Geography terms in Geophysical hazards

A stage in a scale of warnings, often numbered or coloured, that tells authorities and the public how active a volcano or other hazard is and what actions, such as evacuation, to take.

The size of the area affected by a hazard event, from a single slope or valley to a whole region or, for ash clouds and tsunamis, an ocean basin.

The design of buildings and infrastructure so that they bend, absorb or isolate earthquake shaking without collapsing, for example through flexible frames, cross-bracing, shear walls, dampers and base isolation.

The weak, partly molten layer of the upper mantle beneath the lithosphere, on which the rigid plates move.

A method of aseismic design in which a building rests on flexible bearings, such as layers of rubber and steel, that absorb ground motion so that less shaking reaches the structure above.

The zone of earthquake foci that dips from the trench into the mantle along a subducting plate, reaching depths of up to about 700 km.

An approach to recovery that uses reconstruction to reduce future disaster risk, for example by rebuilding to stronger codes, moving homes out of hazard zones and improving services.

A set of legal standards that buildings must meet in design and construction, for example to resist earthquake shaking.

The ability of people, organisations and systems to use their resources and skills to manage a hazard event and its effects.

A chain of hazards in which one event triggers others, such as an earthquake causing landslides, liquefaction and a tsunami, or an eruption followed by lahars.

A bond sold to investors that pays a government or insurer a set sum if a defined disaster, such as an earthquake above a given magnitude in a given area, occurs; otherwise investors keep their money and interest.

A small, steep volcanic cone built from loose fragments of tephra, usually during a single short-lived eruption.

An approach in which local people identify the hazards they face and plan, carry out and monitor their own risk reduction actions, often with support from governments and NGOs.

A steep-sided cone built from alternating layers of ash and lava erupted from viscous magma, often explosively, such as Mount Fuji.

The thick, less dense layer of mainly granitic rock, about 25–70 km thick, that forms the continents; it is too buoyant to subduct and parts are about 4 billion years old.

A circulation in which hot, less dense material rises, spreads and cools, then sinks again; in the mantle it transfers heat outward over millions of years.

A plate boundary where two plates move towards each other, so that one plate subducts or the crust is crumpled in a collision.

Mapping in which many volunteers add data, such as roads, buildings or damage seen on satellite images or reported by phone, to a shared online map used by responders.

A fast-moving, water-saturated mixture of mud, rock and vegetation that flows downslope, often along a channel.

How cut off a place is from transport routes, communications and outside help, which affects how quickly rescue and relief arrive after a hazard event.

The distance from the Earth's surface down to the focus of an earthquake, classed as shallow (0–70 km), intermediate (70–300 km) or deep (300–700 km).

The unequal susceptibility to harm of different groups within the same place exposed to the same hazard, because of differences in income, gender, age, disability, ethnicity, legal status or housing.

An effect caused immediately and physically by a hazard event, such as deaths, injuries, collapsed buildings and destroyed roads or crops.

A serious disruption of a community caused by a hazard event, involving loss of life, property or livelihoods beyond the community's ability to cope with its own resources.

A model, devised by Park in 1991, that shows how quality of life falls during a hazard event and recovers through the phases of relief, rehabilitation and reconstruction.

The policies and actions that aim to prevent new disaster risk, reduce existing risk and manage what remains, so that losses of life, livelihoods and assets are reduced.

The change over time in the number of recorded disasters, the deaths they cause, the people affected or their economic losses, usually shown by year or by decade.

A plate boundary where two plates move apart and new crust forms from rising magma, as at mid-ocean ridges and continental rifts.

The length of time for which a hazard event lasts, from seconds for an earthquake's main shaking to months or years for an eruption or a slow landslide.

In the PAR model, the processes such as rapid urbanisation, population growth or a lack of training that turn root causes into unsafe conditions.

A system that detects the first, faster seismic waves of an earthquake and sends automatic alerts that arrive seconds to tens of seconds before the strongest shaking reaches more distant places.

The monetary value of the damage and disruption caused by a hazard event, including destroyed property and infrastructure and lost production, whether or not it is insured.

A volcanic eruption in which low-viscosity magma flows out gently as lava because gases escape easily.

An explanation of earthquakes in which rocks on either side of a locked fault bend as stress builds, then break and spring back, releasing energy as seismic waves.

The Emergency Events Database, a global record of disasters since 1900 kept by the Centre for Research on the Epidemiology of Disasters in Belgium, which lists events that meet set thresholds for deaths, people affected or calls for help.

The point on the Earth's surface directly above the focus of an earthquake, usually close to where the shaking is strongest.

A volcanic eruption in which gas trapped in viscous magma expands violently, shattering the magma into ash and tephra.

An attitude to hazards in which people accept losses as inevitable or as the will of a higher power and believe that little can be done to prevent them.

A fracture in the Earth's crust along which rocks on either side have moved relative to each other.

The sudden collapse of a large part of a volcano's side, producing a fast debris avalanche and sometimes a tsunami.

The point inside the Earth where a fault first ruptures and an earthquake begins, and from which seismic waves spread out.

A hazard caused by processes in the solid Earth: earthquakes, volcanic activity and mass movements.

The ways in which a society is managed and decisions are made and enforced, by governments and other institutions, including how laws are applied and how accountable those in power are.

The use of fixed satellite navigation (GNSS) receivers on a volcano or fault to measure ground movement of a few millimetres, such as the uplift caused by magma collecting below.

The increase in the strength of earthquake shaking where seismic waves pass from hard rock into soft sediments.

The rule that, in a region, the number of earthquakes falls by about ten times for each one-unit rise in magnitude.

A potential threat to people, property or the environment from a natural or human process.

Adjusting land use, buildings, behaviour and institutions so that people can live with a hazard and suffer less harm from it, without trying to stop the hazard itself.

A diagram or table that compares the physical characteristics of a hazard event, such as magnitude, frequency, duration, areal extent, speed of onset and spatial predictability, on a scale from low to high.

Estimating the probability of a hazard event of a given size occurring in an area over a period of time.

How often a hazard event of a given magnitude occurs in a place, often expressed as a recurrence interval or an annual probability.

The size or energy of a hazard event, measured on a scale such as moment magnitude, the VEI or the volume of a landslide.

The way in which individuals or groups understand and judge the threat posed by a hazard, shaped by experience, knowledge, culture, religion, wealth and trust in authorities.

Stating precisely when and where a hazard event will occur and how large it will be.

The probability of harm or loss from a hazard event, which depends on the hazard, the exposure and vulnerability of people and assets, and their capacity to cope.

An area of volcanic activity fed by a mantle plume, often far from a plate boundary, such as Hawaii or Yellowstone.

The quality of people's lives in all its dimensions, including health, safety, housing, income and livelihoods, education, social relationships, culture and the ability to make choices.

A measure of a hazard's impact divided by the population of the area affected, such as deaths per 100,000 people, so that impacts on places of different size can be compared fairly.

An effect that follows from the direct damage of a hazard event, often later or further away, such as lost income, disease, interrupted schooling or disrupted supply chains.

Interferometric synthetic aperture radar: a satellite technique that compares radar images taken at different times to map ground uplift or subsidence of a few millimetres over large areas.

The part of the economic loss from a hazard event that insurance companies pay out to policyholders; it is usually a large share of losses in high-income countries and a small share in low-income countries.

The heat generated inside the Earth by the radioactive decay of uranium, thorium and potassium isotopes, together with heat left over from the planet's formation.

An earthquake that occurs within a tectonic plate, far from a plate boundary, on an older zone of weakness.

A map showing lines that join places that experienced the same intensity of earthquake shaking.

A volcanic mudflow of ash, rock and water, formed by heavy rain or melting ice, that follows river valleys far from the volcano.

The everyday term for the rapid downslope movement of rock, soil or debris, used for most falls, slides and flows.

Changing the path of a lava flow away from settlements and infrastructure by building earth barriers or channels, or by cooling the flow front with water.

A process in which shaking raises the pore-water pressure in saturated sand or silt so much that it loses strength and behaves like a liquid.

The water content of the material in a mass movement, from dry falls and slides to wet flows; one of the criteria used to classify mass movements.

The rigid outer shell of the Earth, made of the crust and the uppermost mantle, about 100 km thick on average and broken into tectonic plates.

The inverse relationship between the size of hazard events and how often they occur: small events are frequent and large events are rare.

A narrow column of unusually hot rock rising from deep in the mantle that melts as it nears the surface and produces a hotspot.

The downslope movement of rock, soil or debris under the influence of gravity, from very slow creep to sudden falls and flows.

A very large earthquake on the gently dipping fault between a subducting plate and the overriding plate, which can exceed magnitude 9.

A scale from I to XII that measures the intensity of earthquake shaking at a place from what people feel and the damage it causes.

A logarithmic scale that measures the energy released by an earthquake from the fault area, the slip and the rock strength; each whole number is about 32 times more energy.

A fast flow of fine-grained, water-saturated material such as clay and silt down a slope or channel.

A hazard caused by a natural process in the Earth's surface, atmosphere or interior, such as an earthquake, eruption, landslide or tropical cyclone.

The thin, dense layer of basaltic rock, about 5–10 km thick, that forms the floor of the ocean basins; it is younger than about 200 million years because it is recycled at subduction zones.

A compressional seismic wave that pushes and pulls rock in its direction of travel; it is the fastest wave and passes through solids and liquids.

The belt of subduction zones, volcanoes and earthquakes that surrounds the Pacific Ocean, where about two-thirds of the world's volcanoes and most large earthquakes occur.

The capacity of individuals and households to prepare for, cope with and recover from a hazard event, for example through emergency plans, savings, insurance and knowledge of what to do.

The emotional bond between people and a place, built through family history, community, identity and livelihoods, which can lead people to stay in or return to hazardous places.

The zone where two tectonic plates meet and move relative to each other, where most earthquakes and volcanoes occur.

The pressure of water held in the spaces between particles of soil or rock; when it rises it pushes the grains apart and reduces friction.

The actions taken after a hazard event to save lives, meet urgent needs and restore and improve communities, usually divided into rescue, rehabilitation and reconstruction.

A mental health condition that can follow a frightening or distressing event such as a disaster, with symptoms including flashbacks, anxiety, sleep problems and avoidance that can last for months or years.

The measures taken before a hazard event to reduce its impact, including monitoring, warnings, land-use planning, building design, defences, education and evacuation plans.

The knowledge, plans, resources and practised actions that individuals, households and communities have in place to respond effectively before and during a hazard event.

A model of disaster risk in which root causes, dynamic pressures and unsafe conditions build vulnerability, and a disaster happens when this vulnerability meets a hazard.

A hazard caused directly by an eruption or earthquake, such as lava flows, ash fall or ground shaking.

A map showing the level of a hazard, such as ground shaking, that has a stated chance of being exceeded in a given period, used for building codes and insurance.

A fast, ground-hugging current of hot gas, ash and rock that flows down a volcano at hundreds of degrees Celsius and often over 100 km per hour.

The long-term phase of post-event management, lasting months to years, in which buildings, infrastructure and the economy are rebuilt, ideally to a safer standard than before.

The phase of post-event management, lasting weeks to months, in which temporary housing is provided and basic services, infrastructure and livelihoods are restored.

The emergency provision of food, water, shelter, medical care and other essentials to people affected by a disaster, mainly during the rescue phase.

The first phase of post-event management, lasting hours to days, in which survivors are found and freed, the injured are treated and urgent needs for shelter, water and food are met.

Earthquakes triggered by the filling of a large reservoir, where the water's weight and raised pore pressure along faults allow them to slip.

A wall built at the foot of or across a slope to hold back soil or rock and prevent it from sliding, often with drains to let water out from behind it.

The force that pushes plates away from a mid-ocean ridge as elevated new lithosphere slides down the gentle slope from the ridge crest.

The process in which the lithosphere is stretched and thinned by rising heat until it cracks along faults, forming a rift valley and sometimes a new ocean.

A steel rod drilled and fixed deep into a rock face or slope to tie loose surface rock or soil to stable material underneath; a soil nail does the same in soil.

In the PAR model, the deep economic, demographic and political processes, such as poverty and limited access to power, that make people vulnerable.

A mass movement in which material slides along a curved, spoon-shaped surface, so the moving block tilts backwards as it moves.

A shear seismic wave that moves rock at right angles to its direction of travel; it is slower than a P wave and passes only through solids.

A dam built across a valley on a volcano or mountain slope to trap the boulders and sediment carried by lahars and debris flows while letting water pass through.

A hazard triggered by a primary hazard, such as a lahar, landslide, liquefaction or tsunami.

A section of an active fault that has not ruptured in a large earthquake for a long time, where stress is building and a future earthquake is likely.

Strengthening an existing building or structure so that it can better withstand earthquakes, for example by adding steel bracing, concrete jackets around columns or shear walls.

A United Nations agreement adopted in Sendai, Japan, in 2015 that sets seven global targets and four priorities for reducing disaster deaths, people affected and losses between 2015 and 2030.

The internal resistance of slope material to moving downslope, provided by friction, cohesion and plant roots.

The force acting down a slope that tends to move material downslope, mainly the downslope component of the material's weight.

A broad volcano with gentle slopes built from many flows of runny basaltic lava, such as Mauna Loa in Hawaii.

The force exerted by the weight of cold, dense lithosphere sinking into the mantle at a subduction zone, which drags the rest of the plate behind it.

Engineering and vegetation measures that increase the shear strength of a slope or reduce the shear stress on it, such as drainage, regrading, retaining walls, rock bolts and planting.

The very slow downslope movement of soil particles, typically millimetres a year, caused by repeated wetting and drying or freezing and thawing.

How accurately the location of a future hazard event can be known in advance, for example along a mapped fault, on the flanks of a known volcano or on a slope with a landslide history.

The time between the first signs of a hazard event and its peak impact, from seconds for a rockfall to years for soil creep.

The process in which denser oceanic lithosphere sinks beneath another plate at a convergent boundary and descends into the mantle.

The displacement of the ground surface where a fault breaks through to the surface during an earthquake.

A seismic wave that travels along the Earth's surface; Love and Rayleigh waves are the slowest seismic waves but have the largest amplitude and do most damage.

A large, rigid slab of lithosphere that moves slowly over the asthenosphere, carrying oceanic crust, continental crust or both.

Fragments of rock and solidified magma thrown into the air by an eruption, from fine ash to volcanic bombs.

An instrument that measures very small changes in the slope of the ground, used on volcanoes to detect swelling as magma rises and deflation after it erupts or drains.

A plate boundary where two plates slide horizontally past each other along a fault, with no crust created or destroyed.

A mass movement in which material slides downslope along a roughly flat surface such as a bedding plane or joint.

A series of waves caused by the sudden displacement of a large volume of water, usually by an earthquake on the sea floor, a landslide or an eruption.

A structure built to reduce the height or force of tsunami waves reaching the land, such as a seawall, breakwater or embankment, or a natural barrier such as a coastal forest or mangroves.

A network of seismometers, sea-level gauges and deep-ocean pressure sensors linked to warning centres that detect a possible tsunami and send alerts to coastal populations.

In the PAR model, the specific forms of vulnerability in a place and time, such as weak buildings on unstable slopes or a lack of warnings.

The specialised location, extraction and first medical care of people trapped in collapsed buildings, carried out by trained teams with equipment such as listening devices, cameras and cutting tools.

Escaping a tsunami by climbing to the upper floors or roof of a strong, tall building or a purpose-built tower or mound, used where high ground is too far away to reach in time.

The resistance of a fluid to flow; viscous magma is thick and sticky, mainly because of its high silica content and lower temperature.

A logarithmic scale from 0 to 8 that measures the size of a volcanic eruption mainly by the volume of material ejected and the height of the eruption column.

The continuous measurement of a volcano's earthquakes, ground deformation, gas emissions and temperature to detect rising magma and forecast eruptions.