Y1,S1 - Natural Hazards Flashcards
What is magma generated by? (HINT- 3 factors)
1.Increased temperature
2.Decreased Pressure
3.Adding Volatile elements
What is the upper mantle made of?
Peridotite
What happens to the temperature of the mantle as its depth increases?
Temeprature increases
What happens when the pressure or temperature of peridotite (upper mantle) is altered, causing it to pass the geotherm?
Lithospheric mantle is created
Where is water trapped in the upper oceanic crust?
Crystals
What happens to temeprature and pressure as depth of the mantle increase?
They both increase
Why does subducting oceanic crust alter the liquedous conditions of the upper mantle?
Water is released as crystals, it was previously stored in,melt
What does increasing the liquedous conditions of magma result in?
Magma ascension (volcano formation) over long time scales (millions of years)
In most places does magma rise to the surface due to an increase in temperature or decrease in pressure, as it reaches the geotherm?
Decrease in pressure
Give an example of where magma is reaching the geotherm due to a decompression?
East African rift valley
What causes hotspot volcanism?
Upwelling of magma (mantle plume) from the mantle-outercore boundary, despite being a solid it moves up towards the surface over tens/millions of years, driven by byouyancy
what is magma?
Multiphase mixture of melt (liquids), crystals (solids) and gas (bubbles)
Why do volatiles in magma drive the eruption?
Because they from bubbles which accelerate the magma upward
What is magma mixing?
Hot magma replenishment which disrupts the chamber
Why deoes magama mixing cause eruptions?
The increased temperature of magma replenishment mixes with the cool magma sitting in the reservoir, destablising the chamber. Pressure and volume changes as crystals and vesicles mix, causeing acceleration.
Ultramafic lava contains…
Peridotite
Low SiO2 content
As crystal content in lava increases, what else increases (3 factors) ?
Silica content
Na and K content
As silica content increases does temperature increase or decrease?
decrease
Name the 4 different magma compositions from highest SiO2 content to lowest…
1.Felsic
2.Intermediate
3.Mafic
4.Ultramafic
What main factor (mineral) changes the viscosity of magma?
Silica content
Name the 5 types of magma in order from highest to lowest viscosity?
1.Rhyolite
2.Andecite
3.Dacite
4.Basalt
5.Komatite
Describe basaltic volcanoes?
-Typical temp. 1000-1200C
-SiO2 45-52 wt% [mafic]
-High conc. of Fe, Mg, Ca
-Low conc. of Na, K, Si & H2O
-Typically contains olivine, plagioclase (Ca-rich) & pyroxene
-Low viscocity (flows easily)
-E.g. Kileaua , Mt Etna & Bardabunga
Decribe Andesitic volcanoes?
-Typical temp. 800-900C
-SiO2 52-63 wt% [Iintermediate]
-lower conc. of Fe, Mg, Ca than basalt
-higher conc. of Na, K, Si & H2O than basalt
-Typically contains amphibole, plagioclase & pyroxene
-E.g. Tungurahua, Fuego & Soufiere hills
Describe dacitic volcanoes?
-Typical temp. 800-900C
-SiO2 63-70 wt% [Iintermediate]
-Often crystal rich
-Viscuous - eruptions can be highly explosive
-Typically contains amphibole, plagioclase (Na-rich), mica & pyroxene
-E.g. Mount St Helens & Pinatubo
Describe rhyolitic volcanoes?
-Typical temp. 600-800C
-SiO2 more than 70 wt% [Iintermediate]
-low conc. of Fe, Mg, Ca
-high conc. of Na, K, Si & H2O
-Viscuous - often highly explosive
-Typically contains amphibole, plagioclase (Na-rich), mica & quartz
-E.g. Yellowstone, Chaiten & Nemrut Dagh
What are polygenetic volcanoes?
-Magma erupted repeated from same plumbing system
-E.g Stratovolcanoes, caldera volcanoes, sheild volcanoes
What are monogenetic volcanoes?
-Magma plumbing sytem only used once
-E.g Lava shields, Volcanic fissures, Scoria cones
What are stratovolcanoes characterised by?
-Steep conical shape
-Small summit crater
-Layers of lava and tephra
What are sheild volcanoes characterised by?
-Low profile
-Summit crater, rift zones
-Layers of fluid lava flows
What are caldera volcanoes characterised by?
-Large cauldron-like crater
-Formed by evacuation of large magma chamber
List the eruption classifications following the general trend of increasing intensity, explosivity and duration…
1.Hawaiin
2.Strombolian
3.Vulcanian
4.Plinian
What are some generic direct volcanic hazards?
Generic:
-Caldera and sector collapse
-Volcanic gases
-Earthquakes
What are some effusive direct volcanic hazards?
Effusive:
-Lava fountains and flows
What are some explosive direct volcanic hazards?
Explosive:
-Tephra fall
-Pyroclastic density currents
-Ballistic projectiles
What are some indirect volcanic hazards?
-Lahars, mudflows, debris flows
-Debris avalanches
-Landslides
-Acid rain and pollution (SO2 ->H2SO4)
Tsunamis
What affects the speed of lava at effusive eruptions?
1.Type of lava erupted and its viscocity
2.Steepness of the ground over which it travels
3.Whether the lava flows as a broad sheet, through a confined channel or down a lava tube
4.Rate of lava production at the vent
Describe and list affects of low viscocity lava flows at effusive eruptions:
-Can be sustained for days-weeks or even longer
-Mostly a threat to infrastructure
-Can dam rivers
-Long recovery period
List 3 examples of effusive eruptions:
-Hawaii (low viscosity lava)
-Mt Etna (high viscosity lava)
-Soufiere hills (high viscocity lava)
Describe and list affects of high viscosity lava flows at effusive eruptions:
-High viscociyy lavas extrude more slowly, but can be highly charged with volatiles
Lava domes:
-Advance slowly
-Shorter flows
-Eruptions continue for months to decades
-Long recovery periods
Describe and list affects of explosive eruptions:
-Range of eruption types up to Pinion
-Sustained for hours to days
-Plumes up to ~10-50km high
-Ejecta may extend hundreds of km from vent (fallout)
-Generates tephra fall and pyroclastic density currents
-Can lead to many indirect hazards
What are pyroclasts?
-Volcanic rock fragments produced by explosive eruptions
-collectively known as pyroclastic deposits or tephra
What is the general size of ash (type of tephra) ?
<2mm
What is the general size of lapilli (type of tephra) ?
2-64mm
What is the general size of blocks and bombs (type of tephra) ?
> 64mm
What are accesssory lithics?
Wall rock (accidental) fragements that are picked up from the magma, hence foreign particles that travel woth the volcanic ash
What are juvenile particles?
Fragmenst/particles that are cooled from the magma itself
Describe tephra fall and its affects?
-Particles decrease in size as distance from the vent increases, hence quite well organised
-Thicker deposits closer to the vent (but sometimes a secondary thickening downwind)
-Blankets the landscape beneath the eruptions plume, continuous layer
Describe the hazards caused by tephra fall (proximal to medial) :
-10-30cm required to collapse on buildings (less when went due to rainfall)
-Depends on; construction type, roof span, pitch angle, water content
-Roof collapse
-Respiratory problems
-Driving accidents
-Smother or poison vegetation and animals (e.g. flurosis in cattle)
Describe the hazards caused by tephra fall (distal) :
Distal ash in the atmosphere has profound impacts for the aviation industry:
-Closes airports
-Closes airspace
-Causes mechanical damage
-Ash is highly abrasive and corrosive
How do isopach maps, map tephra fall deposits?
Based on measurments of deposit thickness over a wide area (contours of equal thickness)
How do isopleth maps, map tephra fall deposits?
Based on the measurement ofthe maximum average (lithic or pumice) clasts at any location (contours of equla maximum clast size)
What are pyroclastic density currents (pdcs)?
Mixtures of hot particles (ash to boulder size) and gas that moved down the flanks at high speeds as density currents
What are the characteristics of pdcs?
-Temperatures up to several hundred degrees C
-Velocity of a few tens of m/s, up to 300m/s
-Typically travel several metres from vent but can travel >100km
-Also referered to as pyroclastic flows (concntrated mixture) or pyroclastic surges (dilute mixtures)
Describe how pdcs form from the collapse of an eruption column and its affects:
-Unstable eruption column (due to insufficient entrainment air)
-Collapse of material
-Condenses on the ground -> density current
-Can produce radial flows
-Contains mostly pumice and ash
Describe how pdcs form from the collapse of a lava dome and its affects:
-Grvitational instability of solid lava dome (due to oversteepening of planks and gap P)
-Collapse and disintegration
-Mixture expands -> density currents
-Produces flows in direction of collapse
-Contains mostly lava blocks and ash
Describe the nature of column collapse pdcs:
-Generally formed during short-lived eruptions
-can be highly mobile (increased height that the flows initiate from)
-Can be roduces radially around vent
Describe the nature of dome collapsed pdcs:
-Repeated inundation of flanks by pdcs during long-lived eruption
-Up to 30km runouts
-Recover period depends on deposit thickness and climate
What are lahars?
A hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and/or river valleys
Describe the characteristics of lahars:
-Vary in size and speed, from a few m/s up to several tens of m/s
-Reach rdistal locations, often far removed from volcanoes
how are lahars formed?
Mix loose volcanic debris with water on a slope by:
-collapse of water-saturated or ice-covered volcano
-Rapid melting of snow and ice during eruptions
-Eruption through crater lake
-Heavy rainfall on fresh pyroclastic deposits
-Overtopping/failure of impounding lakes
How have we learnt to cope with volcanic eruptions?
-Enhanced use of tehnology
-Social media as a critical tool
-Nationwide coordination
-Croos-disciplinary collaboration
-Psychological readiness
-Re-evaluation of hazard zones
-Flexible monitoring strategies
How are short-term hazards monitored?
Monitoring data and activity observations
How are long-term hazards monitored?
Based on field records, datesand occurences over long-time periods
What is shot-term hazard monitoring used for?
-Evacuation orders
-Disaster mitigation
-Crisis Management
What is long-term hazard monitoring used for?
Land use planning
What are geological volcanic hazard maps used for?
Presenting hazards based on their past occurrence
What are integrated qualitative volcanic hazard maps used for?
For simplified communication
What are modelling-based hazard maps?
Maps based on the study of a single hazrd using empirical relationships and/or modelling tools employed either deterministically or probabilistically.
What are possible uncertainties associated with modelling-based hazard maps?
-Digital elevation models
-Input parameters
What are geophysical signals at volcanoes?
-Regional tectonics
-Pathway and recording effects
-Moving fluids
-Movig magma
-Shallow sources
-Edifice instability
-Deep sources of magma
What are different types of volcano monitoring?
-Geophysical/deformation monitoring (using tiltmeter and GPS)
-Volcano seismology (using EQ and lahar sensors)
-Acoustic monitoring (Infrasounds which travel thousnads of km can detect volcanic explosions very well, that humans cannot)
-Geodesy (tool used to measure ground deformation and gravity changes cause by magama movement)
-Thermal remote sensing (detects heating up of the surface, which captures surface radiance of the ground prior to an eruption)
-Lightning detection
-Physical volcanology (elemants and minerals found determine stage and type of eruption)
What are the tectonics of South America?
-Nazca plate subducting eastwards beneath continental South American plate
-Convergence causes uplift of the Andes
-Subduction causes earthquakes and volcanism
What are the tectonics of Equador (Tungurahua)?
-Nazca plate moving Eastward at ~56mm/yr
-Active subduction off west coast of Ecuador
-Intraplate EQs resulting from movement of continental ‘sliers’
At what zone is Tungurahua?
Subduction zone
What type of Volcano is Tungurahua?
Andesitic Volcano
What are some of the hazards created by Tungurhua?
-Pyroclastic density currents
-Tephra fall
-Lahars
How was Tungurhua monitored?
-Geophyiscial monitoring networks
-Local observers
What forces are involved in causinf slope instabilities?
Driving forces and ressiting forces
What is the driving force in slope instabilities?
Gravity
What are the resisting forces involved in slope instabilities?
-Strength
-Friction
-Cohesion
What other factors are invoilved in slope instabilities?
-Excess loading
-Water( increases loading, reduces cohesion)
Describe soil creep:
-Moves slowly downslope, <mm/yr
-Most important in terms of volumes involved
-Creep is enhanced by steep slopes, water in soil, tree fall and bioturbation
What are landslides favoured by?
-Rainfall ( pore pressure reduces frictional resistance to sliding)
-Disappearance of vegetation ( roots provides resistance to slope failure
-Ground shaking (EQs)
What does deep seated landslides involve?
-Soil
-Bedrock
-Curved failure surface
-Large volume
-Usually after extended periods of rain
What do shallow landslides involve?
-Soil ONLY
-Subplanar failure surface
-Small volume
-Usually after intense rain
-Can be induced by creep + freeze/thaw
-No loss of life BUT damage to structures
What is a landslide?
Material moving in contact with underlaying surface as cohesive blocks of material with limited internal shearing
What are debris flows/ mud flows?
-When material breaks up and moves as a viscuous fluid
-Material is transported mostly by gravity, not water
What is the difference between dry rock avalanches and debris/mud flows?
Dry rock avalanches do not involve water whereas the others do
What is required for a debris/mud flow?
-Mass transport of poorly compacted material (e.g.soil)
-High pore water content
-Large quantities of fine material (to prevent water escape)
Can landslides turn into debris/mud flows?
YES and can transport very large particles (fast-moving dense flurry)
What makes a mudflow?
-High clay-silt content
-Common in volcanic terrain (lahars), can happen long after eruption
What are the direct natural causes of mass movement?
1.Saturation of water (pore pressure;lubrication) - Change in groundwater table, heay rains and rain following drought)
2.Over steepening of slopes - Erosion by streams, waves and glaciers
3.Freeze and thaw cycles -Over time creep destabilises slopes resulting in sudden failure
-frost shattering on rocky slopes
4.Vegetation -Loss of vegetation by wildfires, wind erosion, drought destabilises slopes
-Plants with shallow roots and large foliage can be destabilised by rain
What are indirect causes of mass movement?
-Initiated by other hazard events- EQs, volcanic eruptions
-Initiated by human activity (e.g. deforestation, rainfall caused by climate change)
Is the energy recievd by Earth evenly distributed?
NO:
-More solar energy arrives at the equator than poles
-As solar radiation arrives directly at the equator, whereas it arrives on a slant at the poles
-This generates an Equator-pole tempearture gradient
What is the Equator-Pole temperature gradient?
-Heat flows from hot towards cold
-This transfer of energy from Equator to Pole generates weather
What drives the diurnal cycle?
Rotation of the Earth
What drives the seasonal cycle?
Tilt of rotation axis
Describe the single-cell model:
-Convection cell
-Hot air (less dense) rises at the Equator, moves towards poles at high altitude, then cools and sinks at the Pole
-Circulation completed by cold air moving towards Equator near surface
-This is partly correct, but is too simplified
-This configuration of the global circulation is unstable
-Eartyhs rotation means the Coriolis force must be included
Describe the three-cell model:
-The coriolis force deflects flows to the right in the Northern hemisphere (and to the left in the SH)
-Hadley cell in the tropics and polar cells at high lattitudes have the same sense of circulation as single cell model
-The ferrel cell in mid-lattitudes in opposite sense
-Westerly (flow from W to E) jet streams ( at ~10km altitude) between cells
-Easterly surface winds in the tropics
-This configuration is more stable, but not completely and produces weather variability
Whhat is the Chaos theory and the Lorenz attractor?
-A set of physical equations that describe the weather
-These are non-linear and have sensitive dependence on the initial conditions
-They behave chaotically
-This is a physical explanaition for the variability of the weather (it is random but constrained within limits)
What is climate?
The long-term temperature mean
What is used in the Probablility density function?
Mean and SD
How else can you express the probablity distrubution of temperature?
the Return period
What do climate extremes generate?
Hazards- even for an unchanging climate
When does climate NOT change?
Under ‘Normal’ conditions
What is the effect of the (fast) changing climate?
-Changing PDFs and hence hazard frequencies/magnitudes
-This is fundamentally different to seismic and volcanic hazards
-Exposure is also increasing as global population rises
Whst is a heatwave?
Prolonged period of excessive heat
What does prolonged mean?
Typically at least 3 consecutive days in duration
What are the two types of excessive heat?
-Above a specific temperature threshold (e.g. 30C)
OR
-Above a percentile based on climatology for that location and time of year
Why is the met office no longer using the older climatology to establish thresholds?
-As heatwaves would occur more frequently
-We are adapting to cope with higher temperatures
-Hence dont need a temperature warning so often
Where do heatwaves tend to occur?
In mid-lattitudes (30-60 degrees N/S)
What type of pressure is usually associated with heatwaves?
High surface pressure (Anticyclones)
What type of weather is often brought by anticyclones?
Sunny and settled weather
What is atmospheric pressure due to?
The weight of the overlying air
What is the typical surface pressure?
100000Pa
Why does the mass of a column of atmsophere vary from play to place?
Due to variations in air density, due to:
-The air being warmer/colder
-How much moisture it contains (moist air is less dnese than dry air)
-Cold/dry column ->High surface pressure
-Hot/wet column -> Low surfce pressure
What creates winds?
Horizontal pressure gradients
What do closer isobars mean?
-Stronger pressure gradient
-Stronger wind required for Coriolis force to balance pressure gradient
What happens to wind when there is a balnce between pressure gradient, Coriolis and frictional forces?
-Wind is reduced and flows across the isobars towards low pressure
-so aire tends to spiral away from areas of H pressure to areas of L pressure
How does low pressure affect vertical air motions?
Leads to convergence of air and hence ascent
How does high pressure affect vertical air motions?
Leads to divergence of air and hence descent
Compare vertical and horizontal velocities:
Vertical velocities (~cm/s) generally low compared with horizontal wind speeds (~m/s)
How does temperature change with air height?
-Tempareture decreases with height
-As air cools as it rises
How are anticyclones associated with heatwaves?
-An AC has air spiralling away from it at the surface and this creates decent
-Descending air gets warmer
-Assuming total water content is conserved, this means the air gets less saturated, so any clouds (liquid H2O) will evaporate (to water vapour)
-So anticyclones tend to be cloud-free and sunny
-In the summer they generate hot and sunny weather
-AC are quite stable and slow-moving, so they can persist for several days and lead to heatwaves
-weel spaced isobars-low winds-can accumulate air pollution
Describe the affect of air temperature on saturation vapour pressure:
-Warmer air can accomodate more water vapour molecules than colder air
-So when air rises and cool it cant hold as much water as vapour so it condenses and forms clouds and rain
What are the several measures of heatwaves?
Frequency,duration and Intensity
Where are the steepest temperature gradients?
-Mid-lattitudes
-This is equivalent to a weather ‘front’ ; a boundary between warm and cold air
-Small disturbances (‘waves’) alog this front grow into storms
Where does air pick its characteristics from?
The surface that it flows over
What air is met at mid-latitudes?
Cold air from the poles meets warm,moist air from the tropics
What are weather fronts?
Boundaries between air masses
What can develop into warm sector depressions?
Waves on the polar front
What are the stages of a life cycle depression (in the Northern hemisphere) ,using the Norweign model?
-Small low P develops-anticlockwise circulation
-Cold front is moving faster than the warm front
-Cloud/rain associated with frontal ascent
-Peak winds and tight isobars
-Low fills and storm dissipates
-Typical development and decay over a timescale of a few days
What is needed for a cyclone to develop?
Divergence of air above is needed for surface low pressure to deepen
What occurs when there is a divergence of air above?
When the jet stream bends
What is needed for storm development?
-Divergence downwind of an upper level trough
-So there is an alignment of surface and upper level pressure patterns
-Optimal conditions can lead to very rapid storm development
How do meteorologist try to forecast storm development?
Tracking the jet stream (upper level winds)
What do slower-moving satorm mean for duration?
Higher accumulated precipitation which can last for up to several days
How is Climate change affecting storm precipitation totals?
CC means that the atmosphere contains more water vapour, so higher rsinfall potential
What are the storm characteristics which have evidence of being changed by climate change?
-Intensity
-Size
-Speed of passage
Why do tropical storms not from from the Equator to +/_ N/S?
Lack of Coriolis force
At what temperature do TS form?
> 27C
Where do TS not form?
SE Pacific and S Atlantic as it is too cold
Where do most TS form?
W pacific as it is a warm pool
What makes an extra-tropical cyclone?
Wheb tropical cyclones travel into mid-lattitudes
What classes as a cat. 1 storm on the Saffir-Simpson scale?
P>980hPa
64-82 knotts
What classes as a cat.5 storm on the Saffir-Simpson scale?
P<920hPa
>135 knots
Describe the formation of a tropical storm:
-Ocean evaporation
-Air spirals inwards and convects upwards
-Water condenses to form clouds
-Latent heat is released
-Results in warming the core of the TS
-Lowers pressure
-Pulls more surface inwards (positive feedback loop)
Can a TS be self-sustaining?
Yes, it can be a self-sustaing heat engine as long as environmental conditions are favourable
What happens in the centre of a Tropical storm?
-Descent at centre tends to dissipate clouds (eye)
-Surface winds spiralling inwards angular momentum
-So highest winds NEAR the centre of the storm
-High pressure in the eye
What are scattomeres used for?
-Infer wind speed and direction from looking at sea roughness (from satellite data)
-They transmit a pulse of EM radiation and measure the returned backscatter, which varies with sea state.
What type of wind shear do Tropical storms need to from?
Low wind shear
At what side storm surges worse in the Northern hemisphere?
Right hand side
How does El Nino affect Hurricanes?
-Fewer Atlantica hurricanes
-More Pacific hurricanes
How does La Niña affect Hurricanes?
-More Atlantic hurricanes
-Less Pacific hurricanes
How does Saturation vapour pressure chaneg with temperature?
Saturation vapour temperature increases with temperature
How does climate change affect water content in the atmosphere?
Incraeses water in the atmosphere leading to more precipitation
What do the IPCC predict will increase with climate change regarding tropical cyclones?
-Increase in average peak TC wind speeds
-Proportion of cat.4-5 TCs will increase in limited region over the Western-North Pacific
-Average TC rain rates will increase
-The global frequency of TCs over all categories will decrease or remain unchanged
Can high winds mix Ocean?
Yes, high winds vertically mix ocean - and can leave a ‘cool trail’
When does runoff peak?
Some time after peak rainfall
What is lag-time determined by?
Intervening catchment hydrology:
-Forested hills (G)
-Poorly drained peatlands (G)
-Bare rock with thin soils (B)
-Urban areas (B)
What are sustainable urban drainage systems?
Aim to increase retention of water in ponds to increase transit time
What factors can be used in aiding flood prediction?
-Precipiation distribution and magnitude in time and space
-Transit time from where precipitation falls to the receptor location
What models are used to ensemble weather forecasts?
Coupling rainfall and hydrology models
Define drought:
-A prolonged absence oe marked deficiency of precipitation that results in water shortage
OR
-A period of abnormally dry weather sufficiently prolonged for the lack of precipitation to cause a serious hydrologicla imbalance
How is drought severity assessed?
-Standardised Precipitation Index (SPI) measures how proabable the observed level of precipitation is compared to long-term data (i.e ver low prob. -> rare precipitation deficit)
-The Palmer Drought Severity Index (PDSI) measures the soil moisture content
-Planners may consult one or more indices before making desicisions
Give an overview of the SPI:
-Based on the likelihood of precipitation (ONLY) for any time scale
-Used by many drought planners
-Can be computed for different time scales
-Can provide early warning of drought
-Assess drought severity
-Less complex than the PDSI
Give an overview of the PDSI:
-A soil moisture index that differs with region
-Based on the concept if water balance: water supply precipition), demand (eavpotransipiration) and loss (runoff)
-It is used by many U.S government agencies and states rely on the PDSI to trigger drought relief programs
-Pros: The fist comprehensive drought index developed in the U.S
-Cons: Palmer values may lag emerging droughts (meteorological) by several months; complex
What are droughts, heatwaves and high windspeed all relted to?
Wildfires
How can large fires generate their own storms?
-Air is sucked in
-Smoke plume rises and cools
-Water vapour condenses froming a cloud
-Can cause downburst of rain and lightning
-And can add smoke to the stratosphere (15+ Km altitude)
What is easily ignited by wildfires?
Dry vegetation
What is short-term waether prediction?
A forcasting problem
Describe wreather forecasting:
-The evoloution of weather is a chaotic system, so has sensetive dependence on initial conditions
-Some situations are more predictabel than others
What is ensemble forecatsing used for?
-Used to quantify forecast probabilities
-Probabilities guide forecasts of hazard warning
What does it mean if all the members look similar in ensemble forecasting?
The forecast is more predictable and hence more reliable
With global warming, what warms faster, the Polar regios or the tropics?
The polar regions warm faster than the tropics
What are the impacxts of global warming on global circulation?
-Not yet fully understood
-Predicted that the Tropics (Hadely cells) will widen
-Energy drirving the mid-lattutude cyclones will reduces, making weather systems move more slowly
-Small shifts in the global circulation could have major impacts e.e on monsoon rain systems
What is sound?
-A wave (transport of energy)
-Simple pressure waves made from vibations
What do vibrations cause air to do?
Cause air to expand and compress, creating series of high pressure -low pressure areas
How do you find the source of sound?
-Measure time
-Know velocity
-Caulcaluate distance using:
Distance= velocity x time
How many types of seismic waves are there?
4 types of seismic waves (one is also a pressure waves)
How and where do Seismic waves travel?
-Travels mainly in solids (rocks)
-More complicated propagation
Describe Body waves (1 type of seismic waves):
-Can travel through Earth’s interior
-P and S waves
-Arrives first
Describe Surface waves ( 1 type of seismic waves):
-Only propagate at the interface between two different media (e.g Earth’s surface)
-Rayleigh and Love waves
-Arrive after Body waves
-Cause most destruction
How are seismic waves recorded?
3-Component seismometer; one vertical component and two hprizontal components
What is the arrival order of seismic waves, shown on a seismogram?
-P waves
-S waves
-Love waves
-Rayleigh waves
What is a seismogram a representation of?
Ground motion:
-Z (Vertical)
-North-South
-East-West
What do seismic waves from an EQ look like on a seismogram?
-P waves are the strongest on the veryical component
-S waves are strongest on the horizontal comment, often stronger on one than the other
-The faster the Love waves vibrate on the horizontal components only
-Rayleigh waves on both the vertical and horizontal components
What are the propoerties of waves?
-Amplitude
-Wavelength
-Frequency
-Velocity
What is amplitude?
Height of crest or trough, measure of the energy of the wave
What is wavelength?
-Distance between peaks or troughs (metres)
What is frequency?
The number of waves passing per second (1/sec = Hz)
What is wave velocity?
Frequency x Wavelength (m/s)
What alters seismic wave velocity?
The ealstic properties of the medium
Do P waves or S waves have a higher velocity?
P waves
What is seismic wave attenuation?
Attenuation is the decay of amplitude with distance travelled
What are the two types of sesimc wave attenuation?
1.Geometric spreading
2.Scattering
What is geometric spreading?
-A decay in amplitude ,A, with distance ,d
-A~1/d for bosy waves, which spread out on a spherical wave front
-A~1/root d for surface waves, which decay more slowly (they don’t propagate vertically)
What is scattering?
-Amplitude is rduce by ultiple reflections, refractions and mode conversions through 3D heterogeneities
-Results in a chatoic ‘coda’ to the seismogram, as well as making the S-wave arrival sometimes hard to pick
Explain surfacw wave dispersion:
-Longer wavelengths (lower frequenices) penetrate deeper and arrive first
-This lead to different frequencies travelling at different speeds in a layered Earth. This ‘stretches out’ the waves packet, a process known as dispersion
How can you calculate Epicentral distance?
Travel time:
S travel time - P travel time
What is the general rule of thimb for erpicentral distance?
D~ (tS - tP) x 9
Does an EQ fault slip all at once?
NO
What shows EQ magnitude?
The area that moved (and the amount that it moved) is the size, magnitude, of the EQ
Describe the nature of EQ rupture along a plane?
Although EQs are rupture of an area of a fault plane, they are often approximated (and plotted) as a point
What is the hypocentre?
The origin point where the rupture started
What is the epicentre?
The point on the surface above the hypocentre
What is the centroid?
The centre of energy release, usually near the centre of the rupture area
What does EQ magnitude measure?
The amount of energy released
On which type of scale is EQ magnitude measured on?
-Logarithmic
-Increase in magnitudfe of 1, is 10 times increase in ground shaking (seismic amplitude)
What is the relationship between magnitude and number of EQs?
Logarithmic
What was the first EQ magnitude scale?
-Richter scale, but not in use today
-Only applicabble for Southern California only
What are the principles behind all seismic magnitude calculations?
-Convert seismic amplitude to EQ magnitude
-Correct for distance
-Scale is logarithmic
What are some problems that the richter scale has?
-Only valid for one type of seismometer
-Dependent on local geological conditions
-Only valid for local EQs (called a local magnitude scale)
What are the disadvantages of modified scales?
-It depends on having a seismometer
-Strong saturation effect becasue large EQs release low-frequencies that are not measured notably
What is the universal magnitude scale?
Moment magnitude scale
What is a moment?
A physical quantity proportional to the slip on the fault multiplied by the area of the fault surface that slips
How can moments be estimated?
The moment can be estimated from seismograms (and also gfrom geodetic measurements)
How to measure moment magnitude?
M0= rigidity of rock x rupture area x displacement
Describe the dirtrubution of frequency and intensity of EQs over time?
There are small- and moderate-sized EQs that occure frequently around the world that release far less energy than a single great EQ
What is the largest theoretical EQ magnitude?
If all subduction zones and other structures in the world ruptured at the same time, it would be an EQ of ~Magnitude (Mw) 10.5
Why is Mw 10.5 EQ deemed impossible?
EQ rupture areas are thought to be limited by barriers, such as:
-Changes in the geometry of a fault
-Changes in rock composition along a fault
-Changes in temperature
What is the Guternberg-Rihter relationship?
Log 10 (N) =a - bM
-N is the no. of EQs of Mw > M
-a is a measure of th total number of EQs (activity level) in the region
-b decribes the relative proportions of large and small EQs and is close to 1.
-b depends on regional stress state in the crust
What does a low b value in a Gutenberg-Richter relationship mean?
-Low b value -> more large EQs relative to small EQs -> Possibly higher crustal stress
What is the completeness magnitude?
The magnitude above which all EQs occuring in a region are recorded
What do a and b provide information on?
They provide crucial infromation on the seismicity rate of a region (how many EQs if a certain Mw happen over a specific period of time) and are an important part of seismic hazard analysis
Describe Earthquake recurrence:
-The rate of EQs on faults is governed by the size of the fault and the rate that one slide moves relative to the other
-Larger faults can have larger EQs
-Faults with higher slip rate have more frequent EQs
What are ‘characteristic’ EQs using the idealised ‘Characteristic’ EQ model?
Large EQs on a fault are always the same magnitude and occur at a constant recurrence interval
What can be EQ recurrence be estimated from?
-Historical data
-Paleosismic data (geomorphology, geology)
Why does the idealised ‘charactertistic’ EQ model not work?
-Stress loasing rate varies over time
-Fault strength (failure stress) changes over time
-Stress drop in each EQ is different
-Faults influence each other
-Small EQs can release stress during interseismic cycle
What are seismic hazards measured in terms of?
Ground motion probabilities
What influences seismic hazards?
-Distance from seismic sources
-Magnitude of potential EQs
-Frequency of occurence of EQs (as a function of size)
-Attenuation of ground shaking (with distance from source)
-Frequency content of ground shaking
-Duration of round shaking
-Local side effects
What is a time-independent seismic hazards assessment?
-There is no memory of past events: EQs can happen at any time at a constant rate of probability
-Based on long term-averages of EQ sources
-Has 4 main components; sources, recurrence, ground motion, probability of exceedance
Describe the positives of using Seismic sources?
-Identifictaion of sources locations EQs, active faults, areas of strain accumulation
-Group areas into zone (can be subjective)
Describe the negatives of using Seismic sources:
-Not all events may be recorded
-Not all faults produce large EQs (creeps)
-Not all faults are known
Describe the positives of using Recurrence:
-Known as the Gutenberg-Richter relation
-The maximum magnitude is estimated from the largest previous event or from fault length
Describe the negatives of using Recurrence:
The largest events often dominate the hazard rate, but they are so rare, so their recurrence frequencies are very uncertain
What are ground motion curves?
-Plot ground motion as a dunction of distance for different magnitudes
-The largest events have highest ground motion and propagate to larger distances
-Larger scatter in the attenuation data
-Scatter is due to; geological inhomogeneities along the ray path (wave scattering), local site conditions
What are the positives of using Ground motion prediction:
-Predict fround motion based on distance, magnitude and style of faulting
-Often measured in terms of acceleration or gravitational acceleration
-Immensely useful since simple to use
What are the negatives of using Ground motion prediction:
-Limited to regions with sufficient data (EQs and seismic stations)
-Treats EQs as a point
-Doesn’t count for complex seismic wave propogation
-Don’t always account for local site effects
What do ground motion using physics based simulations show?
Include site-specific geological structures (e.g. sedimentary basins) and source-specific parametres (e.g. slip distribution, rupture directivity, rupture velocity)
What is probability of exceedance?
-Combines sources,recurrence and round motion as a composite plot of the probobility of a given site experiencing a given level of ground shaking
-Green lines show how cost-benefit analysis determines a probability (relative to ither risks) , which in turn determines an acceleration for design purposes
What do seismic hazard maps show?
Produce probability of exceedance curves at every location in a region
What are the uses of time-independent hazards?
-Town and country planning -> Land use
-Setting building design codes
-Eductaion
-Setting EQ insurance premiums -> spreading the risk
What are time -dependant hazards?
-Hazards given by background rate multiplied by a conditional probability that depends on the size and time of previous events
Why are time-depend predictictions more accurate than assumung time-independent hazards?
Where there has recently been a big EQ, seismic energy has now been dissipated and there should now be a long period before it builds up again
What do strong contiguous clusters at a given time reveal?
Reveals the source rupture length from the early aftershocks
Why can neighbouring fault segments be triggered at the same time?
Due to stress transfer
What is a main shock?
Identified by its size, as the largest event in an EQ sequence
What is a foreshock?
A smaller evnt that preceded the main shock. Not all mainshocks have foreshocks
What are aftershocks?
-Events that come in the months, years or even decades following the mainshock.
-They occur with 1-2 fault lengths distance from the mainshock faults ans are smaller, often one Mw less
What is stress transfer?
-EQs can increase/ decrease stress on neighbouring faults
-Aftershocks tend to occur in areas with a stress increase
Are EQs more disastrous now then they were in the past?
Yes, not becasue EQ frequency is increasing but due to increasing fatalities related to urbanisation
What is increasing urbanistaion leading to?
Greater exposure and vulnerability
What is strengthening of buildings?
Basic strengthening of buildings are required by law - and still experince violent shaking and some damage
What is damping of buildings?
Dampers absorb energy - lead to reduced shaking
What is isolation of buildings?
Most expensive option, building isolated from the ground -to result in ONLY a slow shaking
Describe the realtionship between the frequency of buildings and the ground?
-Buildings have a natural resonance frequency: the frequency at which maximum amplitude occurs
-If the ground motion matches the natural resonance frequency of a building it will undergo large oscillations and suffer greatest damage
-Small buildings oscillate at higher frequencies than larger ones
-The ground also has a resonance frequency
How can risk be calculated?
Risk = Hazard x Exposure x Vulnerability
-Combines hazard (ground motion probabilities) with exposed structures/facilities (e.g. buildings, bridges, electricall power switching stations) and their vulnerability (or fragility)
-Calculated in terms of probabilities for economic damage or casualties
-Acquiring and analysing the specific data for an individual building or facility
What are the properties of exposure?
-Physical components are buildings and infrastructure
-Location
-Economic value
-Occupants
-Number of stories
-Use
-Construction type
What can be done to mitigate and prepare?
-Better building codes, that are enforced
-Map seismic site conditions to prevent building in certain regions
-Education; EQ drills, museums, EQ ‘monuments’ , cultural legacy
What are EQ early warnings?
-Quickly identify an EQ and issue and alert before widespread shaking is felt
-Relies on detecting fastest (and weakest) waves, P-waves, then computing an events location and magnitude
-Depending on distance, the alert may reach people before the more destructive S and Surface waves, giving people time to prepare (drop, cover, hold)
How does MyShake operate?
Uses smartphones as EQ sensors, a lot of smartphones and a lot less expensive than a traditional seismic network- hence can be used anywhere in the world
What are the problems of EQ early warning systems?
-Cannot tell magnitude very accuraetkey from the first few seconds of P waves
-Smart phones are concentrated i cities, but EQ often happen in remote areas, so still need conventional seismometers
-False alert reduces the confidnece of users
-Needsb to be implemented along with public eduction (no use warning people if they don’t do anything)
What are cascading hazards?
-New, secondary hazards triggered by the primary hazard
-Secondary hazards may begin in small area but impact a wider region than the primary hazard
What is liquefaction?
-The phenomena when solid soil behaves temporarily as a viscuos-liquid
-Occurs when loosely packed, water-logged sediments lose their strength in response to strong ground-shaking
-Soil loses ability to support foundations of buildings
-Most commoly obsered in low-lying areas near bodies of water
How does agriculture affect susceptibility to liquefaction?
Agricultural (rice) irrigation elevated the water table, increase the liquefaction susceptibility. -Can cause landslides to occur on very shallow slopes that would otherwise be stable
What are tsunamis caused by?
Caused by a sudden vertical discplacent of ocean water
What are the principles of tsunamis?
-Tsunamis travel at a speed that is realted to water depth (D)
-Tsunami wave velocity = root (gD)
-Tsunamis slows as the water depth decreases
-The tsunamis energy flux, which is dependant on both its wave speed and wave height, remains nearly constant -> speed reduces and height grows in shallower waters
What are meteotsunamis caused by?
Caused by fast-moving pressure waves in the atmosphere
What are ways in which to achieve Tsunami risk reduction?
-Sea walls
-Awarness and education; if feel shaking and close to coastal area then get to high ground, clearly marked tsunami evaxuation routes, tsunami drills
How can tsunamis be warned from EQs?
Government monitering agencies often issue tsunami warnings based on whether an EQ the has just been detectd is likely to cause a tsunami. Warnings are iften issued on public systems installed in coastal areas.This is based on rapidly finding out:
-Loction of the EQ (offshore)
-Depth of the EQ (shallow depth to move the seabed)
-Magnitude of the EQ
-The focal mechanism of the EQs (thrusts are most likely to cause EQs)
What are Tsunami early warnings?
-Monitoring network that detects tsunami waves and can provide early warning
-DART (Deep-ocean Assessment and Reporting of Tsunamis) focused on the Pacific Ocean
-Signal sneer via satellite to the tsunami warning centre
-Only useful for places sufficiently far from the source fopr warning to be used
