The type of hazard depends on the strength of seismic activity, along with such factors as local topographic and built features, subsurface geology and groundwater. A large earthquake will always be followed by a sequence of aftershocks.
If an earthquake generates a large enough shaking intensity, structures like buildings, bridges and dams can be severley damaged, and cliffs and sloping ground destabilised. Perched or stacked objects may fall and injure or bury anyone close by. In the largest earthquakes whole districts can be devastated by the multiple consequences of ground shaking.
Groundshaking will vary over an area due to such factors as topography, bedrock type, and the location and orientation of the fault rupture. These all affect the way the seismic waves travel through the ground. For an explanation of the exceptional high energy of the Christchurch earthquakes in 2011 have a look at this video.
Tsunamis are long wavelength oceanic waves generated by the sudden displacement of seawater by a shallow earthquake, volcanic eruption or submarine landslide. What is it like to face a tsunami? Watch this video!
A number of waves may be produced and they can travel long distances at high speeds to flood far-off shores. The height of a tsunami varies and may be affected by the sea floor depth and shape, and other factors. New Zealand is susceptible to tsunamis originating from distance sources around the Pacific Ring of Fire as well as from very close to our coastline. Near source tsunamis will allow for very little warning.
Large earthquakes may generate tsunami waves in enclosed water bodies such as lakes. In New Zealand there are large lakes that could be affected, for example Lakes Wakatipu and Wanaka that are near to the Alpine Fault.
Groundshaking due to earthquakes destabilises cliffs and steep slopes, causing landslides and rockfalls as a significant side-effect. Heavy rain and unconsolidated or fractured rock are exacerbating factors.
Check out this video about the Rockfall Impacts of the Christchurch Quake,
and this one: Laser Scanning Christchurch Rockfalls
Subsidence, or lowering of the ground surface, often occurs during earthquakes.
This may be due to downward vertical displacement on one side of a fault, and can sometimes affect a huge area of land. Coastal areas can become permanently flooded as a result.
Subsidence can also occur as ground shaking causes loose sediments to “settle’ and to lose their load bearing strength (see liquefaction, below) or to slump down sloping ground (see Landslides and Rockfalls).
Lateral spreading occurs where sloping ground starts to move downhill, causing cracks to open up, that are often seen along hill crests and river banks.
Liquefaction occurs when waterlogged sediments are agitated by seismic shaking. This separates the grains from each other, reducing their load bearing capacity. Buildings and other structures can sink down into the ground or tilt over, whilst underground pipes and tanks may rise up to the surface.
When the vibrations stop the sediments settle down again, squeezing groundwater out of fissures and holes in the ground to cause flooding. The aftermath of liquefaction can leave large areas covered in a deep layer of mud.