Ground shaking
The second primary earthquake hazard, ground shaking, is the result of rapid ground acceleration. Ground shaking can vary over an area as a result of factors such 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. If an earthquake generates enough shaking intensity , built structures can be severely damaged and cliffs and sloping ground can be temporarily or permanently destabilised. In large earthquakes, whole districts can be devastated by the consequences of ground shaking.
- Ground displacement is how far the surface moves during the earthquake. It can cause the ground to change position in both horizontal and vertical directions and move relative to objects or other areas of land nearby.
- Ground velocity is a measure of how quickly the ground was displaced – the speed and direction that the ground moved to get from its original location to its new location. Ground that moves with a higher velocity is also displaced more quickly.
- Ground acceleration is a measure of how quickly the ground changes velocity during the earthquake. Ground acceleration is responsible for the classic earthquake shaking effect where the ground rapidly changes direction in a violent back and forward and up and down motion.
Displacement, velocity and acceleration are also responsible for several secondary effects on the ground, including liquefaction, settlement and lateral movement, which can compromise the soil’s ability to support objects on the surface.
Structural stress
Ground shaking is also the primary way an earthquake affects buildings. The rapid acceleration of the ground beneath the building creates inertial forces in the structure. This can cause damage if they become too large or the building is not designed to withstand them.
Under normal conditions, a built structure is subject to steady gravitational loads. However, during an earthquake, ground shaking can impose strong lateral loads. If the structure is not designed to withstand them, the additional lateral loads associated with seismic shaking can cause it to fail.
Earthquake shaking causes movement on all three principal axes (up and down, left and right, forward and back). Lateral movement in the horizontal plane (left and right, forward and back) can place additional stress on structural elements normally intended to only carry vertical loads, such as walls, columns and beams.
Several factors influence the degree of damage a building can sustain when subjected to ground shaking. (BRANZ)
In buildings, these elements are usually designed to withstand an appropriate degree of lateral movement, such as that caused by wind or seismic loading. However, if the earthquake shaking force exceeds the downward force, in beams, for example, due to a combination of gravity and vertical earthquake acceleration, it may place the element under excessive stress.
When this occurs, unreinforced structures may lose integrity and distort, crack or collapse. Elements that do not undergo a catastrophic failure may still be weakened, reducing their ability to carry their original design loads.
The degree of movement and stress a structure can withstand during an earthquake depends on several factors, including:
- the age and state of repair of the structure
- the inherent strength, rigidity and stability of the structure’s design
- design characteristics intended to reduce the damaging effects of shaking
- the properties of the materials used to build the structure (concrete, steel, timber and so on)
- the quality of building construction
- any seismic resilience devices that have been added to isolate, dampen or transfer damaging effects
- the size of earthquake that the building was originally designed to withstand.