Bazı depremlerin çeşitli özelliklerine göre karşılaştırılması
The Analysis of some earthquakes according to various aspects
- Tez No: 83101
- Danışmanlar: DOÇ. DR. NECMETTİN GÜNDÜZ
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1999
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Yapı Analizi ve Boyutlandırma Bilim Dalı
- Sayfa Sayısı: 141
Özet
BAZI DEPREMLERİN ÇEŞİTLİ ÖZELLİKLERİNE GÖRE KARŞILAŞTIRILMASI ÖZET Meydana geldiği her yerde büyük can ve mal kayıplarına neden olan depremler en büyük doğal afetlerden biridir. Yıllardan beri oluş zamanı ve yerinin tahmini, oluşturduğu yatay ve düşey kuvvetlere karşı dayanımlı yapı tasarımı çabalan durmaksızın sürmektedir. Bu çalışmada ise Türkiye'den Erzincan (1992), Dinar (1995) Amerika Birleşik Devletlerinden Loma Prieta (1989) ve Northridge (1994) depremleri sırasıyla meydana geldikleri bölgelerin tektonik yapı-zemin durumu, sismolojik bilgiler-yer hareketi kayıtlan ve hasar durumuna göre incelenmiş, her bölüm sonunda karşılaştırmalar yapılmıştır. Görülmüştür ki, depreme dayanıklı yapı tasannu ve inşası, bölge tektonik ve zemin yapısı düşünülerek hazırlanmış şehir planı can ve mal kayıplannı en aza indirmektedir. xı
Özet (Çeviri)
THE ANALYSIS OF SOME EARTHQUAKES ACCORDING TO VARIOUS ASPECTS SUMMARY This study examines 1989 Loma Prieta and 1994 Northridge earthquakes in the USA, 1992 Erzincan and 1995 Dinar earthquakes in Turkey, respectively, considering three major issues: tectonic and soil characteristics of the regions, seismic data and records regarding earth movements and, finally, damages caused by the earthquakes. Furthermore, this study aims to reach to a general understanding on the subject through analyses of similarities and differences between the earthquakes in the respect of the aforementioned, major issues. The 1989 Loma Prieta earthquake occurred near San Francisco, California, through rupture of a 40 km-long portion of the San Andreas fault, which separates Pacific and North America tectonic plates, running approximately 1300 km from California bay to Cape Mendocino, the north of San Francisco. On the contrary of previous records explaining a typical, horizontal movement from the southwest to the northwest along this fault line, during the Loma Prieta earthquake, vertical movements from the southwest to the northeast were recorded, as well. In addition, the soil of the California bay area and surrounding areas is generally formed by soft, saturated sand and clay, which are one of the major factors causing great damages during an earthquake. Another major earthquake along the San Andreas fault line occurred in Northridge, which is located in San Fernando valley, 32 kilometers northwest of Los Angeles city center and 100 kilometers west and south of the San Andreas fault line. The 1994 Northridge earthquake was caused by a 3-meter vertical movement in the northwest direction of a 15-km fault line, which was an extension of the San Andreas fault line and unknown upto that date. Due to the earthquake, San Fernando valley moved several centimeters in the northwest direction, and the Santa Susanna mountains lying on the northwest of the valley rose 70 centimeters. However, after the 1989 Loma Prieta earthquake, occurrence of such an earthquake with high magnitude along the same fault line was forecasted with high probability. Furthermore, the soil of San Fernando valley is formed by coarse alluvium deposits. The 1992 Erzincan earthquake occurred on the 50 km-long and 12 km-wide Erzincan plain in the eastern region of Turkey. The approximately 1500 km-long North Anatolia fault line located in the east-west direction from Karliova, Turkey to Greece runs from the southeast to the northwest of the Erzincan plain. In addition, the approximately 350 km-long Northeast Anatolia and the approximately 120 km-long East Anatolia fault lines are located in the northeast and southwest of the Erzincan plain, respectively, running to the direction of the North Anatolia fault line. Having examined records of the region, it is concluded that the Northeast Anatolia fault zone including the Erzincan plain is full of seismic activity throughout the last 2000 years. xnHowever, the soil of this area consists of a 300 meter-thick alluvium deposit layer formed by silt, sand and gravel, and the underground water level is approximately between 25 and 30 meters. Another major earthquake occurred in Turkey is the 1995 Dinar earthquake. Dinar is a district within the Afyon county limits in the western Anatolia, where an important horst-graben system called“the region of lakes”is located. A horst-graben system is formed by vertical movement of a fault line, thus collapsing layers and rising layers are named graben and horst, respectively. The region of lakes was formed by the movement of a 20-25 km-long portion of the 75 km-long Dinar fault line, thus the collapsed western part and the risen eastern part of the area were named Dinar graben and Akdag horst, respectively. Furthermore, the soil of the Dinar graben and the Akdag horst is formed by soft, max. 100 meter-thick alluvium deposit layer and hard rocks, respectively. In respect of the above-mentioned tectonic and soil characteristics, it is concluded that all of the four earthquakes occurred in regions nearby active fault lines such as the San Andreas, the North Anatolia and the Dinar fault lines, which remain active throughout the centuries due to the movements of the tectonic plates such as the Pacific tectonic plate towards the North America tectonic plate, the Arab tectonic plate towards the Asia tectonic plate and the Africa tectonic plate towards the Anatolia tectonic plate. Surface deformations, except those in Dinar, cannot be observed with clear and continuous conditions since seismic waves were mostly absorbed by the soil due to the soil formation with alluvium deposit layers and the large distance between the origin and the surface. In addition, soft, saturated soil such as that in San Francisco area and alluvium deposit layers such as those in Dinar graben, the Erzincan plain and San Fernando valley increase the intensity of earth movements, causing greater damages. The 1989 Loma Prieta earthquake occurred on a 50 km-long and 25 km-wide area with a magnitude of 7. 1 on the Richter scale and an intensity of VIII on the Mercalli Intensity Scale. The epicenter of the earthquake was 16 km northeast of Santa Cruz, and the origin was 18 kilometers below the surface. The rupture on the surface began spreading to both directions in the middle with a velocity of approximately 2.5 km/s and lasted 8 seconds, reaching a total length of 40 kilometers. 2.5 minutes after the main shake, an earthquake with a magnitude of 5.2 occurred, and in the following week, it was succeeded by 20 earthquakes with a magnitude of 4 and 300 earthquakes with a magnitude of 2.5. In the region of the epicenter, maximum horizontal and vertical accelerations recorded were 0.65g. In the neighbour cities such as San Francisco and Oakland, accelerations of 0.20g-0.33g were recorded, which is very high, considering the distance between the epicenter and the cities. It is forecasted that the high accelerations recorded in the distant places were caused by the soft soil formed by deposit layers, which increases the intensity of earth movements. The 1994 Northridge earthquake had a magnitude of 6.9 on the Richter scale and an intensity of VÜI on the Mercalli Intensity Scale. The epicenter of the earthquake was 1.6 km south of Northridge, and the origin was 18 kilometers below the surface. The xinearthquake lasted 15 seconds, and it was succeeded by 2 earthquakes with a magnitude of 5 and 35 earthquakes with a magnitude of 4 in the following week, by 3000 earthquakes with a magnitude of 1. 5 in the following three weeks and then by 6300 earthquakes in the following six months. Maximum horizontal and vertical accelerations recorded were 0.3g-1.8g and 0.2g-1.2g, respectively. The 1992 Erzincan earthquake had a magnitude of 6.8 on the Richter scale and an intensity of VTA on the Mercalli Intensity Scale. The epicenter of the earthquake was 8 km northwest of Erzincan, and the origin was 26-28 kilometers below the surface. The earthquake lasted 15 seconds, and in the following 50 days, 120 earthquakes with magnitudes of greater than 3 were recorded in the region. The maximum magnitude recorded during this period was 5. 8. Maximum horizontal accelerations in the east-west and north-south directions and maximum vertical accelerations were 0.50g, 0.40g and 0.25g, respectively. The 1995 Dinar earthquake had a magnitude of 6 on the Richter scale and an intensity of DC on the Mercalli Intensity Scale. The epicenter of the earthquake was 9 km southwest of Dinar, and the origin was 29 kilometers below the surface. The earthquake lasted approximately 25 seconds. The main shake was preceded by 9 earthquakes with magnitudes of greater than 3.5 in the preceding week and succeeded by 34 earthquakes with magnitudes of greater than 3.5 in the following week. Maximum horizontal accelerations in the east-west and north-south directions and maximum vertical accelerations were 0.34 g, 0.29 g and 0. 15g, respectively. In respect of the above-mentioned seismic data and records, the earthquakes studied are arranged in order by their magnitudes on the Richter scale, their intensities on the Mercalli Intensity Scale and their maximum accelerations. In conclusion, the Dinar earthquake is the first one according to the Mercalli Intensity Scale although it is the last one according to the Richter scale and the maximum accelerations recorded. This situation explains that the structures in the region of Dinar are the weakest among the structures in the earthquake regions studied. However, the Erzincan earthquake has the highest rate in dead people (653). It is followed by the earthquakes in Dinar (90), Loma Prieta (63) and Northridge (57), respectively. It is thought-provoking that the Erzincan and the Dinar earthquakes take the first and second places in the order made according to the numbers of dead people although they rank below the other earthquakes in the orders made according to the Richter scale, the acceleration records and the number of people living in the region. In order to take the necessary measures, the damages caused by the earthquakes should be analyzed. Soil Conditions are one of the great factors causing damages. The soil formed by alluvium deposit layers of various thickness in Loma Prieta, Northridge, Erzincan and Dinar increased the intensity of the earth movements, causing greater damages on the surface. For example, the Akdag horst in Dinar, where the soil is formed by hard rocks, suffered not as much damages as the Dinar graben did, where the soil is formed by soft, 100 m-thick alluvium deposit layer. Similar situation occurred during the Loma Prieta earthquake, where San Jose, which is closer to the epicenter, but has good soil conditions, suffered not as much damages as San Francisco and Oakland did, which are distant from the epicenter, but have weak soil conditions such as soft, saturated sand and clay. xivStructures, whose natural periods are equal to the default period of an earthquake, are likely to suffer great damages. Natural period of structures is approximately equal to one tenth of the total number of stories. This situation was observed in the wooden- frame structures with 1, 2 stories in San Francisco, the structures with six or less stories in Northridge, the structures with 3, 4 and 5 stories in Erzincan and the structures with 2-5 stories in Dinar. Landslides occurred during the earthquakes were another factor causing damages in Northridge and Loma Prieta. Soft storey behaviour is also a factor damaging structures under earthquake loads. It means that stories, whose rigidity are less than the upper stories, may cause partial or total collapse of structure. Often due to architectural purposes, stories have different heights, or shearwalls do not exist in a continuous, vertical form, or longer sides of vertical elements such as columns or shearwalls are located only in one direction, leaving the structure vulnerable to loads coming from the other direction. For example, most structures with 1, 2 stories in Loma Prieta had garage doors instead of shearwalls on the first floor, and in Dinar and Erzincan, some buildings had stories with different heights. The damages caused by the defects in the connection area of columns and beams such as use of insufficient quantities of stirrups and insufficient anchorage lengths of longitudinal rebars were observed in Loma Prieta, Erzincan and Dinar. In addition, the use of incorrect information from the old edition of building codes such as the frame-design data, a lower constant for lateral loads and insufficient dimensions given for columns and beams was one of the factors causing damages in the old buildings in Dinar and Erzincan. The insufficiency of the code for the earthquake design-work regarding all elements in a structure was a factor causing damages in Northridge, as well. Furthermore, the buildings in Erzincan and Dinar, which were not built according to the building codes, suffered great damages. The strength of some materials used in structures was not as per the required standards for such earthquake loads. For example, wooden frame-works and lime mortars used in Loma Prieta lost their strengths due to the effects of the atmosphere, and the compressive strength of the concrete used in Dinar and Erzincan was less than that required as per the codes. The short columns used in some reinforced concrete structures in Erzincan and Dinar were affected with great shear loads, and they collapsed or were damaged heavily without being capable of doing any horizontal displacement. The insufficient connection between the slabs and walls in precast structures prevented them from working as a complete system and caused their collapse during the earthquakes in Northridge and Loma Prieta. Equipment, which were not installed correctly, damaged the buildings during the Northridge and Loma Prieta earthquakes. Furthermore, the insufficient connections of sandwich panels and other architectural elements outside of the buildings caused damages during the earthquakes in Erzincan and Loma Prieta. xvBecause the quantity and the quality of welding in some steel structures did not comply with the required codes, related damages occurred in the steel structures in Northridge and Erzincan. Due to the long duration of the construction work in Erzincan and Dinar, weak adherence of concrete and decrease in cross-sectional rebar area due to corrosion resulted weak structures. Therefore, joints formed under the columns caused damages due to the horizontal movement of the upper parts of columns under great moments formed by earthquake loads. Besides, workmanship failures during the construction work were another factor of damages in Loma Prieta, as well. Since reinforced concrete buildings are heavy structures, they were affected by greater earthquake loads compared to other structure types. In conclusion, a thorough analysis of common causes of major damages occurred during earthquakes has great significance in identifying and taking the necessary measures in order to minimize the number of casualties and damages. xvi
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