9.1 Hazards resulting from tectonic processes
What are the type types of seismic waves that occur after an earthquake?
Body waves Surface waves
What is the definition of interplate?
Earthquakes that occur at plate boundaries.
What are primary impacts resulting from tectonic hazards?
The immediate results of an earthquake shockwave or of a volcanic eruption.
What is a plate?
Massive rocks that make up the outer layer of the earth's surface and whose movement along faults triggers earthquakes.
What is the function of the Richter scale, the Moment Magnitude Scale M and the Modified Mercalli Intensity Scale?
Measure the strength if an earthquake. They do not predict earthquakes.
What are love waves?
Move sideways. Slowest waves. Cause most of the damage.
What are tsunamis?
Origin Caused by earthquakes (usually in subduction zones) Caused by volcanoes and landslides Characteristics Harmful effects (impacts on lives and property) Loss of life
What are P waves?
Primary/pressure waves Fastest and shake the earth backwards and forwards. Arrive at a place first. They travel by compression and expansion. Able to travel through liquids and solids (rocks) and gases.
What is vulnerability?
The degree to which conditions make a population more likely to experience a hazard event, which they do not expect, cope with or recover from. The geographic conditions that increase the susceptibility of a community to a hazard or to the impacts of a hazard event.
What is a risk?
The exposure of people to a hazardous event. The probability of a hazard event causing harmful consequences (expected losses in terms of deaths, injuries, property damage, economy and environment).
What is an epicentre?
The point at the surface of the earth directly above the focus of the earthquake.
What will the level of adjustment depend on?
The risk caused by the hazard. This includes: - identification of the hazards - estimation of the risk (probability) of the environmental hazard - evaluation of the cost (loss) caused by the environmental hazard
What is the magnitude of an earthquake?
The total amount of energy released. The number of earthquakes increases as their magnitude decreases.
What is a focus/hypocentre?
The point within the earth where an earthquake rupture starts. Deep focus - 300-700km - below that the rocks are so hot they bend rather than break - deep focus earthquakes are not as destructive as shallow focus ones because the thickness of rocks above the focus absorbs the shockwave - its strength reduces with increased distanced from the focus Intermediate focus - 70-300km Shallow focus - 0-70km
How can human activities trigger earthquakes or alter the magnitude and frequency of earthquakes?
Through underground disposal of liquid wastes. By underground nuclear testing and explosions. By mining and fracking. By increasing crustal loading.
How does the nature of rock and sediment beneath the ground influence the pattern of rocks and vibrations during an earthquake?
Unconsolidated sediments e.g. sand shake in a less predictable way than solid rock - means damage is greater to foundations of buildings.
What is an active volcano?
A volcano that has erupted in the last 80 years. A volcano that has erupted in historic time. A volcano that continues to erupt or is at risk of erupting.
What is the definition of intraplate?
Earthquakes that occur within plate interiors.
What is a dome volcano?
Has a steep, convex slope from thick, fast cooling lava.
What is hazard mapping?
It shows: - areas of ground that are likely to liquify fault locations - dates of past movements, past earthquake epicentres with magnitudes - dates and areas where landslides and tsunamis might result Geologists look for fault locations to map - these are unsuitable locations for development. It is a body of theory that includes risk, prediction, prevention, event and recovery.
What is the Moment Magnitude Scale M?
Measures the amount of energy released and produces figures that are similar to the Richter Scale. For every increase on the scale of 0.1, the amount of energy released increases by over 30. Every increase of 0.2 represents a doubling of the energy released.
What is the origin, characteristics and harmful effects of atmospheric effects?
Origin Eruption columns which may extend hundreds of kilometres into the atmosphere allowing e.g. ash to be transported by high level winds Characteristics Ash and dust particles including acidic aerosol Light-scattering leads to unusual optical effects Harmful effects (impacts on lives and property) Clouding reduces sunlight reaching earth's surface, cooling the troposphere, but release of greenhouse gases contributes to tropospheric warming
What are body waves?
P and S waves They travel through the earth's interior. These are a series of oscillations at right-angles to the direction of movement. These are long waves with large amplitude - cause most of the damage as they shake the ground more violently.
What is the order of waves caused by an earthquake?
P waves S waves Love waves Rayleigh waves
What is the earthquake activity (frequency, magnitude, depth, distribution etc) associated with different types of plate boundary?
Conservative Frequency Magnitude Depth - shallow Distribution Divergent/constructive Frequency Magnitude Depth - less than 30km Distribution - earthquakes are common, but restricted to a narrow zone close to the the ridge - they are located along the transform faults, rather than along the spreading segments, there are clusters of earthquakes at some of the ridge-transform boundaries, some occur on spreading ridges - small and infrequent Convergent/destructive (subduction) Frequency - earthquakes near the upper side of the subducting place, earthquakes are greatest near the surface and around the area where large subduction earthquakes happen Magnitude - larger earthquakes take place at the subduction zone (most M9 or higher earthquakes take place here) Depth - increasingly deep on the landward side of the subduction zone - extents to at least 400km Distribution - get deeper with distance from the trench Along subduction zones earthquakes are abundant Convergent/destructive (collision) Frequency Magnitude Depth - significant concentration of both shallow and deep (greater than 70km) earthquakes Distribution Intraplate locations - related to the build up of stress due to continental rifting or the transfer of stress from other regions - examples - Great Rift Valley area of Africa, the Tibet region of China, and the Lake Baikal area of Russia
What are the socioeconomic impacts of volcanic hazards?
Destruction of settlements Loss of life Loss of farmland and forests Destruction of infrastructure - roads, airstrips and port facilities Disruption of communications
What are the factors influencing volcanic hazards?
Different types of plate boundaries - conservative margins have little vertical movement and lack volcanic activity - earthquakes result from horizontal movements
What is increasing risk perception?
Earthquake drills - allows people to practice what to do in an earthquake - they are regular events in schools and workplaces in many tectonic areas Japan hold an Earthquake Awareness Day each year so everyone will be able to follow emergency procedures without panic. Evacuation points with water, food and medical supplies are placed in every neighbourhood. People are given maps of routes to follow and leaflets about how to dress and what to take. They are informed how to improve safety in the home e.g. securing furniture to the walls and knowing how to switch off the gas supply. In japan, gas supplies automatically shut off in a earthquake to reduce the fire risk. People are advised to keep a survival pack near an exit and to have food and drink available to last a least three days.
How are earthquake hazards managed/how can we respond to earthquakes?
Earthquake prediction Monitoring of earthquake zones Hazard mapping Hard engineering Strengthening buildings Land-use zoning Increasing risk perception
What is hard engineering?
Engineer geologists know what infrastructures ned replacing, moving or strengthening. It is cost-effective to make the necessary changes rather than meet the expense of restoration after a strong earthquake.
What is strengthening buildings?
Example - Japan - little space for building in mountainous Japan - so skyscrapers are built in a pyramid shape for strength - but these measures add to the cost and builders in some countries make profit, not building regulations, their aim - even poor rural areas can have safer buildings, using thatch for roofs and mud and straw for walls
At an individual level, what are three important influences upon an individual's response to any hazardous event?
Experience - the more experience a person has of environmental hazards, the greater the adjustment to the hazard. Material well-being - those who are better off have more choices. Personality - is a person a leader or a follower, a risk taker or a risk-minimiser? There are three choices: 1. Do nothing and accept the hazard. 2 Adjust to the situation of living in a hazardous environment. 3 Leave the area.
What are the main types of volcanoes?
Fissure volcanoes Shield volcanoes Dome volcanoes Ash-cinder volcanoes Composite (stratovolcano) Caldera volcano
What are parasitic cones?
Form on the sides of volcanoes when the main vent becomes blocked and magma fins another way out.
What is the global distribution of volcanoes?
Found at plate boundaries. Found in the Pacific Rim or Ring of Fire - related to the subduction beneath either ocean or continental crust. Exceptions - volcanoes that occur over hotspots. Hotspots can be found beneath oceans and continents. 3/4 of the Earth's active volcanoes lie along the Pacific Ring of Fire e.g: - Mt Pinatubo (Philippines) - Mt Unzen (Japan) - Mt Agung (Java) - Mt Chichon (Mexico) - Mt St Helens (USA) - Nevado del Ruiz (Columbia) Other areas of active vulcanicity: - Iceland - Montserrat in the Caribbean - Mt Nyiragongo in Democratic Republic of Congo
What are secondary impacts resulting from tectonic hazards?
Happen after the shockwaves or eruptions have finished and are the consequences of the primary impacts. Some continue for a long period.
What are pelean eruptions?
Highly viscous magma and a long period of inactivity cause an explosion out of a weakness in the side of the volcano, with nuee ardente falling down the slope. A Pelean eruption is associated with explosive outbursts that generate pyroclastic flows, dense mixtures of hot volcanic fragments and gas. Examples Pelean eruptions are named for the destructive eruption of Mount Pelée on the Caribbean island of Martinique in 1902.
What is monitoring of earthquake zones?
Horizontal movements are detected by measuring the change in the time it takes a laser beam to move between two fixed points on either side of a fault. Changes in slope are detected by a tilt meter. Variations in the Earth's magnetic field, resulting from changes in stress in the rocks, are measured by a magnetometer. Seismograph records are monitored. Clusters of small earthquakes often precede a larger one. The amount of radon gas rising to the surface is measured, as it can increase before an earthquake.
What is resilience?
How well a population recovers from a disaster.
How do earthquakes occur?
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How do tsunamis form?
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What are safe houses?
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What are some earthquake-proof building methods in Japan?
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What are some ways in which volcanic hazard are reduced?
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What is earthquake prediction?
It is not possible to predict exactly when an earthquake will occur or where its epicentre will be. Short-term predictions cannot be made. Geologists use the seismic gap theory to work out where a strong earthquake is likely to occur. Gap - the section of an active fault that has not experienced earthquakes for a considerable time compared with other segments along that fault. All parts of the fault are expected to experience the same degree of displacement over time. The longer the period without an earthquake, the greater the accumulated stress and the greater the likelihood of a strong earthquake. Example The location of the 1989 earthquake in the San Francisco Bay area along the San Andreas fault was at a known gap. Using past records, the frequency of earthquakes along part of a fault is calculated to predict the next one and the rare of movement of sections along the fault is also used. Reliable prediction will be impossible.
What is land-use zoning
Land uses (e.g. oil storage depots) that are potential fire/explosion risks are positioned away from homes, and are built on solid rock. Land likely to liquify is used for playing fields, nature reserves and parks (not buildings).
What is the origin, characteristics and harmful effects of lava flows?
Lava - both the liquid material that flows out of the crater of the volcano and the rock it forms when solidified. Origin Any eruption Originates from magma which rises from a magma chamber beneath the volcano through a vent to reach the surface Characteristics Flow rate dependent on temperature - as lava cools, its viscosity increases and speed reduces until it is less than walking speed As it cools, a solidified surface is produced Volume and range of flow variable - may extend up to 100km from source Harmful effects (impacts on lives and property) Can cause ignition of fires and burial of land and objects, but slow flow rate means it poses little risk to life
What are the six main categories of volcanic hazards?
Lava flows Ballistics and tephra clouds Pyroclastic flows/nuees ardentes - glowing clouds Gases and acid rain Lahars (mudflows) Glacier bursts)
What are the factors that make hazards a disaster? How are hazards classified?
Level of development/wealth Frequency - how often an event of a certain size occurs - the larger the event, the less frequently it occurs Education Geographical location Natural/physical factors Magnitude - the size of the event. Time (of day/year) Population density and distribution Spatial concentration - distribution of hazards over space - where they are concentrated in certain areas e.g. coastal locations, tectonic plate boundaries etc Areal extent - the size of the area covered by the hazard - can range from small scale (an avalanche chute) to continental (drought). Regularity - some hazards occur more frequently (cyclones) - others occur more randomly (earthquakes and volcanoes) Speed of onset - the time difference between the start of the event and the peak of the event - rapid events e.g. earthquakes - slow events e.g. droughts - rapid onset hazards are more dangerous than slow onset ones Duration - the length of time that the hazard exists - can vary from hours (volcanic eruptions) to years (drought) Causes - tectonic (geological) - earthquakes and volcanic eruptions - geomorphological (processes acting on the land surface) - mass movements - atmospheric (metrological) - tropical cyclones and tornadoes
What are the impacts of earthquake hazards?
Loss of life Loss of livelihood Total or partial destruction of building structure Interruption of water supplies Breakage of sewage disposal systems Loss of public utilities e.g. electricity and gas Floods from collapsed dams Release of hazardous material Fires Spread of chronic illness
What are surface waves?
Love and Rayleigh waves They travel along the earth's surface. They travel slowly. They do the most damage in an earthquake.
What is the Richter scale?
Measured by seismographs. It measures the magnitude of earthquakes. It is logarithmic - every 1.0 point on the scale is a ten fold increase i.e. A 3 to a 4 is a ten-fold increase and a 3 to a 5 is a 100 fold. It records the amount of energy released by a single earthquake (but it doesn't say what that earthquake did)
What is the Mercalli scale (Modified Mercalli Intensity Scale)?
Measures intensity (amount of physical damage) of an earthquake. 12 point scale Used to understand how the earthquake felt on the surface. It measures how it feels in a earthquake. Uses Roman numerals - I to XII (1 to 12). It is subjective. II earthquake - weak - felt only by a few persons at rest, especially upper floors of buildings VI earthquake - strong - felt by all, many are frightened. Some heavy furniture is moved; a few instances of fallen plaster. Damage slight. It allows eyewitnesses to provide information on how strong the earthquake was. Isoseismal lines on a map can be drawn using Mercalli scale measurement.
How can we respond to volcanoes?
Modify the cause - N/A (a volcanic eruption cannot be prevented) Modify the event - environmental control - primary hazard - lava flow management and volcanic gases management - secondary hazard - lahar management Modify vulnerability - prediction - monitoring - warning Modify the loss - aid - insurance
What are the problems caused by earthquakes?
Most problems are associated with damage to buildings, structures and transport systems. The collapse of building structures is the direct cause of many injuries and deaths. It also reduced the effect of the emergency services. Buildings can be destroyed both by: - shaking - the ground tilting or subsiding as the shock wave passes
What is the difference between a natural hazard and natural disaster?
Natural hazard - a natural event (originating in the lithosphere, atmosphere or hydrosphere) that has the potential to threaten both life and/or property. (threatens harm) Natural disaster - a realisation of a hazard. Extreme events that normally cause loss of life and/or damage to the built environment and create severe disruption to human activities, which, the affect community is unable to deal with adequately without outside help. (actually causes harm)
What are floods?
Occur if a shock wave causes: - a dam wall to collapse - a large landslide to fall into a lake These result in sudden flooding down valley. Outbreaks of disease occur when survivors are forced to live in camps with insanitary conditions.
What is the global distribution of earthquakes?
Occur in clearly defined linear patterns - these linear chains follow plate boundaries. Examples - clear line of earthquakes along the centre of the Atlantic Ocean in association with the Mid Atlantic Ridge (a constructive plate boundary) - distinct lines of earthquakes around the Pacific Ocean Some occur in broad linear chains. Examples - line of earthquakes along the west coast of South America - around the eastern Pacific associated with the subduction of the Nazca Plate beneath the South American Plate (destructive plate boundary) Broad belts of earthquakes - subduction zones - collision plate boundaries Narrower belts of earthquakes - constructive plate boundaries - conservative plate boundaries Isolated occurrences of earthquakes - these may be due to: - human activities - hotspots Tectonic - occur on all types of plate boundary - at hot spots away from plate boundaries - at fault lines Most earthquake foci occur in narrow zones along plate boundaries but their effects extend beyond the plate boundary. Non-tectonic - caused by human activity putting too much stress on faults
What is the origin, characteristics and harmful effects of poisonous gases?
Origin Any eruption Characteristics Ash-laden gases including: - carbon monoxide - carbon dioxide - hydrochloric acid - hydrofluoric acid - sulphur dioxide - can corrode aircraft - hydrogen sulfide - toxic Water vapour (up to 80% of the gases emitted) Harmful effects (impacts on lives and property) Many are directly toxic and contribute to acid rain May also cause long term starvation and disease
What is the origin, characteristics and harmful effects of eruption columns?
Origin Explosive eruptions of silica-rich and gaseous magmas - gases decompress rapidly to produce up thrusting gases and tephra Characteristics May include a white cloud column from emission of steam, dark masses of pyroclastic material and clouds of fine ash. "Mushroom cloud" produced due to the perturbation of atmospheric temperature and pressure - moist air near the column is drawn up and condenses to form the "mushroom" Harmful effects (impacts on lives and property) Fallout may be destructive and widespread
What is the origin, characteristics and harmful effects of pyroclasts?
Origin All explosive eruptions Characteristics Hot broken fragments of rock ejected with great velocity Harmful effects (impacts on lives and property) Rocks may be very large and cause damage on impact
What is the origin, characteristics and harmful effects of tephra?
Origin All explosive eruptions Characteristics Tephra is all airborne or ground flowing pyroclasts including solidified magma (it is airborne pyroclastic material) Tephra is classified by its size and shape: Volcanic blocks (large angular fragments resulting from the shattering of solid lava during an eruption) and bombs (rounded because they form as molten lava cools while spinning through the air) - largest Lapilli - pebble-sized Cinders - 4-5mm in diameter Ash - less than 4mm diameter, smallest Harmful effects (impacts on lives and property) May be spread over distances of 1500km or more causing major and minor damage Fine ash particles can damage people's lungs Ash is heavy - can cause roofs of buildings to collapse Ash damages machinery, and electronics Lighter particles can damage aircraft engines
What is the origin, characteristics and harmful effects of landslides?
Origin Caused when magma forces its way to the surface and pushes outwards, causing the rock to break/over-steepen and collapse by gravity down the volcano Also triggered by earthquakes beneath the volcano Characteristics May form huge flows of rocks, mud and tephra Can be wet or dry (drier than lahars) They can transform into lahars (if water is added) They originate as large slabs of rock but disintegrate into smaller particles as they move downslope Landslides that do not stop moving but change gradually into a lahar are the most dangerous They can block tributary valleys - creates a natural dam - can cause lahars and flooding Harmful effects (impacts on lives and property) Destruction of property and land
What is the origin, characteristics and harmful effects of pyroclastic flows?
Origin Explosive eruptions May also be caused by collapse of eruption column A long period of inactivity and highly viscous magma causes a solid plug to grow in the vent of a volcano that highly-gas-charged lava explores sideways out of a weakness in the flank of the volcano. Characteristics Hot (up to 700C), and often gas-charged, high velocity flow of tephra Composed of a mixture of bombs, lapilli, ash and extremely hot gases The resultant deposit, high in pumice, is an ignimbrite High density, fast speed of travel, long distances covered and intense heat Density of materials is lowest at the tip of the cloud and greatest in its base flow Harmful effects (impacts on lives and property) May extend many kilometres from source and travel at high velocity (average 300kmh-1) May represent a lethal mix of bombs, lapilli, ash and hot gases Fine ash particles can damage people's lungs Ash is heavy - can cause buildings to collapse Knocks down trees Heat sets buildings, forests and crops on fire People and animals die from breathing in the hot gases Nuees ardentes are a type of pyroclastic flow - they are characterised by their ashes that glow in the dark
What is the origin, characteristics and harmful effects of lahars?
Origin Rain or meltwater may loosen tephra Form when water mixes with loose pyroclastic material The source of the water can be: - the intense rainfall accompanying the eruption - glacier melt - snow melt - dam failure Characteristics Volcanic mud-flows which may move downhill very rapidly, as determined by topography Combination of heavy rain and unstable ash increases the hazard of lahars They are dense an viscous but flow fast - the more volcanic material they contain, the faster they flow and the more destructive they are Most common on the steep slopes of stratovolcanoes where the downward pull of gravity is strong Increase in size as they gain more water and erode more material to carry Die out as they move away from the volcano Harmful effects (impacts on lives and property) Extensive destruction to property Loss of life
What is the origin, characteristics and harmful effects of lateral blasts?
Origin Rapid decompression of dissolved gases due to exposure of a mass of magma by a landslide Characteristics Sideways and sudden release of pulverised rock and hot gases Harmful effects (impacts on lives and property) May travel at speed of sound Lethal within blast zone
What is the origin, characteristics and harmful effects of flooding?
Origin Submarine eruptions displacing large volumes of rock and hence water Blockage of rivers by lahars or lava flows Characteristics Inundation (overflow) of fresh or salt water May be gradual or rapid Harmful effects (impacts on lives and property) Changes in erosion and deposition pattern of rivers Destruction of property and agricultural land
What is the global distribution of tsunamis?
Pacific Ocean - they are associated with subduction zones - most subduction zones are found in the Pacific Ocean.
What are the different types of volcanic eruption?
Plinian Vulcanian Pelean Strombolian Hawaiian Icelandic Vesuvian Hydrothermal eruption Phreatic eruption Phreatomagmatic eruption
What are the factors affecting vulnerability?
Population characteristics - density - growth rate - migration - health - structure of population Buildings - style of construction - building codes - urbanisation Public education - awareness of hazards Preparedness - effectiveness of lines of communication - the existence of early warning systems - availability and readiness of emergency personnel Socio-economic status - poverty - level of employment - gender Cultural factors
How are volcanic hazards managed?
Predicting and monitoring volcanic eruptions Action taken during eruptions as a result of risk perception
How can tsunami hazards be managed/how can we respond to tsunamis?
Prediction Hazard mapping Monitoring When a seismic event is detected that could cause a tsunami, a tsunami watch starts. Sea level height detectors confirm if one has been generated. Example - Hawaii - An International Tsunami Early Warning System, operated by NOAA in Hawaii, covers the Pacific Ocean - seismograph stations and tidal stations spread across the ocean send automatic warnings to Hawaii as soon as the water reaches a certain height - satellites monitor the tsunami as it crosses the ocean - communications systems at threatened coasts, warned by the centre in Hawaii, make residents aware of the near danger by various methods e.g. text messages, and evacuation procedures are put into place.
What are the impacts of earthquakes?
Primary hazards - ground shaking - surface faulting (surface displacement, along fault lines - may lead to the fracture of gas pipes, cause damage to lines of communication) - cost of repairing fractures is considerable Secondary hazards - secondary hazards result from the shaking of the ground - tsunamis - soil liquefaction and ground failure - mass movements (landslides and rockfalls, debris flow and mudflow) - fire - general effects on people and the environment The types of impacts depend on where the earthquake is located - whether is it: - urban or rural - densely or sparsely populated - highly developed or underdeveloped - the ability of the infrastructure to withstand shaking
What are primary and secondary impacts resulting from an earthquake?
Primary impacts - death, injuries, panic and shock caused by collapsing bridges and buildings - roads are destroyed as vertical movements cause fault scraps to develop or the group is offset horizontally Secondary impacts - soil liquefaction - caused by shaking - leads to buildings sinking - homelessness - lack of shelter - loss of livelihoods - people living in temporary tented accommodation or sleeping outside in areas with cold winders or very cold nights can die from hypothermia - water becomes contaminated with sewage - leads to disease - fires and lack of power are caused by the destruction of electricity cables and gas pipes - water shortages and thirst result from broken water mains - breakdown in telephone and internet communications - mudflows - landslides - tsunamis - famine theft and other crime
What are primary and secondary impacts resulting from a volcanic eruption?
Primary impacts - lava flows - ash - tephra falls - pyroclastic flows - release of gases and nuee ardentes Secondary impacts - lahars - landslides - tsunamis from undersea eruptions - crop damage and famine - loss of homes and livelihoods - disruption to flights where ash clouds occur - long-term cooling of the atmosphere
What are the primary/direct hazards associated with volcanic activity?
Pyroclastic flows Volcanic bombs (projectiles) Lava flows Ash fallout Volcanic gases Earthquakes Tephra (including ash, lapilli, blocks and bombs) Eruption Columns Lateral blasts
What are Rayleigh waves?
Radiate from the epicentre in complicated low frequency rolling motions (up and down).
How are volcanic eruptions predicted and monitored?
Remote sensing by satellite sensors detects deformation of the ground surface and temperature changes caused by heating in the ground before an eruption. Gravity measures on the ground detect gravitational changes due to magma movement. Geochemical changes suggest an eruption. Thermal waters e.g. hot springs on volcanoes have increased levels of CO2, H2 and He dissolved in them before an eruption. Chemical sensors measure increased sulphur levels.
What are transverse S waves?
Secondary/shear waves Slower and have a sideways motion. Travel with a side-to-side motion. Able to pass through solids (rocks) but not liquids and gases (they have no rigidity to support sideways motions).
What are tectonic hazards?
Seismic activity (earthquakes) Volcanoes Tsunamis
How do P and S waves travel after an earthquake?
Shock waves are reflected or refracted when they meet rock with different densities. If the shock waves pass through denser rocks, they speed up. If they pass through less dense rocks, they slow down. When P and S waves reach the surface, they become surface waves.
What are lava domes?
Small domes with steep sides and rounded tops. They can form on the slopes of, or in the craters of, stratovolcanoes. They are composed of silicic lava which is too viscous to flow far, so it solidifies quickly, forming a thick crust. Domes swell as they grow from within. Very explosive eruptions result when they collapse.
What are the factors affecting earthquake damage?
Strength and depth of earthquake and number of aftershocks - the stronger the earthquake, the more damage it can do - the more aftershocks there are, the greater the damage that is done - earthquakes that occur close to the surface (shallow-focus earthquakes) do more damage than earthquakes deep underground (deep-focus earthquakes) as more of the energy of the latter is absorbed by overlying rocks Population density - an earthquake that hits an area of high population density could inflict more damage than one that hits an area of low population and building density The type of buildings - HICs have better- quality buildings, more emergency services and the funds to recover from disasters - people in HICs are more likely to have insurance cover than those in LICs The time of day - an earthquake during a busy time, e.g. rush hour, may cause more deaths than one at a quiet time - industrial and commercial areas have fewer people in them on Sundays - homes have more people in them at night The distance from the centre (epicentre) of the earthquake - the closer a place is to the centre (epicentre) of the earthquake, the greater the damage that is done The type of rocks and sediments - loose materials may act like liquid when shaken (liquefaction) - solid rock is safer - buildings should be built on fat areas formed of solid rock Secondary hazards An earthquake may cause: - mudslides - tsunamis (high sea waves) - fires - contaminated water - disease - hunger - hypothermia Economic development - this affects the level of preparedness and effectiveness of emergency response services, access to technology and quality of health services
What are the four general ways in which we can respond to hazards?
The choice of strategy will vary during different phases of the hazard. Modify the cause Modify the event Modify vulnerability Modify the loss PPP - prediction - protection - preparedness The choice of response depends on a complex and interlinked range of human and physical factors. As people and organisations have limited resources and time to make decisions, the relative importance of the physical risk from natural hazards, compared with other priorities such as providing jobs, education, health services and defence, will be a major factor in influencing how much resource is devoted to reducing hazard impacts.
What is soil liquefaction?
The conversion of unconsolidated sediments into materials that act like liquids. Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. The soil body of sediment is transformed into a liquid mass that can flow. Liquefaction can lead to the collapse of buildings and other structures that might be otherwise undamaged. Occurs during ground failure as a result of earthquakes. Liquification occurs - When water-saturated sediments are shaken, the grains become rearranged to the point where they are no longer supporting one another. Loose saturated sand tries to become denser by reconfiguring particles more tightly, but during this quick transition, there is not enough time for water to move out. Instead, the water between the grains is holding them apart and the material can flow. This causes rapid increase in pore pressures which triggers a large decrease in effective stress. The soil loses most of its shearing resistance and behaves like a liquid as it loses strength and stiffness
Why do people live near tectonic hazards?
The hazard may not have been predicted. People may be unaware of the danger if the volcano is dormant or there has been no large earthquake in recent time. They are reluctant to leave their place of birth, family or friends. Their perception of risk is that it will happen to others but not to them, so they do not accept that they need to take action to reduce risk. People interviewed after a hazard comment that it is something that happens elsewhere but was not expected to happen to them. Some take a cynical attitude that the hazard cannot be controlled, so it has to be accepted. Economic reasons, as people are reluctant to leave their jobs: - sulphur is mined from active volcanoes - basic lavas weather to fertile soils suitable for intensive agriculture - geothermal energy in volcanic areas leads to industrial developments and cheap heating for homes and for crops grown in hothouses - volcanic landscape attracts tourism - encourages population growth - people believe that the benefits from living in a hazardous area will be greater than the costs of doing so
What are the range of scales that response to hazards can occur?
The individual within the local community Regional level National level International level Global scale e.g. climate change
What are vesuvian eruptions?
The viscous magma gains a high gas content during long periods of inactivity and a deep plug forms, which can only be blown off after large amounts of pressure has built up. They send a wide, dark ash cloud into the stratosphere. Ash fall over a wide area Examples The AD 79 eruption of Vesuvius buried the town of Pompeii under 25m of tephra.
What are vulcanian eruptions?
These are more violent because the more viscous lava solidifies more quickly and traps gases. Ashes and cinders are emitted when pressure is released. Dark, ash laden cloud rise to form a cauliflower shape. Ash columns can be up to 20 km in height, and lava blocks and bombs may be ejected from the vent. Examples - Sakurajima in Japan - Krakatau, Indonesia
What are stombolian eruptions?
These are the least violent types of explosive eruptions. Have explosions causing a shower of lava fragments. Less fluid lava and gases escape with moderate explosions in which lava bombs are ejected. Examples - Stromboli is known as 'The lighthouse of the Mediterranean' because of its very frequent activity - Stromboli volcano, Eolian Islands, Italy
What are hawaiian eruptions?
These are the least violent types of explosive eruptions. Have fire fountains and lava flows. Fluid basalts issue from vents in volcanoes. Gases escape easily and quietly with occasional spurts of gases causing lava fountains. Examples - Mauna Ulu, a vent of Kilauea Volcano in Hawaii
What is an ash-cinder volcano?
Throws out- besides lava- much ash into the air. Through this the volcanic cone is built up from alternate layers of ash and cinder. Cinder cones form when blobs of gas-charged lava are thrown into the air and break into fragments.
What are mass movements?
Various soil compaction techniques which densify soil and enable buildings to avoid soil liquefaction: 1. vibro compaction (compaction of the soil by depth vibrators) It densifies clean, cohesionless granular soils with a downhole vibrator. The energy from the vibration reduces the inter-granular forces between the soil particles, making them denser as the vibrator is raised in lifts. 2. Dynamic compaction - Dynamic compaction is a ground improvement technique that densifies soils and fill materials by using a drop weight. The ground is subjected to repeated surface tamping using a heavy steel and concrete weight. 3. vibro stone columns - This technique involves the improvement of weak soils by the installation of densely compacted columns made from gravel or similar material with a vibrator. The displacement process reinforces all soils in the treatment zone and densifies surrounding granular soils. 4. Existing buildings can be mitigated by injecting grout into the soil to stabilize the layer of soil that is subject to liquefaction.
What are Plinian eruptions?
Very violent eruptions of gas, ash and pumice results in a narrower cloud that extends into the stratosphere. The high ash cloud from Mt Pinatubo's 1991 eruption circulated the world several times in the stratosphere causing temporary global cooling of about 0.5C. Plinian eruptions have 20 to 35 km tall columns which may collapse to form pyroclastic density currents (PDC's). Examples - Mt St Helens 1980 - Mt Vesuvius AD79
What are the differences between volcanoes at subduction zones and mid-ocean ridges or hotspots?
Volcanoes at subduction zones: - produce more viscous lava - erupt explosively - produce much ash Volcanoes found at mid-ocean ridges or hotspots: - produce fluid basaltic lava The style of eruption is influenced by the processes operating at different plate boundaries which produce magma of different composition.
What are dormant volcanoes?
Volcanoes that are inactive but may become active again in the future. Volcanoes that have not erupted for a long time but could still erupt.
What are extinct volcanoes?
Volcanoes that will not erupt again. Volcanoes that have stopped erupting.
What are seismic waves?
Waves that transmit the energy released by an earthquake.
What is partial melting?
When some minerals melt before others. This alters the composition of molten rock produced. It produces basalt. At subduction zones, the older and deeper slabs experience greater partial melting - produces a silica-rich magma.
What is soil liquefaction?
When the shaking of the ground makes weak or unconsolidated rocks act as a liquid and flow - leads to the sinking and destruction of buildings. Occurs when groundwater is near the surface and soft sediment like sand mixes with water.
What are the four types of seismic zones?
1. Destructive (convergent) plate margins have shallow, intermediate and deep earthquakes inclined along the Benioff zone on the subducting plate where slab pull occurs. 2. Collision boundaries of two converging continental place. 3. Conservative plate margins where faults have earthquakes but no volcanic activity. Powerful earthquakes from a shallow focus. 4. Constructive plate margins at mid-ocean ridges where ridge push (slab push) occurs. The lithosphere is too weak and thin for a lot of stress to build up, so large earthquakes do not occur. Shallow focus earthquakes and occur with volcanic activity.
What is the continental fracture zone?
A belt of activity following the mountain ranges from Spain, via the Alps, to the Middle East, the Himalayas to the East Indies and then circumscribing the Pacific.
What is the Ocean fracture zone (OFZ)?
A belt of activity through the oceans along the mid-ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea rift and California.
What is a shield volcano?
A broad, shallow volcanic cone, which arises because the running lava, which is fluid and hot, cools slowly. Formed from basic lava. Lava flows long distances and solidifies slowly. Occur when there is a central vent. Large volcanoes with wide bases and gentle slopes. Form mainly along constructive plate margins
What is a fault?
A fracture in the rocks that makes up the earth's crust.
What is a disaster?
A hazard that causes so much damage and injury that recovery without help is impossible. It is a major hazard event that causes widespread disruption to a community or region, with significant demographic, economic and/or environmental losses.
What is a tsunami?
A high, long-period wave in the ocean. Result from a sudden displacement of the sea bed along a fault. In the open ocean tsunami wave crests are small and wave lengths long - makes them difficult to monitor. When the wave crests reach shallow water at the coast, they slow and rise in height. The waves then retreat, pulling water back out to sea, before returning to hit the coast with force. Coastal areas near earthquake epicentres have little or no warning because a tsunami travels rapidly.
What is an earthquake?
A sudden and violent movement or fracture within the earth followed by the series of shocks resulting from this fracture. A series of vibrations or seismic (shock) waves that originate from the focus.
What is a hazard?
A threat (natural or human) that has the potential to cause: - loss of life - injury - property damage - socio-economic disruption - environmental degradation An earthquake in an unpopulated area is a physical event - it only becomes a hazard when people put themselves in danger by moving into the area. Natural hazards only occur where humans interact with the physical world and their severity and frequency depends on human activity.
What is an aftershock?
Aftershock - an earthquake that occurs after a previous earthquake (the main shock). An aftershock occurs in the same region of the main shock but is always of smaller magnitude strength. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are smaller earthquakes formed as the crust around the displaced fault plane adjusts to the effects of the main shock. Aftershocks can be devastating - they damage buildings that have already been damaged by the first main shock. They can cause more damage than the main earthquake as they shake the already weakened structures - buildings partly damaged during the earthquake may be completely destroyed by the aftershocks. They are more soft but longer lasting and more frequent than the main tremor.
What is a foreshock?
An earthquake that occurs before the main shock.
What are phreatic eruptions?
An eruption driven by the heat from magma interacting with water. The water can be from groundwater, hydrothermal systems, surface runoff, a lake or the sea. They pulverise surrounding rocks and can produce ash, but do not include new magma. Examples - Biscuit Basin, Yellowstone USA
What are hydrothermal eruptions?
An eruption driven by the heat in a hydrothermal systems. They pulverise (crush) surrounding rocks and can produce ash, but do not include magma. Very small eruptions. Examples - Rotorua- New Zealand, Yellowstone USA
What are phreatomagmatic eruptions?
An eruption resulting from the interaction of new magma or lava with water and can be very explosive. The water can be from groundwater, hydrothermal systems, surface runoff, a lake or the sea. Examples - Mount Etna 2001
What is a caldera volcano?
An older volcano with a larger crater which can be 62 miles (100km) wide, In this crater many littler new crater are formed.
What are the secondary/indirect hazards associated with volcanic activity?
Atmospheric ash fallout Landslides Tsunamis Acid rainfall Lahars Flooding Atmospheric effects/climate change Famine?
What is ground shaking?
Buildings can be destroyed by: - shaking - the ground tilting or subsiding as the shock wave passes
What is a composite (stratovolcano)?
Built up from alternative layers of lava and ash but, besides its main crater, it has many little craters on its slope. Dangerous - have long dormant periods. During dormancy a think plug of solidified magma builds up in the vent and, as the magma has a high viscosity, a large amount of pressure has to build up to unblock the vent - leads to an explosion which shatters the plug into pyroclastic fragments and outpourings of lava.
How is the epicentre of an earthquake figured out?
By triangulation Able to get a general idea of where the epicentre is based on looking at the severity of damage, eyewitness reports etc. To calculate the epicentre a minimum of three seismic stations are needed - at these locations there are seismometers actively monitoring the shaking in the Earth's crust. Data is taken from the seismograms from each of these stations and their epicentre distances are determined (finding how far point 1, 2 and 3 are from the earthquake). This is calculated by finding the difference in arrival times for the p-waves and s-waves. Then a travel time graph is used to calculate the distances. Because we don't know the direction of the earthquake (epicentre), a radius is draw around point 1, 2 and 3 - the epicentre is where the three circles intercept.
What are icelandic eruptions?
Characterized by effusions of molten basaltic lava that flow from long, parallel fissures (e.g. a mid-ocean ridges). Such outpourings often build lava plateaus. Examples - 2014 eruption of Bárðarbunga volcano in Iceland - 2011 Kamoamoa eruption, Kīlauea, Hawai'i.
