Gps Ölçümlerinde Ve Fay Hareketlerinde Deprem İlişkisi

Abstract

Kabuk deformasyonları belirleme çalışmaları jeoloji, jeofizik, jeodezi, inşaat ve maden müdendislikleri gibi farklı disiplinlerde çalışan bilim insanları tarafından yürütülmektedir. Günümüzde, levhalar üzerinde seçilen koordinatların deformasyon ölçümleri yapılabilmektedir. Bu tür ölçümler vasıtasıyla ilgili kabuk içerisinde birikmesi öngörülen gerilme değerleri ve belirlenen bu gerilme değerleri altında litosferin elastik özelliklerine bağlı olarak nasıl bir davranış sergileyeceğini tespit etmek mümkündür. Böylece plakaların hareketinden yola çıkarak plaka içerisinde birikmesi öngörülen basınç değerlerinin doğru bir şekilde yapılabilmesi için, GPS ölçümlerinin yüksek hassasiyette yapılması gerekmektedir. Benzer araştırmalar tektonik hareketlerin zamana bağlı davranışlarını anlama ve depremlerin meydana gelme olasılığını kestirme açısından hayati önem taşımaktadır. Ele alınan bölge için kabuk yapısında meydana gelen değişimlerin belirli bir zaman aralığında tespiti için kullanılması gereken ölçümlerin de sürekli olması gerekmektedir. Ancak bu şekilde deprem öncesi, deprem anı ve deprem sonrası ölçümleri yardımıyla fay hareketlerinin tanımlanması mümkündür. Bu çalışma kapsamında farklı bölgeler için hem sürekli GPS verileri hem de yıllık ortalama GPS verileri kullanılarak 3 farklı veri seti ile çalışılmıştır. Birinci çalışma alanını oluşturan, Türkiye ve çevresini kapsayan bölge için 7 adet sürekli ölçümleri bulunan GPS istasyonu ile bir GPS ağı oluşturulmuştur. 17 Ağustos 1999 Gölcük Depremi’nin oluşturulan GPS ağı üzerindeki etkisini belirlemek için 7 Mayıs 1998 ile 27 Temmuz 2008 tarihleri arasında kalan yaklaşık 10 yıllık bir zaman dilimini kapsayan GPS ölçümleri analiz edilmiştir. İkinci çalışma alanı yine Türkiye ve yakın çevresi için 1988-1997 yılları arasında yapılan GPS ölçümlerinin yıllık ortalaması kullanılarak toplam 32 adet GPS istasyonunu içeren ve ilk çalışmaya göre daha geniş kapsamlı olan bir ağ oluşturulmuştur. Üçüncü çalışma alanı Marmara Denizi ve yakın çevresini kapsayan GPS ağı 28 adet istasyon kullanılarak oluşturulmuştur. Elde edilen sonuçlar ile ilgili çalışma alanlarının asal gerilme haritaları konturlama yöntemi ile farklı temalar kullanılarak elde edilmiştir. Yapılan hesaplama ve haritalama çalışmalarının hepsinde MATLAB programından faydalanılmıştır. Elde edilen sonuçlar Türkiye’nin genel plaka tektoniği ile karşılaştırılmış ve sonuçların tutarlı olduğu görülmüştür. Sonuç olarak, günlük yaşamımızın bir parçası olan GPS sisteminin uzaktan algılama ve ölçme sistemlerinde son derece etkin ve gelişmeye açık bir sistem olduğu düşünülürse, yerkürenin dinamiğinin anlaşılması mümkündür. Bu çalışmada kurulan GPS ağının seyrekliği, kullanılan dataların kesikli ve kısa bir dönemi kapsaması bir dezavantaj gibi görülmesine rağmen daha yoğun GPS ağları ve daha zengin ve sürekli istasyon verileri kullanarak çok daha doğru sonuçlara gidilebileceği anlaşılmıştır.

Turkey tectonically is part of the famous Alpine belt that extends from the Atlantic Ocean to the Himalaya Mountains. There are many large and small faults endowed with different mechanisms as a result of the collision of Arabian and African plate with Eurasian Plate. North Anatolian Fault Zone (NAFZ) is one of the most active region where very destructive earthquakes took place in the past, situated from Karlıova to Mudurnu. As a consequence of those tectonic movements East Anatolian Fault Zone (EAFZ) generate many important earthquakes as well, starting from Antakya - Amik Basin to Karlıova junction. In addition to this situation, Eagen Graben System is very known active area in terms of earthquake occuring in the East Mediterranean Sea. Regarding these three very important earthquake zone, it can be said that Turkey is at risk in the sense of destructive earthquakes occuring obviously In the last century, Turkey was land owner of several catastrophic earthquakes like 1939 Great Erzincan Earthquake (M=7.9), 1943 Tosya Ladik Earthquake (M=7.2), 1999 Marmara Earthquake (M=7.5) and the last example is 2011 Van Earthquake (7.2). Earthquakes which come with loss of life and property affect negatively to the countries in terms of materiality and spirituality. Due to that reason, prediction of some parameters like the time of the occuring, magnitude and location of earthquakes are investigated by many scientists from different disciplines. In the earthquake prediction researches there is not only deterministic approaches but also there are many stochastic approximations. For that purpose, great numbers of parameters are investigated just like, alteration of magnetic field and electrical field, flactuations of radon gas, changes of water level and temperature for water wells and anomaly behaviour of animals. Earthquake source parameters, fault slip length, hypocentral depth, spreading of the stress fields and reaction with other faults provides improved estimations of earthquake risk analyses. Reliably presenting of earthquake threat is enabled by constucting monitoring local, regional, national networks. Before, after and at the time of the earthquake, description of the physical and chemical changes that occured as a result of crustal deformation contribute to definition of active faults, reaction of the faults with eachothers, earthquake prediction research. With the new scientific approaches, scientists tried to protect from natural disasters by determining them more accurately. It is a major step that human beings enabled monitoring the planet from space by means of satellite technology. Technological improvements not only provide making more precise analysis about understanding of planet but also come with new approximation methods that should be interdisciplinary. In this manner, studies of earthquakes, fault mechanism, earthquake parameter determination and earthquake effects on surface and undergound are leaded by geology, geophysics, geodesy, civil and mining engineers. Firstly, GPS is used by USA-Ministery of Defence for military aims like finding directions, military attacts and rockets firings. After its permission for civil usage, it has got several application field such as geodesy, geophysics, archeology and mining. The Global Positioning System has been the most applicable to understand crustal movements, fault behaviours, periodicity of the earthqukaes. GPS consist of satellites spin around orbit continously and receiver-controler system situated definite position on plates. The satellites porpagate radio signals to receivers and GPS receivers enables positionings of stations. GPS technology is widely use by geodesy and photogrammetry for positionings borders, structures and map theme. By taking data from a GPS device repeatedly, movement along the faults and subduction zones between plates can be determined. Earthquakes can be describe as a complex elastik wave propagation due to relaxion of accumulated energy (flexibal deformation) in the faults. After the earthquakes, two type deformation emerge, static and dynamics deformation. Static deformation is a permanent (plastic) deformation depending on offset quantity and mechanism of the fault. Dynamic deformation propagates by elastical waves during and after fracturing of the fault and it cause to pressure, rotation and distortion. Reid, source of inspiration in this monitoring technology, is first person leaded to study about earthquake mechanism solution and crust deformation by geodetic measurements on San Francisco Earhquake back in 1906.Elastic rebound theory is an clarification for how energy is spread during earthquakes. According to Reid s theory, the geodetic network and triangulation station is being deformed when it is subject to any seismic forces, and faults behaviours can be modelled with the help of this theory. Determination studies of crustal deformation are conducted by many scientists work in different disciplines. Nowadays, deformation measurements of specific coordinate which take place on the plates is available. By the help of these measurements, expected stress value that accumulated in crustal and under these stress values ascertainment of litosphere s behaviour that depends on its elastical properties is available. In order to do these calculation correctly GPS measurements have to have high presicion. These researches have vital importance from the view point of understanding time dependent tectonic movements and prediction of occurrence probability.The biggest disadvantage of GPS technology is being dependant to USA decision as seen 2003 Iraq War. Due to these manipulations, GPS stations don t have continous data. These kind of shortages removed by linearization. Regarding the area to be considered, the measurements which are to determine the changes in structure of crustal that occurred in specific time should be continual. Only when this applied, the fault behaviours can be defined by means of the measurements that are taken before the eartquake, at the time of earthquake and after the earthquake. Three different data sets are worked by using the continous and year average of GPS data sets for different regions in the extent of this study.Referring to the results of the study areas, the principal stress maps are obtained by making use of contouring methods with using different themes. All of this application is executed in MATLAB. Anayzing, visualization and mapping application for all of the study areas are composed of some hierarchy coding in MATLAB. Firsly, latitude and longitude values of stations are executed for the interested study area. After that, lateral displacement component of the stations are executed to the MATLAB Workspace as inputs. And then coordinat values of coastal lines for the interested study area are entered to the MATLAB. After that input execution, decisive parameters like unit deformation velocity tensor, stress velocity tensor, principal velocity values and rotations of this values are obtained by doing some linear assumption. Seven GPS stations which have continous measurements and a GPS network are created for the region of the first study area and its surroundings. To determine the effect of 17 August 1999 Gölcük Earthquake on GPS network, GPS measurements that cover approximately 10 yers time period between 7 May 1998 and 27 July 2008 are analyzed. To determine whether there is any relationship between changes in GPS datas and earthquakes or not, 1356 earthquake record is defined that covered the study area between 7 May 1998 and 1 January 2006. Earthquake data sets are obtained from Kandilli Observatory and Earthquake Research Institution. Taking into consideration of finite element network for the first study area, for the most affected element by 17 August 1999 Gölcük Earthquake, displacement and stress alterations are compared with definite earthquake records. According to the comparisons, after the 17 August Gölcük Earthquake, there is a strict harmony between seismicity of the area and strain velocity and unit deformation velocity. In so far as roughness of the GPS network and because of linear behaviour assumption of the crust, creep behaviour that should be observed in especially Ismetpasa segment of the North Anatolian Fault can t be obtained. When principal stress maps are investigated for the first study area, stress values are compatible with tectonics of the areas. Most affected element of the network takes place in Aegean region, hence this region is one of the most famous area in terms of earthquake activity. Principal stress maps refer that there is not only a decreasing of the stress values but also shows that there is a counter clockwise rotation of the stress values. For second study area which take place on Turkey and surrounding, by considering yearly avarage GPS measurements between 1988 and 1997 which involde 32 GPS station and 51 element, more detailed than first study network is designed. The measurements cover all area from Cucasus Mountains to Adriatic Sea and from north of the African plate to south margin of the Europian plate. After analyzing of the second study area, when distribution of the principal stress values are investigated, regions that contain the highest stress values like Erzincan and surrounding area, Eagen costlines and Saros Gulf are at the same time most active regions in terms of sesimic activity. Also, there is a obvious lineament between the distribution of the shearing stress values and North Anatolian Fault. This Lineaments proofs right lateral strike baheviour for the NAFZ. As it is well known, after Marmara and Düzce Earthquake,Marmara region is a center of attention for many scientists who are from different disciplines . Especially, there are many studies for the Marmara region about determination of the faults mechanism and periodicity for taking some precaution to be protected from destructive effects of earthquakes. In the third study area, the most appropriate GPS network is constructed and using the finite element method proposed by Aydan (2000), the variation of the quantity and the direction for the principal stress distribution is executed in Marmara Sea and the vicinity of it.The last study area GPS network that cover Marmara Sea and nearby is created with using 28 GPS station and 43 element. According to maps show distribution the principal stress values, the most notable areas in terms of high shearing stress are compatible with clustering of epicentres.The results obtained are compared with Turkey’s plate tectonic and they are consistent with each other. In conclusion, as a part of our daily life GPS which is a system of remote sensing and measurement is very effective and open-to-improvement instrument for defining earth dynamics. With the widespread usage of GPS scientif researchers would be able to understand nature easily. Although in this study, analyzied data and density of the networks for all case studies were rough, by using more dense network and more continous GPS time series, better result could be obtained.