Hazards caused by Earthquakes

Shaking

In New Zealand, the intensity of shaking caused by earthquakes is measured by the Modified Mercalli intensity scale, which assesses the severity of an earthquake using a descriptive scale that incorporates the earthquake's effects on people and the degree of damage caused to the environment. ³⁸⁰⁶  

The intensity of the ground shaking will vary depending on three major factors:

1. The magnitude of the earthquake;
2. The distance from the fault; and 
3. Soil conditions.

The magnitude is measured by the Richter scale and is given a numerical value representing the amount of energy released at the source of the earthquake (epicentre). ³⁸⁰⁷  The intensity of the seismic waves generally decreases the further the distance from the source; however the specific soil conditions at a site may amplify the effect of the shaking, with softer soils more readily amplifying the seismic waves than hard rock.

Landslides

Ground shaking caused by earthquakes can destabilise slopes causing landslides. These can be widespread during a major earthquake. The 7.8 Hurunui/Kaikōura earthquake in 2016 triggered between 80-000 to 100,000 landslides. ⁴⁷¹³  Landslides can also be caused by heavy or prolonged rainfall, which can exacerbate the damage if an earthquake occurs. 

Landslides can cause significant damage to roads and other infrastructure and rock fall and debris can dam rivers causing lakes to form. These can destabilise over time, due to further earthquakes or heavy, prolonged rain, breaking the dam and causing downstream flooding.

Liquefaction

There are three requirements for liquefaction to occur:

1. Saturation of susceptible soils, such as silt or sand;
2. A high water table; and
3. An earthquake with a Peak Ground Accelation of 0.1 or greater. ⁴⁷¹⁴  

During an earthquake, the seismic waves cause the water molecules in saturated soil to become pressurised and the soil particles begin to flow, behaving like a liquid. This liquefied soil is forced up and out of any available crack, weakness or fissure to the surface. The purging of the liquefied soil at the surface creates sand volcanoes and large cracks filled with silt rivers. ³⁸¹¹

The soil loses its strength and is no longer able to support the weight of surface layers or structures above, and the destructive effect on buildings can be extensive.  Damage to underground sewage and water pipes can be wide-ranging, and sewage can become an environmental contaminant.

When the liquefaction settles and dries, subsidence may occur where the liquefaction has been ejected from the ground. Later the dried silt may become a further hazard to those with respiratory illnesses as the fine particles are picked up by the wind as a pervasive dust.

During the devastating 2010/11 earthquake sequence in Christchurch, liquefaction was a significant contributor to the widespread damage. The Hurunui/Kaikoura earthquake in 2016 triggered liquefaction in coastal areas causing extensive damage to buildings and infrastructure. 

Subsidence and lateral spreading

During earthquakes the ground may lower due to land subsidence caused by vertical or horizontal ground movements or a combination of both. Large areas of land may sink and become flooded. ³⁸¹²

When liquefaction occurs adjacent to waterways it can cause lateral spreading where the liquefied soil forces a stronger layer sitting above to collapse sideways into a river or stream. This process opens cracks running parallel to the waterway. The effect on buildings can be severe structural damage or even collapse.

Fire

Fires can be triggered by earthquake damage to gas mains and electrical wiring, heat sources or reactions from spilled chemicals. The damage caused by fire maybe increased by the unavailability of water for dowsing due to destroyed water mains, pipes and roads.

Tsunami

Tsunamis ³⁸¹³  are a series of waves caused by the rapid displacement of water that can occur during shallow earthquakes, volcanic eruptions or landslides. Earthquakes create tsunamis by:

1. Vertical displacement (uplift) of the seabed that results in displacement of the water column to trigger a tsunami; and 
2. Submarine landslides caused by loose sediment, accumulated at the top of submarine canyons, becoming unstable and flowing down into the canyon as a mass landslide that displace the water column and create a tsunami. ⁴⁷¹⁰

Due to New Zealand’s position on the Ring of Fire, and the fact that is an island nation surrounded by the pacific ocean,  its coastline is vulnerable to risk of tsunami from near or distant terrestrial and undersea earthquakes, undersea or coastal landslides, or volcanic eruptions. 

Whilst the majority of tsunami that arrive at our coastline are relatively small waves, the effect of larger scale tsunami can be substantial. Unlike a normal ocean wave which occurs at surface of the water, a tsunami wave incorporates the whole water column which is why they can be described as a ‘wall’ of water. Their destructive power is magnified by the debris that is propelled along with it. The 2004 Boxing Day Indian Ocean tsunami was caused by a 9.3 magnitude earthquake which raised a 1200 km stretch of seafloor by several meters. ³⁸¹⁴ This thrusting upwards of the crust caused the displacement of an enormous volume of water which, as it sped towards land became successive walls of water which then surged inland in some places as much as several kilometres. The tsunami waves carried fast-moving debris and sand which destroyed buildings and vegetation in their wake. Over 225,000 people in were killed in 12 countries from this devastating tsunami. ³⁸¹⁵

Last updated at 1:09PM on February 8, 2018