Why do buildings collapse in earthquakes? Building for safety in seismic areas. Robin Spence
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huge loss of life. The initial ground shaking was destructive throughout Aceh Province of Indonesia, particularly in the main city of Banda Aceh, and also in the Andaman and Nicobar Islands. But the tsunami carried the earthquake's energy over a much wider region, causing destruction throughout coastal northern Sumatra, and in all the countries bordering the Indian Ocean. Casualties caused by the tsunami were reported in 12 different countries, but most of the tsunami‐related deaths occurred in Indonesia (165 000), Sri Lanka (36 000), India (16 000) and Thailand (8000). In Aceh Province of Indonesia, it destroyed virtually every village, town, road and bridge along a 170 km stretch of coast that was not more than 10 m above sea level. The death toll was over 16% of the entire population of the northernmost six districts of the province. Inundation depths reached up to 20 m in parts of Sumatra, 5–8 m in Thailand, and 2–5 m in South‐eastern India and Eastern and Southern Sri Lanka (EEFIT 2006).
The tsunami was devastating to small buildings wherever the inundation depth was 2 m or more, and huge numbers of buildings of timber or traditional masonry were destroyed in Indonesia, Thailand and Sri Lanka (EEFIT 2006) (Figures 2.7 and 2.8). RC buildings of several storeys often survived but with serious damage, although there were cases of collapse through scour under the foundations. The huge loss of life was primarily due to the direct effects of the tsunami itself. Victims were either drowned directly or as a result of injuries caused by impact with debris from buildings or other objects: ‘falling structures and waters full of swirling debris inflicted crush injuries, fractures and a variety of open and closed wounds’ (WHO 2006). Tens of thousands were swept out to sea, and were ultimately recorded as missing, and were presumed drowned.
Figure 2.7 Damage caused by the 26 December 2004 tsunami at Unawatuna, Sri Lanka where the inundation depth was about 5 m. Damage to a masonry building.
Source: EEFIT. Reproduced with permission.
Figure 2.8 Damage caused by the 26 December 2004 tsunami at Unawatuna, Sri Lanka where the inundation depth was about 5 m. Damage to a reinforced concrete frame building.
Source: EEFIT. Reproduced with permission.
In Sri Lanka and Thailand, many of the victims were foreign tourists. It has been estimated that the death rate in the worst hit areas in Sri Lanka and Thailand was over 10% of the resident population within 1 km of the coast. A study of the life loss in Indonesia, India and Sri Lanka found that in certain locations the disaster claimed four times as many lives among women as men (EEFIT 2006).
It is clear from all accounts that an effective warning system, coupled with a better understanding of the phenomenon of tsunamis among both residents and visitors could have saved many lives, since the tsunami struck the Thai and Sri Lankan coasts more than 90 minutes after the earthquake.
A study carried out by our own team of the experiences of eyewitnesses (Spence et al. 2009) showed very strong correlation between survival and distance from the shore: all of those within 15 m of the shore reported serious injury or fatalities in their group, but less than half of those more than 30 m away did. Most survivors who were in the affected zone attributed their survival either to prompt action in moving to safer ground, or to being in a building which survived.
2.2.4 The 8.10.2005 Kashmir Earthquake in Pakistan: Mw = 7.6, 73 338 Deaths
On 8 October 2005 at 8.50 a.m. local time, an earthquake of magnitude Mw7.6 struck the Kashmir regions of Pakistan and India. The epicentre was a little north of Muzaffarabad, the major town of Pakistan's AJK (Azad Jammu and Kashmir) Province. It was located on the Jhelum Thrust (Tapponier et al. 2006), part of the well‐established thrust fault system associated with the subduction of the Indian plate below the Eurasian plate.
Heavy ground shaking was felt over a very wide area, and was devastating for the nearby towns of Muzaffarabad, Balakot, Bagh and Rawalakot; but damage was severe in towns up to 50 km away, including Murree, Abbotabad and Mansehra in Pakistan, and Uri and Baramulla in India. In the three worst‐affected districts of AJK (Muzaffarabad, Bagh and Rawalakot), 84% of buildings were damaged or destroyed. In Islamabad, 100 km away, a recently built multistorey RC apartment block collapsed (EERI 2005, 2006a) killing 74. As many as 5000 school buildings were seriously damaged or destroyed killing 18 095 students and 853 teachers (Asia Development Bank 2005; EEFIT 2008a). This was the most destructive event in the Indian subcontinent in the last 50 years, causing as many as 75 000 deaths and 68 700 serious injuries in Pakistan, and 946 deaths and 4400 injuries in India (EEFIT 2008a). Altogether, about 450 000 homes were destroyed, and over 2.8 million people were left homeless.
The high death toll was undoubtedly primarily due to the widespread collapse of buildings in the area, most of them of masonry. Because of the harsh climate, buildings have traditionally been made from thick stone masonry, often using rounded riverbed stones in poor‐quality mud mortar, with steel sheet or thick mud roofs (Figure 2.9). In the past, such walls were often tied together with timber lacings and the roof independently supported on timber columns. However, timber is less and less used because of its scarcity and high cost, and the severe ground shaking would have been more than enough to cause overturning or disintegration of the walls, followed by roof collapse. In many places, more modern building types using concrete blocks and RC frames also collapsed (Figure 2.10), and this included many government‐built schools and barracks. Evidence from post‐earthquake field investigations showed poor‐quality building standards (EEFIT 2008a).
Figure 2.9 2005 Kashmir earthquake: damage in the Muzaffarabad district. Aerial view of damage to traditional residential buildings.
Source: EEFIT. Reproduced with permission.
Figure 2.10 2005 Kashmir earthquake: damage in the Muzaffarabad district. Damage to reinforced concrete construction.
A factor which certainly also contributed to the high death toll was the inaccessibility of much of the mountainous affected area, as a result of the numerous landslides triggered by the earthquake blocking roads. The emergency services were thus very slow to arrive, and many of the survivors had to walk long distances in difficult terrain to reach a functioning health centre; this also complicated injuries, bringing on infections and resulting in more drastic medical measures. Many more victims with head and chest injuries from falling masonry did not survive until medical help arrived. Unfortunately, search and rescue capability in the crucial early stages was overly concentrated in Islamabad, where few buildings failed, rather than being sent to the epicentral area (So 2009).
There were no official data from which causes of death could be established, but a survey of survivors in 500 families in one of the worst affected neighbourhoods