Other Physical Factors Affecting Quake Damage
Regional and geological factors in an area afflicted by earthquakes play a part in influencing the final outcome of the event. There can be differences on a major scale with even the type of crust affecting the nature and extent of shaking. Dr. Roger Musson, of the British Geological Survey, told Decoded Science how, for example, in North America, earthquakes to the east of the Rockies are generally felt over a wider area than those to the west. According to Dr. Musson, this is because the crust is older and colder and shock waves propagate more easily through it.
Local topography is also a factor: steep and unstable slopes are prone to failure during earthquakes, resulting in landslides which not only cause direct – and often devastating – damage but which also delay relief operations – as happened, for example, after the Kashmir earthquake of 2005, when many roads leading into mountainous areas were blocked.
Nor should local geology be neglected, in particular in situations where it increases the extent and the duration of the shaking resulting from fault movement. Some rocks are more easily deformed than others: increased deformation leads to an increased likelihood of deformation and damage. Soft sediments are prone to amplifying shock waves: solid rock is less so.
A good example here is the 1985 earthquake in Mexico. The earthquake was large (M8.0) but although it struck offshore, much of the damage and most of the casualties occurred in Mexico City – some 220 miles away. Mexico City is constructed on the site of a former lake, a bowl-shaped depression among the mountains: the lake sediments increased the shock waves and even caused them to turn to liquid – a process known as liquefaction.
The Christchurch, New Zealand, Earthquakes of 2010 and 2011
Perhaps the most striking recent illustration of the different damage caused by different earthquakes occurred in Christchurch, New Zealand. An M7.1 earthquake caused significant damage to buildings, but no casualties was followed some months later by an M6.3. tremor which caused extensive damage and killed 84 people.
The first and larger event was further from the city (40km distant) than the second (10km) and its focal depth was greater (10 km compared to 5km). The nature of fault movement was also significant: Dr Musson told Decoded Science, that the first was largely caused by strike-slip movement (i.e. horizontal) while the second, smaller event had a thrust, or vertical, component.
Other factors also come into play – the timing of the two events differed, with the first taking place at night when few people were about, while the second occurred at lunchtime, but the Christchurch earthquakes provide a good illustration of the unique character, and thus the unique impacts, of different earthquakes, that need not be related directly to their magnitude.
Science Media Centre of Canada. Experts Comment – Christchurch Earthquake. Accessed March 11, 2012.
University of Berkeley, California. Today in Earthquake History: Mexico City 1985. Seismo Blog. Accessed March 11, 2012.Decoded Science