Earthquakes

 

Earthquakes happening within and outside of our region can affect the people and properties of Otago.

 

The risk of damage from earthquakes depends on the magnitude, frequency, nature of the earthquake, distance from earthquake focus, and the type of ground.

Seismic activity can cause ground shaking, surface rupture, liquefaction (settlement of soil), landslides and lateral spread of sediments towards bodies of water. It can also cause tsunamis.

 

How should I prepare in case of an earthquake?

Visit the Otago Civil Defence and Emergency Management website for information on what you should do to be prepared for an earthquake. 

 

Is my property going to be affected by an earthquake?

The Otago Natural Hazards Database provides information about the type of ground, likelihood of liquefaction, and likelihood and intensity of ground shaking at any particular location within our region*. The database also shows locations of known active faults within Otago**.

Properties are classified and mapped into three liquefaction susceptibility categories: not susceptible, low susceptibility or possibly susceptible

*Derived from the Seismic Risk of the Otago Region report

**The location of known active faults is based on information supplied by GNS Science.

 

 

Glossary

 

Magnitude is the size of an earthquake.

 

Frequency is how often ground shaking occurs over a period of time.

 

Types of ground can be classified into strong rock, rock, shallow soil, deep or soft soil, and very soft soil.

 

Seismic activity is the frequency, type, and size of earthquakes experienced over a period of time.

 

Ground shaking, or the movement of seismic waves through the earth’s surface, is the most recognised seismic hazard. The intensity of ground shaking felt at any location during an earthquake depends on many factors, such as the underlying soils, distance to the earthquake focus and the magnitude of the event. The stronger the shaking, the greater the impact on people and the environment, both built and natural.

 

Surface rupture is a seismic hazard most commonly restricted to the location where a fault meets the land surface. The land surface may move horizontally or vertically depending on the type of fault. While there is potential for surface rupture to occur along any active fault, there is low risk of communities to be directly affected. This is because most known active faults are located outside of urban areas (except for Cardrona Fault, which runs close to Wānaka and Albert Town). Surface rupture can cause extensive damages to both built and natural environment, such as transport infrastructure, homes, floodbanks and drainage channels.

 

Active fault is a fracture or discontinuity in rock due to rock movement, that is likely to move again sometime in the future, causing earthquakes. Large faults within the Earth's crust result from action of plate tectonic forces. Energy released from rapid movement on active faults is the cause of most earthquakes.

 

Liquefaction is a natural process triggered by earthquake shaking, causing soil to behave like a fluid. As the soil is shaken, it acts like a fluid or jelly, causing deformation, rupture, settlement, and sometimes lateral spread towards rivers or lakes. It results in temporary loss of soil strength, potentially causing significant damage to land, buildings and infrastructure. Areas with unconsolidated sediments, soils, and high groundwater tables have a high-risk exposure to liquefaction and settlement of soils. Locations close to active faults have a higher risk exposure to liquefaction due to more intense ground shaking. Read more about what liquefaction is through this resource from Water New Zealand.

 

Landslides can be caused by large earthquakes with sufficient ground shaking trigger the movement of existing landslides or generate areas of new movement on slopes with an existing marginal stability.

 

Lateral spreading refers to the spread of sediments, often towards bodies of water such as a lake, as a result of seismically-induced shaking.

 

Earthquake-induced tsunamis are the most likely cause of tsunami. Large earthquakes (magnitude of more than 8) can affect Otago's eastern coastlines, causing vertical movement of seabed along a fault line. A smaller earthquake could also produce tsunami events along sections of Otago’s coast. More information about tsunamis can be found here.

 

Alpine Fault

Learn about the Alpine Fault from Professor Tim Davies Read more

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