Earthquakes occurring both within Otago and beyond its boundary can affect people and property within the region.
The risk of damage from earthquakes, depends on the magnitude, frequency, and nature of the earthquake; its distance from the subject area; and the susceptibility of the underlying ground to seismic shaking.
Seismic activity can generate direct and indirect effects including ground shaking, surface rupture, liquefaction/settlement of soils, landslides, and the lateral spread of sediments towards water bodies. It can also generate tsunamis.
Visit the Otago Civil Defence and Emergency Management website for information on what you should do to be prepared for an earthquake.
The Otago Natural Hazards Database provides information about:
- Ground class (the nature of the sediment underlying a site). The ground class map classifies the Otago region into one of five geological categories – strong rock, rock, shallow soil, deep or soft soil, and very soft soil, based on the underlying geology.
- The susceptibility of the land to liquefaction.
- The likelihood and intensity of ground shaking associated with credible earthquake scenarios.
- The location of known active faults.
The information identified in the first three items above is derived from a ‘regional scale’ report completed by Opus in 2005 (Seismic Risk in the Otago Region).
The liquefaction potential of land was also investigated in the Opus report. Areas were classified and mapped into three liquefaction susceptibility categories: ‘not susceptible’, ‘low susceptibility’ or ‘possibly susceptible’. An updated assessment of liquefaction hazard for the Dunedin District was completed in 2014, which uses a slightly different classification system. The assessment for the Dunedin District draws upon methods developed for liquefaction hazard evaluation in Canterbury following the 2010-11 earthquakes.
Additional information about the susceptibility of land to liquefaction will continue to be incorporated into the database, as it becomes available. This includes local scale, or site specific investigations. The location of known active faults is based on information supplied by GNS Science.
Terms associated with earthquake hazard:
Surface fault rupture is a seismic hazard most commonly restricted to the location where a fault meets the land surface. The land surface may displace sideways (move horizontally) and/or one side of the land surface may rise (move vertically) depending on the type of failure. While there is potential for surface rupture to occur along any of the active faults within the region, the risk that populated communities would be directly affected is low, with most of the known active faults located outside urban areas (except for the Cardrona Fault, which runs close to Wanaka and Albert Town). Surface fault rupture can cause extensive damage to structures and features located across them, such as transport networks and utilities, dwellings, floodbanks, and drainage channels.
Ground shaking, or the movement of seismic waves through the earth’s surface, is the most recognised seismic hazard, due to the propensity for people to feel it. 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 impact on people and the environment, both built and natural, is greater with stronger shaking intensities.
Liquefaction and settlement of soils. Liquefaction occurs when saturated fine grained sediments (such as sand and silt) are subjected to high intensity shaking and lose their ability to stay cohesive. As sediments are shaken, they act like a fluid or shaken jelly, causing deformation, settlement, and sometimes lateral spread towards rivers or lakes. 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.
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 landslides. A large earthquake with sufficient ground shaking could trigger the movement of existing landslides, or generate areas of new movement on slopes with an existing marginal stability.
Earthquake-induced tsunami. The most likely source of tsunami events which may affect Otago’s eastern coastline are large earthquakes (magnitude > 8), which cause a vertical displacement of the seabed along a fault line. A smaller seismic event on a local offshore fault could also produce tsunami events along sections of Otago’s coast. Additional information about tsunami hazard is provided elsewhere.
Learn about the Alpine Fault from Professor Tim Davies Read more