Bazı önemli depremlerin çeşitli yönleriyle incelenmesi

Abstract

İnsanlık tarihi boyunca oluşmuş en önemli doğal afetlerden biri olan depremler, günümüzde de halen önemli miktarlarda can ve mal kayıplarına sebebiyet verebilmektedir. Yerkabuğunun titreşimleri olarak ifade edilebilen depremlerin etkilediği bölgelerdeki şiddet seviyeleri, depremlerin büyüklüğünden başka çok çeşitli parametrelerede bağımlı olarak değişmektedir. Bu çalışmada esas olarak yapılmak istenen ülkemiz içinde ve dışında meydana gelmiş önemli depremlerin çeşitli yönleriyle ele alınıp tektonik yapı, zemin durumu, sismik aktivite, kuvvetli yer hareketi ve hasar durumunun incelenmesidir. Çalışmada güncelliğini koruyan Kobe (1995), Dinar (1995) ve Erzincan (1992) depremlerinden başka Romanya (1977) ve Erzurum (1983) depremleride irdelenmiştir. Bu araştırmalar sonucunda deprem hasarlarına kuvvetli yer hareketi ile beraber bölgenin tektonik yapısının ve zemin durumunun büyük tesiri olduğu gözlenmiştir. Buna paralel olarak deprem yörelerindeki kötü ve bilinçsiz yapılaşmaların hasarı olumsuz yönde etkilediği anlaşılmış ve özellikle ülke sınırları içerisindeki depremlerin şiddetini arttıran en önemli faktörün yanlış, eksik ve umursamazca inşa edilen yapılar olduğu görülmüştür.

The earthquakes, one of the most important acts of God throughout the history of humanity, are still result in sustantial human and property losses at the present time. The level of intensity of the earthquakes, defined as the vibration of the crust of the earth as well, depends on various parameters besides the size of the earthquakes. The aim of this study is to consider the important earthquakes both in and out our country with different aspects and to determine the relationship between the tectonic setting, state of the soil, seismology, strong ground motions and the resulting damage. The earthquakes of Kobe (1995), Dinar (1995) and Erzincan (1992) together with those in Rumania (1977) and Erzurum (1983) are examined in this study. Kobe Earthquake: The Great Hanshin (Hyogo-Ken Nanbu) earthquake, happened on 17th January 1995, is defined as the most serious earthquake of the Japanese history, which directly effected an urban area, and as the second big act of God after the Great Kanto earthquake of 1923. It happened at 05.47 local time. In Japan, the earthquakes of land are less than those at sea. The center of Kobe earthquake, as well, is at the sea (Akashi Strait). The Kobe earthquake presents a right lateral dischargeable fault in northeast- southeast direction. The tectonic tear begins at the north end of Awaji island and has a length of 40-50 km to both directions. Kobe city is located the foot of a mountain, at such foot, there are narrow flood beds, alluvion fans, river deltas and regions gained as filled up areas. Along the coastal region between Kobe and Osaka there are intensive liquefaction and subsidence problems at soft alluvion grounds, filled areas and artificial islands (Port, Rokko). As per USGS-PDE data, the resource parameters of Kobe earthquake are as follows : XVI X Time of happening : 20:46:51, UTC Coordinates of the center : 34,55N, 135,00E Depth : 16 Km. Instrumental Volume : Mb = 6,4 Ms = 6,8 Seismic moment : Mo = 6,3 x lCr Nm After the main shock several successive earthquake were recorded. According to the records of Kobe Station, the greatest acceleration, speed and displacemenet values were 0.82g, 100 cm/sn, 15 cm at north- south direction, respectively, 0.62g, 75cm/sn, 11 cm at east-west direction, 0.33g, 40 cm/sec and 10 cm at vertical direction. The period of strong earth movement has been accepted as 15 sec. The speed increased up to 130 cm/sec on soft filled-uup ground near the fault. As result of Kobe earthquake 0.25 % of the total population died and 16% lost their home. 90% of deaths was the consequence of the collapse of the wooden carcass one or two-floor buildings, not built by engineers. Countrary to the survival performance of high buildings, the intensive damage occurred at the commercial buildings lower than 10-12 floors with steel or, mostly, reinforced steel carrying systems is really interesting. It is very common that there are collapse of ground floors at engineered buildings with 4 and 10-12 floos due to formation of soft layer. Another common type of damage is that only one intermediary floor collapsed of the intermediate floors of reinforced concrete buildings. There are various opinions regarding such damage. For example, in the Japanese regulations prior to 1981 one of the possible reasons of the collapse of intermediate floors it has been suggested that the distribution acceptance of earthquake loads caused smaller floor cutting forces at higher floors. To another opinion, such damages occur at sudden transitions from rigid and strong lower floors to relatively weaker upper floors. Regarding the structural damage at bridges, Kobe earthquake is the greatest earthquake ever since. The heavy reinforced concrete superstructure, different dislocation of neighbour bridge legs longitudinally, the insufficiency of transversally winding fittings and the weakness of construction seams can be regarded as the reasons of such damage. XVlll In Kobe city heavy damages were observed at autoways and harbours due to subsidence and liquefaction problems prevailing especially in filled- up regions and artificial islands. Erzincan Earthquake: Erzincan earthquake happened on 13th March 1992 at 19.19 local time. The center was near city of Erzincan. Erzincan earthquake occurred at the North Anatolian fault region, which is a right lateral fault. The city of Erzincan is located in the middle of the plain, called with the same name, in the upper Fırat basin. The north and south of the plain are surrounded with mountains. These mountains contain paleozoic, metamorphic and mesosoic series. The rocks on the mountains are barren and the rocks faling down have formed heaps of 50-100 m. The deposits in the Erzincan river-basin comprise pebbles, sand and clay. At the eastern side of the river-basin, where the water level is high, there are liquefaction problems. Erzincan plain is covered with a thick alluvion layer consisting of pebbles, sand, silt and clay. The thickness of the layer is suggested to be 300 m. As per USGS-PDE data, the resource parameters of Kobe earthquake are as follows : Time of happening : 17:18:39,9 UTC Coordinates of the center : 39,71N, 39,61E Depth : 27 Km. Instrumental volume : Mb = 6,2 Ms = 6,8 Seismic moment : Mo = 5,2 x 1019 Nm After the main shock several successive earthquakes were recorded. According to the records of Erzincan Station, the greatest acceleration, speed and displacement values were 0.4g, 105 cm/sec, 30 cm at norm- south direction, respectively, 0.5g, 80 cm/sec, 20 cm at east-west direction, 0.25g, 22 cm/sec and 7 cm at vertical direction. The period of strong earth movement has been accepted as 15 sec. As result of Erzincan earthquake 0.57 % of the total population died and 16.7 % lost their home. 90 % of deaths was the consequence of the collapse of multi-floor reinforced concrete residences. XIX The most important reason of the heavy damage of Erzincan earthquake was the errors in technical practice and technical deficiencies. As examples for errors in technical practice and technical deficiencies we can mention the non-compliance to reinforced concrete data given in the project, the. mistakes by the selection of the materials for reinforced concrete, the deficiencies in the hoop frequency, the insufficiencies of supplementary equipments, the continuing order of strong joist weak column, transitory curtain and column order, ground floors without dividing wall causing the formation of soft floors, one-directional curtain construction and decomposition of reinforced concrete as result of wrong casting procedures. Furthermore, especially the dominance of areas with alluvion layers and the very close distance (ca. 0.3 sec) between the natural periods of the buildings at such city wards increased the damage. The damages at the roads, bridges and railways are less than those at the buildings. Such damages, not causing any death, did not hinder the everyday life of the habitants. Although some liquefaction was observed especially the eastern side of the basin, they did not cause damages of larger scale. Rumania Earthquake: The Rumania earthquake happened at the southern part of Rumania on 4th March 1977 at 21.22 local time. The center of the earthquake is near Focsoni town at south of the eastern Karpats. In Rumania continental earthquakes usually happens, both of narrow and deep characteristics. Bucuresti, where the most structural damage and death were observed and which is 160 km away from the center in the south-western direction, is located at a platform with palaezoic, mesozoic and neozoic formations on a ground covered crystal schist. The upper layers contain pebbles, sand, alluvion and clay. The alluvion and loose filling layers are below the earth's surface and of different thickness. Such accumulations reach 20 m at some places. The most important problem in the country is the presence of loose ground, which constitutes 11.4 % of the country. XX The magnitud of Rumania earthquake was M7,2 according to Richter scales. The depth was 110 km, the coordinates of the center were 45,84N and 26,73E. According to the data of the recording device for strong earthquakes at Rumanian Construction Research Center in Bucuresti, the greatest acceleration and 0.1 6g, 33 cm/sec in east-west direction, respectively. The period of stong earthquake is accepted to be as 19 sec. Regarding the Rumanian earthquake, the earthquake waves could go beyond great distances without any energy loss. The wave components became strong towards alluvion filled-up ground in the Northern Rumania and Bulgaria. 0.08 % of the total population lost their lives and 6.8 % their home. The traditional type rural houses, made of brick and sundried brick, suffered heavy damage, but death rate was low thanks to light wooden roofs of such houses. The outstanding reason for heavy damages is mud used for building walls. Some of the multi-floor reinforced concrete buildings either collapsed totaly or were subject to heavy demage. Among reasons for such heavy damage we can mention; the concrete supports being less than those proposed in the project, the insufficiency of the joint distance of the equipment at crucial points, the amount of additional equipments being less than requuired, the design of the buildings, constructed buildings before the n. World War according to only vertical loads and the rigidity differences between the floors. Another reason for the damage is that the dominant period of the earthquake and the natural periods of the high buildings are close to each other. In general, the damage is much more at the southern areas of the center point. It is possible to state that the alluvion layers in these areas increased the damage. No damage was observed at autoways, bridge and dams. Dinar Earthquake: Dinar earthquake happened on 1st October 1995 at 17.57 local time. The center point is at Dinar district. Dinar is located at the transition zone of the plain region of the Middle Anatolia and the opening areas in the Western Anatolia. There are a lot of faults in the region, presenting normal fault movement in the northeast- southwest direction and the directional discharge movement. The X5f1 earthquake is related to the faulting between Dinar and Çivril in northwest-southeast direction and perpendicular to other faults already known. The rupture is against the inclined normal fault mechanism. Dinar is located in the "Lakes Area" at the southwest of Turkey. A part of the centrum is founded at the hills in northwest-southeast direction, however, the majority of the residences, commercial and governmental buildings are located on the plain with alluvion characteristics toward southwest direction. The surface geology of the hills consists of Eosen and Kretose aged limestone, morl and schists. The plain comprises alluvion layers with sand, pebbles and clay. At the studies, it has been found out that the dept of the alluvion layer is up to 150 m. Taking the local geological and geotechnical features, we can divide Dinar and its environment into three main areas. Hilly area is formed by meidum-hard limestones. At the foot of the hills and the transition line between the hills and the plain consists of natural fillings and rough stones of the slopes. The plain area with high level of underground water contains alluvion accumulations. As per USGS-PDE data, the resource parameters of Kobe earthquake are as follows : Time of happening : 17:57:15.9 Coordinates of the center : 38.03 IN, 30. 147E Depth : 29 km. Instrumental volume : Mb =5.9 Ms = 6.0 Seismic moment : Mo = (2-3) x 10L8N.m. Before the main shock around 20 earthquakes happened and after it more than 40. As per the records of Dinar Meteorology Station, the greatest acceleration values were 0.28g in north-south direction and 0.33g at east- west direction and O.lg at vertical direction. Almost 0.26 % of the population died as result of Dinar earthquake Especially in Dinar district heavy damages occurred. The majority of the damages were observed in the alluvion plain at southwest Dinar. In Dinar, where the highest buildings are 5-floor ones, most of the reinforced concrete buildings with 4-5 floors suffered heavy damages XXI 1 collapsed completely. At some of the three-floor buildings similar damages were observed. Among the causes we can mention the concrete supports being less than those proposed in the project, the mezzanines at ground floors, extra floors, the inadequacy of the length of the ores left at the end of columns-joists, wrong and insufEcient steel arrangements and etrier with inadequate frequency. The collapse of ground floor due to formation with sof floor is another example for damage. Additionally, the unsuitabilities at the heap constructions increased the level at damage. The fact that the buildings in the plain with alluvion layer suffered more damage that those at the rocky slopes shows that the alluvion layer increases the strength of the earthquake. No significant damages occured at roads, bridges, railways and the similar. Erzurum Earthquake: Erzurum earthquake happened on 30th October 1983 at 7.13 local time. The center point of the earthquake was 20 km north of Horasan district. The formation area of Erzurum earthquake is outside the fault area of the Northern Anatolia and it happened in a region, seismotectonic features of which are not well known. As per the earth splits found out, it has been determined that it was with left lateral discharge. Erzurum and its surrounding, located in the Western Anatolia, is generally mountanious and contains high plateaus splitted by medium size valleys. The majority of the plateaus and valleys at the area of earthquake are covered with lava layers from time limits of IH. ear. (Neoden territorial deposits and volcanits of same age) As per USGS-PDE data, the resource parameters of Erzurum earthquake are as follow: Time of happening : 7: 12:27.7 UTC Coordinates of the center : 40.290N, 42.171E Depth : 17 km. Instrumental volume : Mb = 6.1, Ms = 7. 1 After the main shock several around 110 earthquakes were recorded. XXlll According to the records of State Meteorolgy Station in Horasan, the greatest acceleration, speed and displacement values were 0.16g, 22.81 cm/sec, 66.27 cm at east-west direction, 0.07g, 12.17 cm/sec and 22.39cm at vertical direction. The period of strong earth movement is more than 30 sec. It is very astonishing that at such a big earthquake the maximum ground acceleration 20 km away from the center point is around 0.1 6g. As result of Erzurum earthquake almost 0.39% of the total population died and 5.9 % lost their home. The local houses with heavy soil roofs and mud mortar suffered heavy damages and a lot of people died due to heavy roofs. Generally, the buildings such as school, health center and mosquue had relatively light damages, since they were constructed with more attention, or those suffering heavy damages didn't cause any death. The few reinforced concrete buildings could survive with low or medium size damages. Generally, some of the causes for these damages can be stated as non-uniform distribution of the system's rigidity, non-entirety and insufficient joint of the column-joist equipment, hoop frequency and insufficient cross section. Much more than usual damages were observed at the buildings erected on alluvion plains saturated with underground water and on grounds with landslide. As result of the earthquake study, following consequences are obtained in short ; Erzincan earthquake is that with heaviest damage regarding the number of dead and homeless people.The interesting point is that 90% the deaths were due to collapse of multifloor reinforced concrete buildings, in contrary to the Kobe earthquake. That is, the actual cause of high mortality rate at Erzincan is the multi-floor reinforced concrete buildings. In contrary, the mortality rate at Kobe earthquake due to reinforced concrete buildings is only 10 %. On population basis, Rumanian earthquake has the feature as causing least mortality rate. The fact that Rumanian earthquake is of deep earthquake nature and the light wooden roofs of the regional houses are among the most important XXIV reasons for such result. At Erzurum earthquake, however, the heavy roofs, being contrary to those in Rumania increased the number of dead people substantially. At Dinar, on the other hand, biggest damages occurred at reinforced concrete buildings, being similar to Erzincan earthquake, and most of them suffered heavy damages, resulting in high mortality rates. hi general, as regard of performances of reinforced concrete buildings, we observe that the damages at Erzincan and Dinar were 2-3 times more than those at Kobe. The collapse of ground floors due to soft floor formation are observed intensively at earthquakes in Erzincan, Kobe and Dinar. As regard of Rumanian earthquake, however, the structural damages are not of high level to result in the collapse of ground floors, as it was the case in Erzincan, Kobe and Dinar, although such damages occurred in conjunction with the sudden rigidity differences between the ground and upper floors. The intensive errors and deficiencies in technical practice, as seen at Erzincan, Dinar and Erzurum, are found out to be less at Kobe and Rumania. It is feasible to state that the alluvion filling layer found at Erzincan and Dinar, at the first place, and Kobe and Bucuresti has strengthened the earthquake and increased the level of damage. The heavy damages at autoways, bridges and harbours occurred at especially filled areas and artificial islands of Kobe due to subsidence and liquefaction problems have not been observed at Bucuresti, Erzincan, Erzurum and Dinar with a few separate exceptions. Although the acceleration and speed values of earthquake at Kobe were higher than those obtained at other places, and there are several similarities between Kobe and Erzincan earthquakes with regard of both source parameters and tearing mechanism, it is really hurting and trouble some to observe that the damages especially at Erzincan and Dinar were higher than that at Kobe. Several regretful experiences have been gained in these areas since long years, nevertheless the negligence of both the people and public authorities have resulted in intensified earthquakes.