Why do buildings collapse in earthquakes? Building for safety in seismic areas - Robin Spence

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to us both. We would also like to acknowledge the late Laurie Baker and the late Paul Oliver, whose understanding of vernacular architecture and its value has been an inspiration for us.

      In the preparation of this book, we greatly appreciate the comments and advice on particular chapters from Andrew Coburn, Antonios Pomonis, Svetlana Brzev, Edmund Booth, Mary Comerio and Rashmin Gunasekera.

      We appreciate also the cooperation of Rajendra and Rupal Desai, Amod Dixit, Lucy Jones Toshitaka Katada, Randolph Langenbach, Tracy Monk and Edward Ng in reviewing our profiles of them and their work.

      We thank our colleagues Hannah Baker and Weifeng Victoria Lee for help in the preparation of maps and diagrams used, as well as for reviewing parts of the manuscript, Yue Zhu for her help with preparation of a number of the images used, and Sandra Martinez‐Cuevas for supplying the diagram of fragility curves used in Chapter 5, based on her analysis of the Cambridge Earthquake Impact Database (CEQID).

      We especially appreciate the painstaking work of Charlotte Airey in developing the building construction type and damage mode drawings used in Chapters 5 and 6.

      We also greatly appreciate the contributions made by the global panel of experts who responded to the surveys of national successes and failures reported in Chapter 8. An acknowledgement to them is given at the end of that chapter. We would like to acknowledge the help of our colleagues at EEFIT, to Allan Brereton and the committee for their help in making available appropriate illustrations. And to our colleagues at Cambridge Architectural Research and the Department of Architecture at the University of Cambridge for their support of the book.

      Finally, we would like to express our personal thanks to our families. From Emily: my gratitude to Alex and Clara for being the perfect lockdown husband and baby during this challenging year. From Robin: my thanks as always to Bridget for her unfailing support and wise counsel through this, and so many projects in the past.

      1.1 Earthquakes – An Underrated Hazard

      Earthquakes have been a threat to human habitation throughout history, but until relatively recently, their causes were poorly understood. In the pre‐scientific era, they were commonly ascribed to divine intervention. By the time of the Lisbon earthquake in 1755, there were many who understood that earthquakes had natural causes, but the mechanism remained unexplained, and the supernatural explanation was widely proclaimed, especially from church pulpits (Udias and Lopez Arroyo 2009). And over 150 years later, according to observer Axel Munthe (1929), the inhabitants of Messina, destroyed by a massive M7 earthquake in 1908, cried ‘Castigo di Dio’ (‘punishment from God’).

      Only with the development of plate tectonics in the twentieth century has it become understood that earthquakes are associated with active faults in the earth's crust, with most of the largest occurring at the boundaries of the tectonic plates as they interact with each other (as explained in Chapter 4). We can now identify with some precision whereabouts on the earth's surface large earthquakes will occur. From measurements of the movements at plate boundaries, and from the historical record, we can make estimates of the largest magnitude event which can occur on a fault section, and approximately, the frequency with which events of different magnitude will occur. But the largest events commonly have return periods of several centuries or more (Bilham 2009), and science is still unable to predict, even to within a few decades, when the next large earthquake on any fault section will occur.

      There is some evidence that the global earthquake mortality rate (deaths per 100 000 of the world's population) has been rather gradually reducing over the last century or so. But it is a very slow rate of improvement, and the variation from decade to decade is very large. The first decade of the twenty‐first century was a bad one, with several earthquakes resulting in more than 50 000 deaths. Yet, over the same timescale, death rates from many other causes, such as infectious diseases and road accidents, have been very significantly reduced (ourworldindata.org/causes‐of‐death 2020). This has been made possible with the introduction of public health programmes and protection measures, backed by government legislation and action programmes, but supported and implemented by the general public. Such programmes could similarly be applied to reduce earthquake risk, but in many countries most at risk, this has not so far happened. Why is this?

      But, as the world's population grows, and urbanisation increases in pace, there are many places where new buildings are being constructed without any reference to good engineering practice for earthquake resistance.

      This is partly because those responsible for constructing the new buildings are unaware or possibly unconcerned that a large earthquake may occur any time soon, and building controls are lax. It is also due to lack of education, information, skill and sense of urgency on the part of builders and building owners (Bilham 2009; Moullier and Krimgold 2015).

      In rural areas of many poor countries, buildings are largely constructed using highly vulnerable materials such as adobe and unreinforced masonry. Poverty and lack of understanding, combined with a vast demand for new dwelling places, are thus fuelling the creation of a series of future disaster scenarios (Musson 2012).

      In order to understand why buildings collapse in earthquakes and to find out what we can do about it, we must look at each of the three ingredients of the problem: earthquakes, buildings and people.

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