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

Both authors have been involved with several EEFIT post‐earthquake reconnaissance missions. Our direct knowledge of the types of buildings affected in earthquakes, and our understanding of their behaviour, is largely derived from these earthquake missions, as well as from some more detailed field investigations and household surveys carried out independently. The following sections give brief accounts of 10 of the most significant earthquakes of the last 20 years, partly based on our own observations, but also making use of the field reports of our colleagues in the EERI and EEFIT teams and other reports. As we are concerned in this book primarily with buildings, these brief accounts emphasise in particular the range of building types which were affected and the levels and types of damage caused, topics which we will return to look at in more detail later in the book. They also touch, where appropriate, on the methods of damage investigation used.

Table 2.1lists the most significant events of the twenty‐first century up to 2018. It includes all those events which, according to the EM‐DAT database, killed more than 4000 people, and also all those which had a damage cost exceeding US$3.9bn. The 10 events briefly described here include the 9 events with the highest casualty tolls of the last 20 years, and one other event, the New Zealand Christchurch event of 2011. This was particularly significant not for its casualties, which were relatively low, but for the very high financial cost of the damage caused, and for its particular impact on the historic masonry buildings of the city of Christchurch.

The chapter concludes with some general observations about earthquake damage, an assessment of global damage trends and the distribution of damage between different regions and country groups. In this way, we aim to approach an assessment of the question: how well are we, as an international community, doing in trying to limit the effects of earthquakes for this and future generations?

At 8.46 a.m. on 26 January 2001, India's 52nd Republic Day, one of the most devastating earthquakes ever to strike India occurred in the Kutch Region of Gujarat State. The earthquake of moment magnitude Mw7.7 and focal depth 23 km was located approximately 70 km east of the historic city of Bhuj. Heavy ground shaking affected an area of tens of thousands of square kilometres, although there was no surface fault rupture observed. The isoseismal map prepared by the EERI team indicates that the area subject to shaking at a level exceeding MM Intensity VIII (‘heavily damaging’) was over 30 000 km 2(Jain et al. 2002). The area has experienced a previous large earthquake (Mw about 8.0) in 1819, and a moderate Mw6.1 one in 1956, and is in the zone with the highest earthquake loading requirements in the Indian code of practice for the design of buildings.

Table 2.1Significant earthquakes worldwide since 2000, ordered by number of deaths.

Sources: CRED (2020); Pomonis, A., 2020. Personal communication.

Income groups are from World Bank data (High, H; Upper‐middle, UM; Lower‐middle, LM; Low, L). See also Table 2.2. Dates are given in DDMMYYYY format. Some casualty and loss data are amended based on more recent estimates (Pomonis 2020).

a Events with significant tsunami impacts are shown.

Load‐bearing masonry is the predominant way of building throughout the affected area, but methods have changed over time. The most common masonry technique is a single‐storey house with walls of random rubble stone masonry set in a mud mortar, with a clay tile roof: these buildings are found everywhere, both in the main towns and in the villages ( Figure 2.1). More substantial dwellings use dressed or semi‐dressed stone or sometimes clay brick walls; these are commonly two‐storey buildings. In recent years, the use of reinforced concrete (RC) slabs for floors and roofs, with coursed masonry walls, has become common in the wealthier parts of Kutch ( Figure 2.2). The main towns have also significant numbers of multistorey apartment blocks in RC ( Figure 2.3). None of these forms of building were spared by the intense and widespread ground shaking.

The major city of Gandhidham, and four large towns Bhuj, Anjar, Bhachau and Rapar, all in the Kutch district, were devastated, as was every village within a wide area. Over 230 000 one‐ and two‐storey masonry buildings and several hundred concrete frame buildings collapsed. However, as pointed out by Sudhir Jain (2016), the collapse rate of buildings in the zone of highest intensity was much lower in this earthquake than in the 1993 Mw6.2 Latur earthquake in India's Maharashtra province where rubble stone walls with heavy mud roof are typical.