Magnitude is not the Only Factor Influencing the Damage Caused by Earthquakes

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Earthquake damage in Christchurch, New Zealand (Photo by Andy Miah)

Earthquake damage in Christchurch, New Zealand: Photo by Andy Miah

Crumpled buildings, piles of rubble, cracks in roads, and twisted iron girders – everyone has a mental image of the kind of damage caused by a major earthquake. But, perhaps surprisingly, it doesn’t follow that the bigger the earthquake the greater the damage – a glance at the USGS list of the world’s most deadly events doesn’t correlate closely with its list of the largest events. Indeed, only two of the top 20 most devastating events feature among the 16 largest earthquakes.

Earthquake damage varies hugely between events, and a number of factors influence the outcome in terms of what we define as a ‘damaging’ earthquake – usually defined by the number of human deaths. While many of these are related to human activity, the physical characteristics of an earthquake and the environment in which it occurs are also extremely significant.

The Physical Nature of the Earthquake

Earthquakes may in essence be easily and simply defined – but each earthquake is different, and its character,its size, and the nature of its damage is determined by very many factors including depth of the earthquake (shallower earthquakes cause more damage than deeper ones) and the type of movement, with thrust faulting (vertical) likely to cause more damage than strike-slip (lateral) movement.

Earthquake location also introduces a significant additional component of damage. As a general rule, onshore earthquakes do not cause tsunamis and the damage is limited to the area directly affected by shaking. An earthquake which occurs offshore, however, can cause a devastating series of waves which will reach areas many thousands of miles away – for example, most of the casualties of the 2004 Boxing Day earthquake were killed not by the earthquake, but by the resulting tsunami.

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