Why do buildings collapse in earthquakes? Building for safety in seismic areas. Robin Spence
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and preparation programme which has resulted in much reduced risks in California over the past decade.
But considerably more devastating consequences face many of the growing cities in other earthquake zones, particularly in Asia. The southern edge of the Eurasian Plate, stretching from the Mediterranean to China, and including Myanmar and Indonesia, is responsible for 85% of the world's historic earthquake deaths. And this is a region in which cities are today growing rapidly both in size and in number, fuelled by global population rise and urbanisation. Seismologist Roger Musson points to the risk in Tehran, today a city of 12 million people. The last major earthquake on the North Tehran Fault, passing close to the city centre, was in 1834 at a time when Tehran was a small town: an earthquake of M > 7 hitting Tehran today could cause as many as 1.4 million deaths. And seismologist Roger Bilham (2009) has estimated that a direct hit on a megacity (>10 million population) somewhere in the world once a century is now statistically probable, with a possible death toll exceeding one million, because of the combination of hazardous locations and structural vulnerability. The World Bank estimates that three billion people will live in substandard housing by 2030. By 2050, the UN projects that two‐thirds of the world's population, around 7 billion people, will live in urban areas.
Unfortunately, because the threat to each city is seen as remote, protection from earthquakes is given a lower priority than other issues. Few households prioritise spending on safety from future earthquakes above pressing immediate concerns, like providing extra space or better comfort, unless required to do so by regulation. And elected governments tend to look for expenditure programmes and new regulations which will give returns within their current tenure of office, despite evidence that money spent on disaster mitigation often avoids much greater losses over time. For this reason, general development expenditure is given priority over disaster risk mitigation. And even within that part of government budgets devoted to natural disasters, those from other natural hazards are often given priority. Windstorm and flood damage are more immediate risks, particularly as these are becoming worse as a result of climate change.
Optimistically and opportunistically, the climate change agenda has provided a global focus on resilience of communities to natural threats. It is recognised that especially in developing countries, cycles of disasters have depleted decades of progress made in development. The deaths and destruction from earthquakes are preventable. Whilst the hazard itself is natural, the disasters are largely man‐made, and completely preventable with proactive interventions.
1.3 The Authors' Experience of Earthquake Risk Assessment
The overall aim of our work over four decades at the University of Cambridge's Department of Architecture and at Cambridge Architectural Research Ltd has been to understand the vulnerability of buildings to earthquakes globally, in order to estimate the damage which is likely to occur from future earthquakes. This knowledge can be used to provide a sound basis to improve the building stock, and reduce damage, loss of life and disruption from future earthquakes. We have developed our knowledge of building vulnerability through a series of collaborative research projects, supported by the European Union and the UK Government and Research Councils, and through work for individual cities, companies managing portfolios of buildings and insurance companies. But the primary source of our knowledge and experience of buildings’ behaviour in earthquakes has been post‐earthquake field missions. We have been involved in EEFIT, the UK's Earthquake Engineering Field Investigation team, since it was founded in 1982, and have between us participated in field missions in Japan, Italy, Turkey, India, Pakistan, Peru, Indonesia, China, New Zealand and the South Pacific. The detailed nature and aims of these field missions are discussed in Chapter 2: but an essential element in all cases is to describe and document the types of building affected and the types of damage observed.
Successive projects have examined in detail the problems of particular regions. In the 1980s, we examined the traditional stone‐masonry construction of rural Eastern Turkey and conducted shake‐table tests in Ankara to investigate simple ways to reduce their vulnerability, the cause of many deaths in earthquakes of the previous decade. In the 1990s, we investigated the options for protecting historic European cities such as Lisbon and Naples from likely future earthquake damage, and we looked at the performance of buildings which had been strengthened following previous earthquake damage. We also developed a method for assessing human casualties from earthquakes based on the level of building damage, and with colleagues in New Zealand applied this to the city of Wellington.
Since 2000 we have worked with others to develop loss modelling approaches to estimating damage and casualties, on a city‐scale (in EU collaborative projects), for insurance companies, or with the US Geological Survey, for rapid post‐disaster damage assessment. And we have applied our knowledge to assist organisations with large portfolios of buildings to identify those which should be upgraded.
We have also worked with teams developing new ways to assess earthquake damage using remote sensing, and led the team developing the Earthquake Consequences Database (So et al. 2012) for the Global Earthquake Model (GEM). And we have applied similar approaches to assessing vulnerability and damage to buildings from other natural hazards such as windstorms and volcanic eruptions. All this work is described in detail in technical project reports and published papers, referred to in the chapters which follow.
1.4 Aims of This Book
The title of this book asks a question: Why do buildings collapse in earthquakes? In exploring the many layers of the answer to this question, and the many answers in differing contexts across the world, we want to demonstrate that this is not just, not even primarily, a technical question, but also a social, organisational and even political question. In this book, we look at buildings not only as assemblages of materials and components put together to achieve certain functional ends, but also as products of a society and a culture. We aim to explain the physical reasons why buildings fail to withstand earthquakes, but also to attempt to understand the social, economic and political reasons why earthquake disasters continue to happen. And through this combined understanding, we want to point to the actions that can be taken to improve seismic safety, and identify who should be taking them.
With this aim, we hope to reach a wider audience than those interested in the purely technical aspects of earthquake protection, who would prefer a non‐mathematical approach to the subject, with limited technical detail. Thus, the book is designed to be read by all those interested in the consequences of earthquakes, or concerned for their own safety as occupants of buildings in earthquake areas. It is also intended for those who have responsibility for ensuring the safety of others in earthquakes, whether as government officials, political representatives, building owners or managers of businesses. The book is written for a non‐technical readership, but will also be of interest to all those professionally involved in disaster preparedness and earthquake engineering, as well as to students and practitioners of architecture and engineering seeking a broad overview of the consequences of earthquakes for buildings.
Some readers of the book will live in an earthquake zone, in which case they will want to know if their homes or workplaces are vulnerable, and what they can do to protect themselves from an earthquake, in advance or when it happens. Other readers may own or manage buildings in earthquake zones, or be responsible for the safety of those who occupy them; they will want to know what steps they as owners might be able to take to provide adequate safety. Other readers may be responsible, as architects and engineers, for the design of new buildings or the refurbishment of older ones in earthquake zones and will want to know what the essential steps in building for safety in such areas are. Yet, others may have a more general interest in natural disasters and need an informed but largely non‐technical account of how buildings have performed and of how the way today's buildings are constructed has been influenced by past earthquakes. The book aims to provide useful and accessible answers for all of these groups of readers.
1.5 Outline of the Book
The remainder of the book is divided into eight chapters. Chapter