduced independently from the 14C dating of stream terrace offsets. Over the short term, GPS data gives a similar rate of 22 ± 3 mm/yr. All our results tend to show that most of the deformation between the Anatolian and Eurasian lithospheric plates has been accommodated along, or very close to, the active trace of the NAF. The difference between the Neogene and the Holocene slip rate may be due to the recent establishment of the current plate geometry after the creation of the NAF. INDEX TERMS: 8107 Tectonophysics: Continental neotectonics; 8158 Tectonophysics: Plate motions—present and recent (3040); 7230 Seismology: Seismicity and seismotectonics; KEYWORDS: North Anatolian Fault, slip rate, total offset, strain localizatio

Seismic hazard in the Sea of Marmara following the Izmit Earthquake

On 17 August 1999, a destructive magnitude 7.4 earthquake occurred 100 km east of Istanbul, near the city of Izmit, on the North Anatolian fault. This 1,600-km-long plate boundary1,2 slips at an average rate of 2–3 cm yr−1 (refs 3–5), and historically has been the site of many devastating earthquakes6,7. This century alone it has ruptured over 900 km of its length6. Models of earthquake-induced stress change8 combined with active fault maps9 had been used to forecast that the epicentral area of the 1999 Izmit event was indeed a likely location for the occurrence of a large earthquake9,10. Here we show that the 1999 event itself significantly modifies the stress distribution resulting from pre- vious fault interactions9,10. Our new stress models take into account all events in the region with magnitudes greater than 6 having occurred since 1700 (ref. 7) as well as secular interseismic stress change, constrained by GPS data11. These models provide a consistent picture of the long term spatio–temporal behaviour of the North Anatolian fault and indicate that two events of magnitude equal to, or greater than, the Izmit earthquake are likely to occur within the next decades beneath the Marmara Sea, south of Istanbul