What damage can earthquakes do?
Ground shaking from earthquakes can cause buildings and bridges to collapse; disrupt gas, electricity, and telephone services; and sometimes trigger landslides, avalanches, flash floods, fires, and tsunami.
Buildings with foundations resting on unconsolidated landfill or other unstable soils are at increased risk of damage, as are homes not attached to their foundations.
In general, damage to buildings is the main cause of financial loss from earthquakes. Collapse of buildings is the main cause of casualties, either through crushing or entrapment. Loss of services is the main cause of people becoming displaced.
Earthquakes can cause damage in the following ways
Fault rupture is a relatively rare cause of damage and injury. However, if a fault ruptures to the earth’s surface, any building extending across it will be severely damaged.
Strong ground shaking is a major cause of landslides in New Zealand. Factors affecting slope stability include the slope angle and height, slope modification, underlying geology, the history of landslides in the area and groundwater content. Properties above and below unstable slopes are also at risk from undermining and burial respectively. In the 1929 Murchison earthquake, 16 of the 17 fatalities were as a result of landslides.
This occurs when saturated sandy ground is subjected to strong shaking; effects range from harmless ‘sand boils’ to serious ground damage such as subsidence. Liquefaction can cause substantial damage to buildings and underground equipment such as tanks and pipelines. Fortunately, not many areas of New Zealand have soils with high liquefaction potential.
Large earthquakes can generate tsunami if they cause major uplift of the sea floor, or trigger coastal or submarine landslides. Tsunami generated by local earthquakes are very dangerous as they can arrive at the nearest shore within minutes.
Post-earthquake fire is a highly variable phenomenon. Most earthquakes are not accompanied by fire, but devastating fires have occurred after earthquakes. In Napier, following the 1931 earthquake, much of the central business district was burned out and the loss due to the fire was about equal to the loss from the ground shaking. The 1906 San Francisco and 1923 Tokyo earthquakes caused fire losses that greatly exceeded the losses from ground shaking.
The critical factors in creating a fire risk are wind, water and weather. If the shaking is strong enough to disrupt the water supply, winds are strong enough to spread the fire across city streets and vegetation is flammable following hot, dry weather then the scene is set for a high level of fire risk.