Earthquakes: Why do some places suffer more than others?

Whilst earthquakes are perhaps the most frequently occurring natural hazard, their impact on people, property and communities varies enormously from one place to another. It is possible to identify a number of factors that cause some places to suffer more than others. Whilst some are large scale and are to do with tectonic location, others are decided at a much more local scale, and relate to building design and levels of preparedness. Perhaps the most significant factor determining why some places suffer more than others is the tectonic location of an area. The distribution of earthquakes is commonly linked to the margins of global plates. Whilst the assertion that earthquakes only occur at plate margins is broadly true, it would be overly simplistic to assume that earthquakes are more common, and more devastating at some margins than at others. Yet it remains apparent that most earthquakes do coincide with the major plate margins, although a smaller number do occur away from plate boundaries.

Generally speaking, earthquakes at destructive plate margins have a greater spread, and therefore affect more places than those at constructive plate margins, however this is a generalization, and earthquakes are subject to individual variation. Regardless of this, cities located along plate margins are typically of higher risk, than those situated within plate interiors. Another significant factor determining why some places suffer more than others is the magnitude and depth of the earthquake. It is quite logical to correlate that the stronger a quake, the more serious its impact will be. Indeed in the majority of cases, there seems to be a direct relationship between magnitude and impact. However, magnitude alone cannot be held responsible for the scale of an earthquake disaster. In addition, the depth of focus is pivotal in determining the level of suffering. Generally, shallow earthquakes tend to result in a greater intensity of surface shaking, and often cause the greatest loss of life and damage to property. Shallow earthquakes are also often associated with destructive margins, where the subducting plate descends at a slight angle, thereby creating stresses close to the surface.

Where the depth of an earthquake is deeper, typically the impact is less destructive. Indeed larger magnitude quakes, with deeper focuses often incorporate less damage than shallower quakes with larger magnitudes. Thus, it is a balance of magnitude and focus depth, rather than one extreme factor, which can prove decisive in the impact an earthquake has on a region. The economic development of a region can further be critical in determining why some places suffer more than others. It is often suggested that LEDCs suffer much more from the effects of earthquakes than MEDCs and, whilst this is a generalisation, broadly speaking it is true, particularly in terms of human impacts. All things being equal, a less economically developed country, with its less restrictive building standards and its inability to cope with the aftermath of an earthquake, will suffer greater loss of life, homelessness and loss of livelihood than a respective MEDC.

In recent years, some of the greatest earthquake disasters have occurred in LEDCs, including the India quake of 1993, where 10,000 were killed, and the Afghanistan quake of 1998 were over 6,000 were killed. Richer MEDCs do not tend to suffer quite so much human loss, although they do suffer extensive financial damage, as insurance companies and governments fund rebuilding programmes and pay compensation. The Kobe earthquake fittingly exemplifies this, where the cost of reconstruction topped $100 billion. The level of preparedness can further be pivotal in assessing why some places suffer more than others. Levels of preparedness can be closely paralleled to the frequency of earthquakes, and levels of development. In wealthy regions where earthquakes are common, such as New Zealand and Japan, much is done to prepare for the inevitability of earthquakes. Indeed in these locations measures are taken in terms of earthquake drills, and educational planning, to ensure public awareness remains at a constant high. People are often informed about potential dangers, and how to respond when earthquakes do occur.

Furthermore, emergency services regularly practice their response procedures, so in the event of an earthquake the reaction is as smooth as possible. Typically, supplies of food, water, medicines and shelter are also stored in recognised safe areas ready for coping with the aftermath of an earthquake. Education and preparation are undoubtedly factors in reducing the scale of a disaster, particularly regarding the response after the event in terms of rescuing injured people and preventing the spread of disease. However, even the best laid plans can fail to live up to expectations. This was exemplified with the Kobe earthquake, when emergency teams reacted slowly, and appeared to be totally overwhelmed by the scale of the disaster. Poorer countries tend to be less aptly prepared. Whilst this is due in part to the lack of money to invest in materials and educational programmes, it is also because earthquakes are often perceived as infrequent problems in a society facing daily struggles for survival of a much more mundane nature. The geology of an area is further decisive in the ultimate significance a quake will have.

Typically, areas of soft clays and silts are likely to succumb to liquefaction during a quake. Harder and more solid rocks, such as granite, generally don’t liquefy, although no rock is entirely immune if an earthquakes magnitude is large enough. The result of liquefaction can be extremely destructive. Generally speaking, building foundations become extremely unstable, and slopes become increasingly vulnerable to mass movement. For instance, many buildings in Mexico City became tilted following the 1985 earthquake when the lakebed sediments on which much of the city was built on became liquefied. Similarly, during the 2011 Christchurch eruption, most of the damage incurred to buildings was a direct result of soil liquefaction. In this sense, a low magnitude quake still has the potential to inflict serious damage if the geology of an area is unfavourable. The population density of a region can be further significant in determining why some places suffer more than others.

A natural event such as an earthquake only becomes a ‘hazard’ when it impacts on human activity. In this way, population density is pivotal in the eventual impact a natural hazard inflicts. There is a considerable overlap between major earthquake zones and areas of high population density. Indeed many of the world’s largest cities are situated in areas of high earthquake activity, particularly around the Pacific Ring of Fire. These extensive conurbations, which include the cities of San Francisco, Los Angeles and Tokyo, are especially vulnerable, with their tightly packed buildings and raised freeways. Some 70 out of the top 100 largest cities in the world, accounting to 10% of the world’s population, lie in earthquake prone regions. Naturally, when an earthquake strikes an area of high population density, the likelihood is that the eventual impact will be greater than the equivalent quake in an area of low population density.

Building structural vulnerability can be equally significant in assessing why some places suffer more than others. Perhaps the largest killer in the event of an earthquake is falling debris, which in many cases is a direct consequence of poor structural design. Indeed, the vast majority of suffering during a quake stems from the collapse of buildings or structures, such as bridges and elevated highways. In the 1989 Loma Prieta earthquake near San Francisco, 41 of the 67 deaths resulted from the collapse of the Nimitz Highway when the top storey collapsed onto the storey below. In wealthy areas where earthquakes are common, building materials and appropriate designs can minimise loss of life. This was certainly the case with the Loma Prieta earthquake where very few people were killed.

However, despite the implementation of strict building regulations in recent decades, older properties remain vulnerable; it was the collapse of many such houses in Kobe, Japan in 1995 that led to the high death toll of over 6,300. In poorer parts of the world building design is often inadequate and, although building standards might be officially in place, regulations are rarely enforced. This was certainly the case in Mexico City, when in 1985 several modern high-rise buildings collapsed as concrete crumbled and thin steel cables tore apart. Conclusively, the impact of earthquakes on human activity varies significantly across the world.

This is partly because the events themselves are unevenly distributed, both in terms of their geographical location and their magnitude, but also because people and societies have reached different levels of preparedness, in terms of building design and construction, and in their ability to educate people and respond after an earthquake event. Two earthquakes of a similar magnitude might be expected to have similar effects on human activity, but this is often not the case. Perhaps more than anything else, it is the ability of a country to respond to earthquake vulnerability that determines the likely impact, and there can be no doubt that the economic development factor is paramount in this respect. Whilst there can be no doubt that tremendously powerful earthquakes will cause destruction wherever they occur, it does seem to be the case that, all things being equal, LEDCs tend to suffer more than MEDCs.