Here is a list of ten major earthquakes: 1. Koyna Earthquake of 10 December 1967 2. Southern Italy Earthquake of 23 November 1980 3. El-Asnam, Algeria Earthquake of 10 October 1980 4. Mexico City Earthquake of 19 September 1985 5. Armenian SSR Earthquake of 7 December, 1988 6. Loma Prieta Earthquake of 17 October 1989 7. Northern Iran Earthquake of 21 June 1990 and Few Others.
1. Koyna Earthquake of 10 December 1967:
The Koyna earthquake of 10 December 1967 of M 6.3 is so far the largest and most damaging reservoir triggered earthquake. It claimed about 200 lives, injured about 1500, rendered thousand homeless and caused extensive property damage. Our team was camping at a site very near to the Koyna dam location on the downstream at an elevated site 2 days after the occurrence of the earthquake when aftershocks were still happening.
During the first night, we were awakened by a roaring sound of an aftershock, which was terrifying and scared us to death. The very thought of the collapse of the dam behind us made us shiver like jelly with fear.
2. Southern Italy Earthquake of 23 November 1980:
The sight in southern Italy after the earthquake of 23November 1980 was more shocking. Eighty percent of the village of Sant Angelo dei Lombardi had been destroyed and hundreds of people were buried under the rubble but nobody had come to their rescue during the first two days after the disaster. A full day after the tremor, the first outsiders arrived in the mountain village of Teora. “Are you the rescuers”? Exclaimed the first man to run up to them. “Thanks God, You’ve come. There are three hundred people under the rubble and some of them are still alive.”
The new arrivals were not rescuers. They were journalists. Two days after the earthquake help arrived in Sinerchia, another village in the Apennine Mountains. But it consisted of food and not the digging equipment. “There are people under there screaming, ‘Help, don’t let me die like this’,” wailed one resident; “and they bring us food”. Affected and helpless residents of the region told such horrifying stories to us.
The earthquake was Europe’s deadliest in 65 years. The quake began at exactly 7.36 p.m.- stopped clocks all over southern Italy confirming to the time. The tremor recorded 6.8 on the Richter scale. Everywhere, buildings collapsed: many victims died instantly, while debris buried others alive. The tremor shuddered from Sicily to the Alps. Around the epicenter in southern Italy, nearly a hundred towns crumbled into rubble. There was misery everywhere. The official count of the dead stood at more than 3000, and some rescue workers predicted that it might reach 10,000.
3. El-Asnam, Algeria Earthquake of 10 October 1980:
The city of El-Asnam and its surroundings in central northern Algeria were struck by two major earth tremors during the afternoon of Friday 10 October 1980. The tremors, which reached 7.3 and 6.4 on the Richter scale, rendered more than 300,000 people homeless, killed or injured close to 11,000 persons and completely destroyed more than half of El-Asnam city. In addition, many rural settlements situated in an area of 900 square killometres were also badly damaged or destroyed.
These earthquakes were remarkable for several reasons. The epicenter was very close to that of the last major earthquake in Algeria, that of 9 September 1954 (magnitude 6.7) which also caused great destruction and loss of life in El-Asnam, known at that time as Orleansville. Furthermore, this earthquake was particularly unusual from others since the strongest movement was in the vertical direction, especially in the epicentral region.
The manifestation of nature’s miracle during the earthquake could be witnessed in the form of a surface fault extending about 40 km on a bearing of N 50 degrees E, from a point about 10 km south-southeast of El-Asnam to the hilly country northeast of El Abbadia. A maximum of 5 m vertical shift during this earthquake has been reported. Initial shock, which was upward, was followed after a few seconds by horizontal oscillations, mainly in an east west direction and lasting several tens of seconds.
In the city of El Asnam and in the small towns and villages to the east and north, damage was severe. A large number of modern buildings including multi-storeyed residential blocks, schools, public buildings and industrial structures suffered severe damage or collapsed during the earthquake. Eighty five schools collapsed in El Asnam alone. One shudders at the thought of what would have happened had the earthquake occurred during school time. Most of these structures were constructed in reinforced concrete frames with masonry infill walls.
In El Asnam itself, detailed examination of the structural damage provided information of considerable interest for earthquake engineering. The older buildings still standing in the city had survived the 1954 earthquake. Some of the new three and four storey residential apartments in reinforced concrete frames with masonry infilling and other medium high structure had been constructed according to the then French code “AS 55”.
The provisions of this building code seem to have provided some safety to low, rigid buildings but failed to provide protection to other buildings such as Al Nasser market complex and hotel Chelif. Al Nasser market complex totally collapsed because the accelerations developed during the 1980 earthquake were almost double than those given in code AS 55. In addition, for the Al Nasser market complex, the local subsoil and topography had the effect of amplifying the effect of ground motion.
Structural Failures:
Many of the structural failures were caused by the unusual vertical high accelerations, improper design provisions: for example, shear failure of short columns between footings and ground floor beams and slabs, long span beam failures, and the buckling of columns and the crushing of infill brick walls in concrete frame buildings.
Other causes of structural Failures are attributed to:
(1) Building Orientation:
The extent of damage in certain buildings appeared to be dependent on the orientation of the building axes with respect to the fault. The damage survey revealed that the buildings having their long axis about northwest to southeast suffered more damage than those having perpendicular to them. Since the direction of future ground shaking cannot be predicted so it becomes obligatory to provide adequate strength about both the axes.
(2) Rigidity Distribution:
Another predominant cause of damage was the distribution of structural and non-structural elements like shear walls and masonry panels in the buildings. This type of construction was typical of several recently built multi-storey blocks, some of which collapsed in a shear mode with plastic hinges in the columns because of insufficient strength in the direction of longitudinal axis along which the ground shaking was excessive.
In the central part of the city, there were four storeyed buildings built from a special structural system consisting of two bay frames in the transverse direction; semi-pre cast floor system, and a sanitary floor of smaller height. The reinforced concrete transverse walls of the stairwell were constructed up to the sanitary floor. During the earthquake, the short columns of the sanitary floors of all the buildings totally collapsed and the structure suffered large deformations.
The sanitary floor having smaller height columns as compared to other floors produced discontinuity of rigidity along the height of the building. The presence of an over strong part in a structure results in the ductility demand to be concentrated into local regions of the structure, and leads to collapse because of very high inelastic deformations enforced there. Unless the structural rigidity is well distributed horizontally and vertically, it is impossible to predict the mode of failure.
A discontinuity of rigidity requires earthquake rotational effects to be taken into design consideration. The location of shear walls and infill panels can be judiciously planned to provide reasonable rigidity along both directions. At the same time, care should be taken to keep the centre of rigidity and the centre of mass close to each other to minimize the torsion effects.
(3) Soft Storey Effect:
Many of the buildings that collapsed during earthquakes suffered the ‘soft storey’ effect because the bottom one or two storeys had fewer partition walls than the upper storeys. A soft storey is a storey in a building (often the first storey) with few partitions and/or open exterior walls (such as large garage doors, large windows, etc).
This openness makes it vulnerable to earthquake damage. Earthquake energy gets concentrated in the more flexible areas and places very high demands on the columns and walls. Soft storeys are universally recognized as dangerous, but if architects and engineers collaborate early in the design, the problem can be eliminated without detriment to architectural objectives.
(4) Column Failures:
Four typical modes of column failure were prevalent:
(1) Column side sway mechanism,
(2) Crushing of concrete at the top under the impact of the vertical load,
(3) Shear and flexure failure, and
(4) Local buckling of longitudinal bars near the top.
The widely spaced ties could not contain the concrete core. All the post elastic deformations were due to plastic rotations at the hinges in the columns at the critical sections. The local buckling of longitudinal bars, and the falling out of the concrete core stresses the need of providing closer ties near to the top and bottom sections of the column.
(5) Construction Related Failures:
Poor detailing of joints, abrupt cutting of reinforcement and insufficient anchorage lengths also reduce the strength of joints considerably when subjected to cycling loading.
Earthquake damage was observed to be dependent “upon the quality of materials and construction as well. It was observed that some parts of the structure came down like a lump of debris whereas the adjoining part was slightly damaged.
4. Mexico City Earthquake of 19 September 1985:
On 19 September 1985, at 7:17 a.m., an earthquake of magnitude 8.1 on Richter’s scale occurred along the Pacific coast of Mexico. The damage was concentrated in an area of 25 km2 of Mexico City, 350 km from the epicenter. Of a population of 18 million, an estimated 10,000 people were killed and 50,000 were injured.
In addition, 250,000 people lost their homes and property damage amounted to 5 billion U.S. dollars. Over 800 buildings crumbled including hotels, hospitals, schools and business centres. Communications between the Mexican capital and the outside world were interrupted for many days.
Surrounding areas affected by the earthquake included the Mexican States of Jalisco, Guerrero and Michoacan. Damage in the epicentral region was restricted to a few tourist resorts and industrial estates along the Mexico Pacific coast. A two meter high tsunami also caused some damage in this area.
There are geologic reasons why Mexico and especially Mexico City are vulnerable to earthquake damage. Along the west coast of southern Mexico and Central America the Cocos Plate dips beneath the North American Plate producing a very active seismic zone. Since the beginning of the twentieth century, 35 earthquakes of magnitude greater than 7.0 have occurred in this zone.
The location of the 1985 earthquake’s epicenter near the coast at the border between the states of Michoacan and Guerrero was not a surprise. Prior to the 1985 earthquake, this area located between two areas that had experienced recent earthquakes was known as the “Michoacan Gap.” The “gap” was filled in 1985 by the main shock and a severe aftershock (of magnitude 7.5) that occurred the next day.
Mexico City itself lies in a broad basin formed approximately 30 million years ago by faulting of an uplifted plateau. Volcanic activity closed the basin and resulted in the formation of the Lake.
The Aztecs chose an island in Texcoco Lake as an easily defended location for their capital. The expansion of the capital (Mexico City) and the gradual draining of the lake left the world’s largest population centre located largely on unconsolidated lake-bed sediments. These soft sedimentary clay deposits in the lake bed amplified the seismic waves and subsided carrying buildings down with them.
Double resonance coupling between the earthquake waves, the sub-soils and the buildings caused intensity IX shaking in some areas, lasting up to three minutes. Earthquakes in 1957 and in 1979 also damaged Mexico City. However, neither of these earthquakes were quite as devastating as the 1985 earthquake.
In the area of greatest damage in downtown of Mexico City, some types of structures failed more frequently than others. In the highest damage category were buildings with six or more floors. Resonance frequencies of these buildings were similar to the resonance frequencies of the subsoil. Because of the “inverted pendulum effect” and unusual flexibility of Mexico City structures, upper floors swayed as much as one meter and frequently collapsed.
Differential movements of adjacent buildings also resulted in damage. A tall flexible building often failed when adjacent more rigid lower buildings held it. Damage or failure often occurred where two swaying buildings came in contact. Corner buildings were also vulnerable to damage. Lessons learnt from different patterns of earthquake damage need to be quickly implemented to prevent future disaster when building in another area along the Mexican coast between Acapulco and Zihuataneio.
5. Armenian SSR Earthquake of 7 December, 1988:
On 7 December 1988, at 11:41 a.m. local time an earthquake of magnitude 6.9 shook northwestern Armenia that was followed four minutes later by a magnitude 5.8 aftershock. Swarms of aftershocks, some as large as magnitude 5.0, continued for months in the area around Spitak. The earthquakes hit an area 80 km in diameter comprising the towns of Leninakan, Spitak, Stepanavan, and Kirovakan in the Armenian Soviet Socialist Republic.
The region is part of a broad seismic zone stretching from Turkey to the Arabian Sea near India. Here, the Arabian landmass is slowly colliding with the Eurasian plate and thrusting up the Caucasus Mountains in the north. The earthquake occurred along a fairly small thrust fault running north-west-south-east, apparently right under Spitak. During the earthquake, the Spitak section to the northeast of the fault rode up over the southwest side.
Geologists have located a 1.6 meter-high, 8-km long scarp just southeast of Spitak where fault movement broke the surface.
The earthquake epicenter was located in the Lesser Caucasus highlands, 80 km south of the main range of the Caucasus Mountains. Historically, this area has experienced damaging earthquakes during 1899 and 1940 that occurred within 100 km of the 1988 earthquake’s epicenter. These events had magnitudes of 5.3 and 6.0 respectively. In 1920 a 6.2 magnitude earthquake, that killed forty people, occurred north of Spitak. In 1926 an earthquake of about magnitude 5.6 occurred 20 km southwest of Leninakan and reportedly caused more than 300 deaths and extensive damage to property.
Despite its moderate size, the deaths and damage that the December 1988 earthquake caused made it the largest earthquake disaster since the 1976 earthquake of magnitude 7.8 in Tangshan, China that killed more than 250,000 people. The Town of Spitak (population 25,000) was nearly leveled and more than half of the structures in the City of Leninakan (population 250,000) were damaged or destroyed. Damage also occurred in Stepanavan and Kirovakan and other smaller cities. Direct economic losses were put at 14.2 billion U.S. dollars.
Twenty-five thousand persons were killed and 15,000 injured. In addition 517,000 people were made homeless. However, 15,000 people were rescued. Most of these rescues were made within the first few hours following the earthquake.
Many factors contributed to this large magnitude of the human disaster for example, freezing temperatures, time of day, soil conditions, and inadequate building construction. A large number of medical facilities were destroyed, killing eighty percent of the medical professionals.
In this earthquake both design deficiencies and poor construction practices were blamed for the large number of building collapses and resulting deaths. Many of the modern multi-storeyed buildings did not survive.
Soil conditions also contributed to building failures. The high death rate may in part be attributed to the way the buildings fell apart. When concrete floor panels about three feet wide collapsed into compact rubble piles, little open space was left where trapped people might survive. The proportion of survivors among people trapped in the rubble of multi-storeyed buildings was approximately 3.5 times higher for the ground floor than for higher floors. The collapse of a large number of apartment buildings which had many occupants on upper floors added to the number of fatalities.
While the earthquake exposed the flaws in the construction, it also brought out the goodness in people around the world. The cooperation of international teams in rescue efforts, the willingness of groups everywhere to contribute financial aid, and especially the undaunted determination of the Armenians themselves to rebuild their cities and their lives are worthy of commendation.
Load-bearing masonry-wall buildings were very common in large towns and cities in Armenia. This type of buildings was unable to resist lateral shaking because of inadequate bond beams necessary for tying the entire building together.
6. Loma Prieta Earthquake of 17 October 1989:
On 17 October 1989, at 17.05, an earthquake of 7.1 magnitude occurred in the Santa Cruz Mountains. The main shock was located 15 km north-east of the city Santa Cruz, and 100 km south-east of San Francisco, on the Zayante secondary fault, closely parallel to the famous San Andrea fault. Movement occurred along a 40 km segment of the San Andreas fault from southwest of Los Gatos to north of San Juan Bautista.
Measurements along the surface of the Earth after the earthquake show that the Pacific plate moved 1.9 m to the northwest and 1.3 m upward over the North American plate. The upward motion resulted from deformation of the plate boundary at the bend in the San Andreas Fault. At the surface the fault motion was evident as a complex series of cracks and fractures.
Recordings made available by USGS show that the Lomo Prieta earthquake produced maximum accelerations at upper ground level equal to 0.6 g in the epicentre zone, and
i. 0.64 g at Corralitos,
ii. 0.55 g at Gilroy,
iii. 0.39 g at Watsonville and Corralitos
iv. 0.38 g at Holster.
Vertical accelerations were often above normal for example, 0.66 g at Watsonville and 0.47 g at Corralitos.
A remarkable feature of the Loma Prieta Earthquake was the revelation of notable amplification of ground motion at a great distance from the epicentre, on sites with poor geotechnical properties. Thousands of landslides occurred throughout the area blocking roads and highways, hampering rescue efforts and causing damage to structures. Landslides were particularly prevalent in the Santa Cruz Mountains where they occur regularly even without earthquakes. These slides resulted in at least two deaths. One slump slide near Laurel took with it several dozen houses damaging them severely.
Thirty percent of the buildings in the Pacific Garden shopping mall in downtown Santa Cruz were damaged severely by amplified ground
Thirty percent of the buildings in the Pacific Garden shopping mall in downtown Santa Cruz were damaged severely by amplified ground shaking and ground deformation. The mall lies on unconsolidated deposits. One hundred thirty buildings (many of which date from the last century) were damaged in this historic section. Many houses that were not bolted to their foundations partially collapsed. Several hundred houses were either severely damaged or destroyed.
The worst ground shaking appeared to occur in the Santa Cruz Mountains close to the epicenter where many buildings were damaged or destroyed by ground cracking and shaking and by land sliding’s. Scores of mountain homes were also destroyed. Initial damages were estimated at 350 million U.S. dollars in Santa Cruz.
In Watsonville two adjacent buildings of a department store sustained extensive structural damage due to a weak first storey, insufficient shear reinforcement of the columns, and possible pounding of the two structures. Whereas, recently constructed buildings with tilt-up walls performed well.
At the Stanford University campus, 30 miles northwest of the epicenter, 60 buildings sustained varying degrees of damage, with an estimated repair cost of $160 million.
Concrete sidewalks and curbs were systematically fractured and buckled on northeast trending streets throughout downtown Los Gatos.
One of the very old houses in Los Gatos, California, was damaged due to its movement off its foundation. Hollister also experienced severe damage. Sand boils appeared in irrigated fields near Hollister. Collapsed and damaged buildings were also reported from Gilrov and San Jose.
These buildings collapsed largely because of the poor reaction of foundation soils, and the see through areas designed into ground floors, as parking spaces or to provide technical access. These were unable to withstand lateral forces induced by tremors.
Boulder Creek, Redwood Estates, Los Gatos, Scott’s Valley, Santa Cruz, and Watsonville all experienced strong ground shaking and had a high percentage of damaged structures. These towns were only 16 to 32 km from the epicenter.
The older structures in these towns were vulnerable for one or more of the following reasons:
1. Deterioration of the structure,
2. Lack of ties to the foundation,
3. Non reinforced masonry (brick or stone),
4. Lack of shear resistance in the ground floor,
5. Pounding of adjacent structures, and
6. Timber diaphragms not tied to unreinforced masonry walls, which allowed separation or pushing out of the walls.
In the epicentral area most of the damage resulted from the strong ground shaking and land sliding. Ground shaking primarily affected unreinforced masonry structures, and was enhanced in areas of fine-grained sand. Landslides occurred on steep slopes where ground shaking was most severe.
Effects in San Francisco and Oakland:
Even though the earthquake occurred in the remote Santa Cruz Mountains, it caused severe damage in San Francisco and Oakland 80 km to the north. This is somewhat unusual for an earthquake of this magnitude. Estimation of results of the disaster were: more than S7 billion in property damage (2.5 billion in San Francisco alone), 414 single-family units destroyed, 104 mobile homes destroyed, 18306 homes damaged, 97 businesses and 3 public buildings destroyed, 2575 businesses damaged, 12000 people displaced from homes and housed in shelters, 3757 injuries and 67 deaths.
The most spectacular damage occurred in the residential neighbourhood of Marina in San Francisco, with the collapse of four storey building built in 1920 on hydraulic fills, reclaimed from the bay following the worst 1906 earthquake. Marina district was one of the areas in San Francisco that sustained major damage. This area was a lagoon at the time of the 1906 San Francisco earthquake. During that earthquake the margins of the lagoon shook violently.
However, after the 1906 earthquake, the lagoon was filled with sand and rubble of destroyed buildings to make a fairground for the 1915 Panama Pacific International Exposition. This fill area later became the site of an expensive real-estate development known as the Marina District. The unconsolidated soils amplified the shaking and became liquefied (behaved like a dense fluid) causing permanent deformation of the ground. This was one of the causes of the increased damage in the area.
Construction practices also contributed to the damage. Some four-storey buildings built above garages had inadequate lateral bracing. Thirty five of the buildings in the Marina district were eventually torn down and 150 others were structurally damaged. The two major causes of structural failures were poor soil conditions and inadequate structural design. In contrast, very carefully designed high rise buildings against earthquakes that form the famous San Francisco skyline were perfectly able to withstand significant accelerations at ground level.
South of Market Street, several buildings between 5 and 10 storeys high were damaged. Old masonry buildings were badly damaged, including a warehouse where collapse of fourth floor exterior walls killed five people parked in cars along the street. There were also severely damaged buildings in the Mission District.
The first storey of a three-story structure failed when ground shaking was intensified by liquefaction. The second storey collapsed, leaving only the third storey.
In Oakland, severe damage occurred to several mid-rise buildings and many old brick buildings in the downtown area. Primarily due to the effects of liquefaction, buried underground utilities such as gas pipelines, water lines, and sewer lines were heavily damaged. This left about one thousand homes without gas or water. As in the 1906 earthquake, fires in the Marina District could not be fought with city water because water mains had failed.
One of the sources for concern produced by the earthquake is the damage or failure of transportation systems at comparatively large distances from the epicenter. A total number of 1500 bridges in five towns and cities were affected by the Loma Prieta earthquake. There was a spectacular collapse of a one and a half mile (2.4 km) stretch of Cypress overpass, a double decker viaduct giving access to the Bay on freeway Nimitz 880, when the upper deck of this Interstate 880 (Nimitz Freeway) in Oakland fell onto the lower roadway causing an official death toll of 41.
Another spectacular failure occurred on the Oakland Bay Bridge, one of the longest bridges in the world connecting San Francisco with Oakland. Interstate 280, the Embarcadero Freeway, and Highway 101 at Fell Street were also damaged.
The Cypress overpass has two levels of reinforced concrete decks resting on a series of concrete portal frames, 80 feet apart, standing on piles sunk in Bay Shore mud. Two types of portal frames have been used on this viaduct. Firstly, portal frames supporting the lower deck, which carries the one-way four-lane highway, are all made of reinforced concrete. Secondly, the upper portal frames, carrying the top deck, are built either of reinforced or pre-stressed concrete.
The two lower nodes of these portal frames contain keys, which form a neck in the cross-section area with four bars that pass through the key. In the case of pre-stressed concrete portal frames, the designer had provided a third “hinge joint” at the top of the upright, so as to facilitate internal deformations resulting from pre-stressing of the upper crosspiece.
The collapse of the Cypress overpass occurred as a result of the earthquake, which caused about 0.26 g acceleration at ground level. The fourth node in the upper portal frame, the only resistant cross-section to balance the force of inertia resulting from the heavy weight of upper deck made in caisson form, could not dissipate all the energy generated without itself turning into a plastic hinge, thereby converting the frame into an unstable mechanism.
Shearing of this cross-section was furthered by the drastic inadequacy of re-bar ties provided at 30cm interval. The bi-jointed column sheared off at the foot, then slid away from its support, causing the upper deck to fall on to the lower deck, crushing all the vehicles there. Other portions of this viaduct, standing on better foundations and consisting of hyper static portal frames, were damaged but did not collapse.
The Oakland Bay Bridge consists of various different spans, ranging from a suspension bridge to truss spans. The Bay Bridge has structural steel framework, and the deck consists of concrete slabs, simply placed on a network of steel girders. Suspension bridges behaved very well during the earthquake since they have varying natural periods – 3 to 5 sees. In contrast, 13 spans each 285 feet long (88 m), made of truss girders (piles E9 to E23) and with double decks suffered serious movements.
Damage occurred to the 15-m long section at Pier E-9 as shown in Figure 4.33. The view is looking west from the upper deck of the 88-m truss (beam) supported by Pier E-9. A 153-m-long truss, also supported at Pier E-9, is shown on the West Side of the opening. The 15-m span of the upper deck was supported at the west-end by bearings attached to the large north-south girder visible on the West Side of the opening. These support bearings allowed expansion movement of the 15-m sections.
Similar supports were provided on a girder on the eastside of the opening, except the bearings were attached to prevent movement. Failure of the bearings of the 88-m (east) truss allowed movement towards the east to the extent of approximately 18 cm exceeding the supporting surface of the slabs equal to 12.5 cm. This pulled the 15-m sections (upper and lower) that were attached to it, off their supports. In addition, the 2.5 cm bolts on the 88 m spans on pile E9 broke, thereby preventing total collapse of the bridge, as a result of a differential movement over 1.6 km between the east edge of the bridge and the rest.
In Oakland, Highway 980 and the MacArthur Maze developed cracks in the support columns. It will cost about $1.5 billion to rebuild state and local roads damaged in the earthquake. In addition to damage to roads, the Oakland International Airport, naval port, and the Alameda Naval Air Station runways were damaged by liquefaction.
It is interesting to compare this earthquake with the slightly smaller (magnitude 6.9) Armenian earthquake of December 1988 that killed 25,000 people and destroyed entire towns. Good construction and engineering practices in the Loma Prieta area obviously contributed to the preservation of property and human lives.
7. Northern Iran Earthquake of 21 June 1990:
Iran, located in a region studded with volcanic ridges and formed by the slow collision of huge geological masses, has always been vulnerable to earthquakes. In fact this region is one of the world’s deadliest earthquake zones and, rests on several tectonic plates. Earthquake fault lines are the edges of the tectonic plates. Fourteen disastrous tremors with magnitudes between 6.0 and 7.7 have been recorded in the area since A.D. 700.
There have been more than fifty significant earthquakes in Iran in the past thirty years alone. These earthquakes occur as Africa and the Arabian Peninsula, which is moving towards Eurasia at about sixteen millimeters per year, compress Iran. During an earthquake in 1968 at least 18,000 died in northern Iran near the Soviet border, in 1978, 25,000 were killed in the southeastern part of Iran, following a tremor that registered 7.4 on the Richter scale.
In December 1988, 25,000 people were killed in Soviet Armenia by an earthquake originating a few hundred kilometers north west of the epicenter of June 1990 earthquake, which was located just off the Caspian shore. A 7.7 magnitude earthquake occurred in the Gilan Province between the towns of Rudbar and Manjil in northern Iran on Thursday, 21 June 1990, at 12:30 p.m. The earthquake largest ever to be recorded in that part of the Caspian Sea region, may have been amplified by two or more closely spaced earthquakes occurring in rapid succession. The event, which was exceptionally close to the surface for this region, was unusually destructive.
The epicenter of the June 1990 earthquake was located in the collision zone between the Arabian plate and the Eurasian plate. This area, the northern Seismic Zone runs east and west along the southern shore of the Caspian Sea. In 1962 an earthquake of similar magnitude in the area killed 12,000.
The June 1990 earthquake caused widespread damage in areas within a one hundred kilometer radius of the epicenter near the City of Rasht and about two hundred kilometers northwest of Tehran. The cities of Rudbar, Manjil, and Lushan and 700 villages were destroyed, and at least three hundred more villages were slightly damaged. There was seven million U.S. dollars in damage in Gilan and Zanjan provinces southwest of the Caspian Sea.
One hundred thousand adobe houses sustained major damage or collapsed resulting in forty thousand fatalities, and sixty thousand injured. Five hundred thousand people were left homeless. Most of the victims were buried beneath concrete walls and ceilings as they slept. Aftershocks rippled through the area that registered 6.5 on the Richter scale. Telephone lines, electricity and water supplies were shut down.
Rescue operations were hampered- (1) by the fact that the earthquake occurred in the middle of the night, (2) by adverse weather conditions and (3) by the rugged terrain of the mountain villages. Roads and highways were blocked by extensive landslides further hampering rescue operations.
About one hundred thousand buildings collapsed or incurred major damage.
The following factors contributed to this extreme damage:
1. Construction Materials:
The use of brittle construction materials like brick, block, adobe, wooden timbers and modern materials inappropriate for use in traditional structures.
2. Construction Techniques and Workmanship:
The use of un- reinforced masonry and un-reinforced shear walls, poor welding of connections in steel frames, failure to tie steel support beams together and the use of heavy masonry without adequate support in flooring, ceilings and roofs.
3. Inadequate Design and Detailing:
Some modern structures lacked the symmetry of earlier traditional structures. Earthquake resistant designs principles were not used. Reinforcement detailing near the joints and in the columns were inadequate. Building codes used were inconsistent or not enforced.
4. Liquefaction and Failure of the Soils:
This was especially prominent on the shores of the Caspian Sea. The soil temporarily lost strength and behaved as a viscous liquid. With no firm support, structures sank or were spread apart by the liquefied soil. The unconsolidated soils may also have amplified the seismic vibrations.
5. Often several factors contributed to the failure of a single structure. The single most important factor in building failures was the use of un-reinforced masonry walls. Use of a number of lessons learnt from the study of this earthquake will result in the saving of lives and property during the next earthquake when it strikes in this highly seismic region.
Wall crack, happened when soils liquefied and building settled during the earthquake. Soils liquefy when ground water near the surface is forced between the grains of sand during an earthquake. The sandy soil behaves like a very thick liquid. Structures then settle or tip in the liquefied soil or are ripped apart as the ground spreads laterally or flows.
The majority of the smaller residential and commercial buildings in the areas of highest impact were of un-reinforced masonry bearing wall construction. This included the towns of Rudbar, Manjil, and Lushan where many of the buildings collapsed or were damaged beyond repair. The brittle masonry materials used in these structures performed poorly under the conditions of strong seismic loading. The heavy weight of the masonry floors and roofs also contributed significantly to the failure of such buildings.
In most of the villages, construction was mainly of irregularly shaped lava blocks set in dried mud, or of sun-dried mud bricks with similar “cement.” The roofs in these villages consisted of thick layers of dried mud spread upon reeds laid across closely spaced horizontal poles – a construction practice that was highly vulnerable to earthquake damage.
Another Study describes damage due to roof collapse in an unreinforced masonry bearing-wall structure near manjil. The roof consisted of a brick infill steel beam system without adequate support for its weight. Roofs, ceilings, and floors constructed with this commonly used “jack arch system” contributed to building failures and to the unusually high death toll. As many as half the buildings built in the early 1970s in Iran had jack arches.
In the “jack arch system” steel beams or a reinforced concrete joist system spanned the distance between the main girders across the length of the building. An arch made of small bricks connected the beams. Each arch had a rise of only about ten centimeters. The “valleys” of this wave-like surface were filled with mortar. The completed ceiling, roof, or floor was thick and heavy. Frequently the steel support beams were not tied together properly or were left untied.
Same type of building as above but with the presence of a reinforced concrete tie or bond beam (a concrete beam poured on the top of walls) that prevented a roof collapse in this building. This type of construction practice was generally effective in reducing the total collapse of structures.
8. Northridge Earthquake of 17 January 1994:
At 4:31 a.m. (local time) on Monday, 17 January 1994, an earthquake of magnitude 6.8 woke nearly everyone in southern California. The earthquake epicenter was beneath the San Fernando Valley, 20 miles (32 km) west-northwest of downtown Los Angeles, near the community of Northridge (34° 13′ N, 118°32′ W). Rock on the south side of the fault surged upward and over the rock on the north side. As a result of the quake, the Earth’s crust south of the San Fernando Valley moved slightly closer to the Earth’s crust north of the valley, and the mountains just north of the valley are slightly higher.
The Damage:
Damage was most extensive in the San Fernando Valley, the Simi Valley, and in the northern part of the Los Angeles Basin. After the earthquake, a total of 24,000 dwellings were vacated. The death toll from the quake was 57. The total cost of the earthquake was estimated to be at least 10 billion U.S. dollars. The Northridge earthquake is significant since it was the most expensive earthquake and one of the most expensive natural disasters in United States history, yet it occurred on a previously unknown fault.
Damage to building structures outside the epicenter area was severe, spotty in geographic distribution, and spread over a large area. Significant damage was reported as far as Fillmore in the west, Valencia in the north, and Anaheim in the southeast. The type and density of the construction and the strength of the earthquake shaking affected the distribution of damaged buildings.
It was no surprise that un-reinforced masonry and older concrete frame constructions suffered structural damage. However, damage and collapses in newer structures – particularly parking garages, commercial buildings, and apartment complexes – were surprising and even alarming, considering the strict compliance with existing building codes.
In the northwestern San Fernando Valley, surface disruptions have been identified. A prominent zone of surface fissures also occurred across Balboa Boulevard in the Granada Hills area. Several pipelines appeared to have pulled apart in approximately the same general area. Generally, the accelerations for this earthquake were higher over a larger area than would have been expected from previous experience with similar-sized earthquakes.
Santa Monica, about 15 miles (24 km) from the epicenter and across the Santa Monica Mountains, was also heavily damaged. Most of the damage occurred in an east-west trending belt within the northern portion of the city, and extended westward into Pacific Palisades and eastward into west Los Angeles and Hollywood. Two hundred million dollars in damage occurred in Santa Monica. One hundred thirty four buildings were made unsafe for occupancy and 396 others were damaged enough to limit access.
Damage to infrastructures and the transportation system during the earthquake is described below:
Infrastructure:
After the quake occurred, 680,000 people in the Los Angeles area were without power, gas, or phone service. Power cuts swept throughout the Los Angeles basin and were reported as far away as Alberta and British Columbia due to the load problems stemming from the quake. Although the phone system survived relatively intact, some long-distance services (into and out of parts of Los Angeles) were lost because of equipment damage and power failure.
Water trunk lines were broken, and water surged down some streets; 40,000 people were without water. Gas from a ruptured gas line ignited, and the resulting fire destroyed several homes. Explosions from ruptured gas mains occurred in the midst of the flooding from the broken water mains.
Transportation:
The earth quake closed several major highways and freeways. At the Fairfax, La Cienaga and Venice Bloulevard intersection with the Santa Monica Freeway, an overpass fell, closing the nation’s busiest freeway. Spans collapsed in the interchange between the Golden State Freeway (I) and the Antelope Valley Freeway (SR-14), in the northern San Fernando Valley.
Rock slides closed roads to Malibu Canyon and Topanga Canyon. The highway and freeway collapses nearly isolated some communities and caused commutes of as much as four hours. A Southern Pacific train was derailed near the earthquake epicenter spilling 8,000 gallons of sulfuric acid and 2,000 gallons of diesel fuel.
9. Kobe (Japan) Earthquake of 1995:
On Tuesday, 17 January 1995 at 5:46 a.m. local time, an earthquake of magnitude of 6.9 on Richter scale struck the region of Kobe and Osaka in south-central Japan. This region is Japan’s second-most populated and industrialized area, after Tokyo, with a total population of about 10 million. The shock occurred at a shallow depth on a fault running from Awaji Island through the city of Kobe, which in it has a population of » about 1.5 million.
Strong ground shaking lasted for about 20 seconds and caused severe damage over a large area. The greatest intensity of shaking was in a narrow corridor of two to four kilometers stretching 40 km along the coast of Osaka Bay. The ground moved as much as five meters in some places. The worst destruction ran along the previously undetected fault on the coast, east of Kobe. Kobe’s major business, industrial and port facilities, and residences are located in this strip.
The Damage:
The earthquake caused extensive damage to the coastal cities that border Osaka Bay and to the northern portion of Awaji Island. Inland cities located near the northern end of the fault rupture sustained significant damage. Osaka (Japan’s second largest city), Kyoto, and Shiga, farther to the northeast, reported extensive damage from the quake. The earthquake caused 5480 deaths, the highest death toll in Japan since the Great Kanto Earthquake of 1923 when about 140,000 people were killed, mostly by the post-earthquake conflagration. About 94,900 people were injured; nearly 317,000 people moved to evacuation centres.
Damage was recorded over a 100-kilometer radius from the epicenter, including the cities of Kobe, Osaka and Kyoto, but Kobe and its immediate region were the area’s most severely affected. Damage was particularly severe in central Kobe, in an area roughly 5 kilometers by 20 kilometers parallel to the Port of Kobe. This coastal area is composed primarily of soft alluvial soils and artificial fills. Severe damage extended well northeast and east of Kobe into the outskirts of Osaka and its port.
Before the ground stopped shaking fire broke out everywhere in the predawn darkness caused by ruptured gas mains and pipelines and the tinder of old wooden houses. Everything except misery was in short supply. In just 20 seconds, Japan’s magnificent postwar rise to economic superpower had been reduced to illusion. The world wondered: could this be wealthy, high-tech Japan, the world’s second largest economy – the ultra-organized Japan that is supposedly prepared for everything?
Many Japanese were shocked to the bones not just by the tragedy but by their nation’s ill-preparedness after years of government assurances about the state-of-the-art earthquake-proofing. What Japan may not recover for a long time, however, is the psychological shock that Japan was not earthquake proof. And the top politicians began to fear that if something like this happened to Tokyo it would paralyse the Japanese economy and affect the economy of the world. Kobe’s earthquake had reminded Japanese how vulnerable their country was to natural disaster.
However, they felt if Tokyo has little cause for comfort, it is at least better prepared to handle a disaster than Kobe. In Tokyo, emergency water reservoirs are in place and special vehicles are ready to deliver clean water if the water mains rupture. Further a computerized command centre stands ready; portable toilets have been stockpiled. Elaborate traffic plans have been made to clear streets for fire engines and ambulances. Ward offices have three day supplies of rice, biscuits and miso paste on hand to prevent the kind of hunger that the people of Kobe suffered.
Buildings Damage:
More than 192,700 houses and buildings were totally destroyed by the earthquake. Most of the damaged buildings were unsafe to occupy and had to be demolished later. Many buildings had either a collapsed first or the fifth floor. Such floor failures often occurred in buildings that appeared from the outside to have floors of equal strength and identical construction.
The first and fifth floors were especially vulnerable since they were at the internodes of the sine wave. Many buildings were out of plumb, or leaning. This was usually caused by partial collapse of a floor on one side of the building or by permanent offset of the structural system.
The highest concentration of damaged mid-rise buildings was observed in the Sannomiya area of Kobe’s central business district. In this area, most of the commercial buildings had some structural damage, and a large number of buildings collapsed on virtually every block. Most collapses were towards the north, which was evidently the result of a long-period velocity pulse perpendicular to the fault. This effect has also been observed in other earthquakes. Failures of major commercial and residential buildings were noted as far away as Ashiya, Nishinomiya, and Takarazuka.
The majority of partial or complete collapses were in the older, reinforced concrete buildings built before 1975. However, significant non-structural damage was also observed for buildings of relatively recent steel or composite construction. The base of larger buildings often appeared to be a couple of inches higher than the adjacent sidewalk or street.
Horizontal gaps between the base of the building and adjacent sidewalks and streets were common. This indicates that building response may have been influenced by soil-structure interaction. Age of construction, soil and foundation condition, proximity to the fault, and type of structural system was major determining factors in the performance of structures.
Damage was worst in the areas bordering the port or streams and rivers – where soils were either poorly consolidated alluvial deposits or fill – and tended to be relatively minor in the foothills of Rokko Mountain, where either soils are very shallow or there are rock outcroppings. Loose and soft soils amplify ground motions in comparison to bedrock, especially ground motions within a certain frequency range. The duration of shaking also tends to be longer on such soils.
In general, many newer structures performed quite well and withstood the earthquake with little or no damage. Most of the heavily damaged wood-frame buildings were traditional one or two storey residential or small commercial buildings of Shinkabe or Okabe construction. These buildings normally have very heavy mud and tile roofs (which are effective at preventing typhoon damage), supported by post-and-beam construction.
Foundations are often stone or concrete blocks, and the wood framing is not well attached to the foundations. The Shinkabe construction has mud walls reinforced with a bamboo lattice. Okabe construction has thin-spaced wood sheathing those spans between the wood posts and is attached with limited nailing. The exterior plaster is not reinforced with wire mesh or well attached to the wood framing, so it falls off in sheets when cracked. In new (post-1981) construction, nominal diagonal bracing is required to resist lateral loads.
Traditional wood-frame construction had the most widespread damage throughout the region, resulting in the largest number of casualties. Collapses led to the rupture of many gas lines.
Large inertia loads typically caused failures in these buildings from the heavy roofs that exceeded the lateral earthquake load-resisting capacity of the supporting walls. The relatively weak bottom storeys created by the open fronts typically collapsed. Unlike most U.S. homes, Japanese homes typically have few if any substantive interior partitions to help resist the earthquake loads.
In this respect, the bottom storeys are similar to the U.S. homes that are supported on unbraced cripple walls. In older homes, many framing members had been weakened by wood rot. Soil failures exacerbated the damage, because the foundations have virtually no strength to resist settlement, and connections between the residences and their foundations were weak.
Impact between buildings occurred often in Kobe’s residential areas. This interaction usually involved the lateral collapse of a traditional housing unit impinging upon a neighbouring structure. The impact of the heavy roof from one collapsing house often caused the destruction of neighbouring buildings that probably would have otherwise survived the earthquake.
The repair costs to buildings were estimated at more than $100 billion (U.S. Dollars). The design code in effect at the time of the construction was a major factor in determining the extent of damage to the commercial and residential buildings. Modern high-rise buildings typically fared better than older residential construction.
Dozens of reinforced concrete commercial buildings partially or completely collapsed at one or more floor levels. Typically, the buildings were 6 to 12 storey tall, and the failure often occurred within the middle third of the building height. One possible reason was that the period of the strong ground motion pulses may have been in a range that generally coincided with higher vibration modes for these buildings. This would have tended to amplify stresses in the middle portion of the buildings.
Another possible factor was that there were changes in building strength or stiffness at these levels. For example, if shear walls or the steel columns encased in concrete that extend up from the foundation discontinue at a floor level, the strength and/or stiffness of the structure above that floor may be significantly less than at the floor below.
Instances of concrete structures with collapses or failures in the bottom (ground) floor were also fairly common. These failures typically resulted from soft or weak storeys created by the need for garages and the desire to have numerous large open windows for storefronts at the bottom floor.
The high land costs and general congestion in Japan exacerbates this problem. Very narrow multi-storey buildings with open storefronts are very common. Irregular distribution of shear walls or concrete frames resulted in substantial torsion, causing the structure to twist as well as sway due to earthquake loading.
Another most commonly observed mode of damage was a brittle shear failure of concrete column elements, leading to a pancake collapse of the floor level above. The brittle failures resulted from inadequate reinforcing details. In general, damaged columns were observed to have horizontal reinforcing (referred to as ties) with relatively large spacing. These ties typically had hooks at their ends that were bent only 90°.
Consequently, when the earthquake struck and the concrete cover outside the ties spelled off ties opened up and could not provide the confinement to the central concrete core. Complete failure quickly followed. Many of the damaged buildings in Kobe were also constructed with un-deformed reinforcing bars.
Similarly, non-ductile concrete construction has been the popular form of building and other transport structures in past earthquakes, such as 1994 Northridge earthquake. Current code requirements include closer and larger ties of deformed steel, 135° hooks that extend into the confined concrete, and cross-ties to supplement the rectangular ties around the perimeter bars. In addition, ties must be closely spaced and extend through the joint created by the beams and columns.
Buildings possessing these enhanced detailing features are referred to as ductile moment frames. “Ductile” refers to a building’s ability to dissipate energy and deform without having brittle or sudden failure. In general, designs produced using the Japanese code tend to result in stronger columns and beams that are detailed in such a way that they have less ductility than do typical U.S. buildings in high seismic zones.
Hundreds of thousands of existing buildings of similar non-ductile construction are present in seismically active areas throughout the world. Unless these buildings are retrofitted, many lives will be needlessly lost in future major earthquakes.
Transportation:
One of the most far-reaching and disturbing aspects of the earthquake was the severe and extensive damage to the transportation system. Kobe is located within the main transportation corridor between central and southern Honshu. The Hanshin Expressway, supported by large hammerhead reinforced concrete piers, failed over 20 kilometer length.
The supporting steel girders of the Wangan Expressway (along the harbour shore) were dislodged from their seats, although few collapsed. Had the earthquake occurred during rush hour, there would have been many hundreds of fatalities on collapsed freeways, and numerous crowded trains would have derailed, in some cases plunging onto city streets.
The Hanshin expressway built in the mid- to late 1960s is the main through road and is almost entirely elevated for more than 40 kilometers. Single, large reinforced concrete piers spaced every 32 meters, many of which failed in shear or bending over a 20-kilometer length support much of the roadway. Similar failures of the roadway occurred at many locations, including complete toppling of large reinforced concrete pillars supporting a 500-meter section.
It was observed that the road deck changed from steel to a heavier concrete section at the location where this collapse occurred. These failures have not only closed the Hanshin Expressway for an indefinite period, but have severely impeded traffic on Route 43, a street-level highway beneath the expressway.
Rail facilities were particularly hard hit. All three main lines through the corridor sustained embankment’s failures overpass collapses, distorted rails, and other severe damage. The elevated viaduct that carries the Bullet Train was severely damaged when supporting columns underwent shear failure.
There was damage to the subway systems, including a rare instance of severe earthquake damage to a modern tunnel for reasons other than fault displacement near the portal. Rail and road transportation disruption affected a number of companies relying on rapid production systems. Due to effects on transportation, automobile and motorcycle manufacturers temporarily shut down factories located far from the earthquake site.
Kobe Subway System:
Damage to underground facilities, such as mines, tunnels or subways is rare in earthquakes. An unusual example of severe damage to this type of facility occurred in the Kobe subway system, a two-track line running under central Kobe, which was generally built by cut-and-cover methods in the mid-1960s.
The double track is typically carried through a concrete tube 9 meters wide by 6.4 meters high, which widens to 17 meters at the stations. The tube typically has about 5 meters of overburden, which is supported by 0.4-meter-thick walls and roof slabs. The walls and roof slabs are supported mid span (between the tracks) by a series of 5-meter-tall, 1.0-meter-long by 0.4-meter-wide reinforced concrete columns.
Port Damage:
The port of Kobe, one of the largest container facilities in the world, sustained major damage. Shipping had to be diverted to other ports. Closure of port functions impeded the shipment of raw materials and parts between businesses in Japan and their subsidiaries or partners overseas. This impacted the electronics, apparel, and auto manufacturing industries.
There was severe and widespread liquefaction as a result of the earthquake. Liquefaction caused subsidence in the range of 50 to 300 cm in some areas; large volumes of silt were ejected. Local lateral spreading of soils occurred along quay walls in many parts of the extensive port facilities.
Lateral ground deformation caused the piers of the Highway Bridge and electric rail bridge between Port Island and Kobe to lean between two and three degrees towards the waterfront. Pile-supported structures remained at their original elevations, while the surrounding ground settled substantially. Significant quantities of sand were ejected because of liquefaction and covered large portions of the pavements. Most gantry cranes were damaged, and one collapsed because the quay wall caissons were displaced.
Damage to the gantry cranes was in the form of leg and cross beam buckling, as well as rupture at the wheels. The extent of buckling varied, depending primarily upon the relative horizontal displacement resulting from the movement of the caissons. Relatively few cranes jumped off the tracks, which can be attributed to most of the cranes being in the stowed position with their pins engaged at the time of the earthquake. Numerous other cranes throughout the port were damaged because of foundation damage. Several cranes collapsed; some collapsed because of structural damage caused by inertial forces generated by the earthquake.
Quay wall caisson displacements, which undoubtedly propagated the major damage in the port, may be attributed to several phenomena. Earthquake accelerations applied to the massive sand-filled caissons resulted in large horizontal forces, which may have exceeded the sliding resistance offered by the base. This can be further aggravated by the rocking motion of the caissons, which may result in excessive bearing pressures at the toe of the base.
The latter coupled with the possibility of liquefaction may explain the observed tilting of many caissons. Since the islands’ fill placed below water was dumped from barges, it was relatively un-compacted. Hence, soil settlements resulted from lateral spreading as well as compaction. Such settlements continued during the first few days after the earthquake and are likely to continue for some time, especially in the event of further aftershocks. Of 186 heavy shipping berths, 179 were inoperative after the earthquake.
Infrastructures:
Electric power and telecommunications services were not disrupted, but most of Kobe lost essential services such as water, water treatment and gas utilities. Electrical power performed well with very little reduction in service during the earthquake, and was completely restored within one week. Underground water pipelines sustained severe damage in the earthquake. Numerous breaks resulted in a general lack of service in Kobe, Ashiya, and Nishinomiya. Water was restored within two weeks and gas was restored within a month.
Fires:
Almost 150 fires started, most within minutes of the earthquake, and primarily in densely built-up low rise areas of the city. The fires destroyed one million square meters of residential area in Kobe.
10. Bhuj Earthquake of 26 January 2001:
On 26 January 2001, a killer earthquake measuring 7.7 on Richter scale and having Mw of 7.6 shook the Indian province of Gujarat at 8.46 a.m. when the people were preparing themselves to celebrate the 52nd Republic day in a holiday mood. The epicentre of this strongest earthquake, that hit the country in the last four decades, was in Bhuj town parts of which disappeared from the face of the earth in the quake aftermath. Buildings collapsed in Ahmedabad, Surat, and Bhavnagar.
Communications were cut off. The trail of death left by nature’s fury in the town and villages of Gujarat estimated that the death toll was up to 50,000 or more. One month after the earthquake official Government of India figures placed the death toll at 19,727 and the number of injured at 166,000. Indications were that 600,000 people were left homeless, with 348,000 houses destroyed and an additional 844,000 damaged.
The Gujarat State Department estimated that the earthquake affected, directly or indirectly, 15.9 million people out of a total population of 37.8 million. More than 20,000 cattle were reported killed. Government estimates placed direct economic losses at $1.3 billion. Other estimates indicated losses as high as 5 billion U.S. dollars.
Earthquakes don’t normally kill people. Design deficiencies, faulty construction and poor disaster management do. We had seen the same cycle of tragedy unfolding at Latur, Uttarkashi, and Jabalpur and now in Kutch. Many of the buildings that collapsed in Ahmedabad were so poorly designed and constructed that they were incapable of resisting lateral loads induced by the earthquake. As many as 80 buildings caved in a span of three minutes, leaving another 50 so badly damaged that they were being demolished.
Over 500 needed urgent repairs to make them safely habitable again. A known builder built none of the buildings that collapsed. The most gruesome incident happened when a brand new four storeyed school building in the city’s Maninagar area collapsed crushing 33 children to death. Many buildings collapsed due to a combination of factors including substandard material, poor execution by non-technical people and faulty design. Many of the collapsed buildings in Ahmedabad stood on reclaimed and soft ground. But some of the reinforced concrete buildings of high quality demonstrated good earthquake resistance.
In Bhuj and nearby regions some villages have been wiped off completely. The reason for such disastrous consequences is primarily very poor quality of construction. The walls of the houses were made of quarry stones, bricks or mud laid in with a weak cement or lime mortar and, no special earthquake resistance measures had been taken. The construction of houses in majority of cases had been made on soft soil and alluvium deposits. The absence of reinforcement and poor bond between mortar and brick (stone) was the main reason for almost total destruction of buildings with load bearing walls.
The Rann of Kutch has been designated as an extremely high-risk earthquake area since the first seismic hazard survey in 1935. Structural engineers have known earthquake resistant design since 1960’s through building codes and by laws. The present devastation in Gujarat was avoidable if only guidelines for earthquake resistant buildings had been followed. Since the engineers in the country have no licensing system, so even a novice engineer can finally certify that a building has been suitably built.
Earthquake safety requires good design and good construction by expert and well-experienced builders. Preventing another catastrophe like this depends on these concerns not being forgotten away with time.