Seismic Hazards Flashcards
Define the focus
Point in the earth’s crust/lithosphere at which the earthquake occurs - where the plates suddenly slips and the energy is released
Define the epicentre
The point at which the seismic waves reach the earth’s surface
Define shockwaves (seismic waves)
Waves of energy released form the jolting/slipping of plates
Radiates from the focus and causes the earth’s crust to shake
Define aftershocks
Usually smaller earthquakes that occur after the main seismic event.
Plates readjust back to their original state causing (usually) smaller earthquakes in the area - minuets, days, weeks post event
What is the Seismic Gap Theory
Predicting the relative size and frequency of an earthquake in a given area, depending on the size and frequency of previous earthquakes in the area
(Areas which experience many small earthquakes will not likely experience a large one)
What percentage of earthquakes occur on the Circus Pacific Belt
80%
How are earthquakes formed
Plate movements produce huge levels of energy and their movements are not smooth, causing high levels of friction
Friction along the plate margins builds up stress in the lithosphere
When the strength of the rocks under stress suddenly overcome the elastic strength of the rock, they fracture along cracks (faults) sending a series of seismic shockwaves to the surface
A lot of the energy is transferred vertically to the surface and then moved outwards from the epicentre
Other than the main cause of plate tectonics, how else are earthquakes formed
Mining
Fracking
Reservoir construction
What was the most powerful earthquake ever recorded
Chilie 1960
9.5 on moment mag scale and 80ft tsunami
How do earthquakes form at destructive boundaries (Collision)
Two continental plates collide.
Frictional stresses build up and energy is released suddenly in the from of an earthquake as the plates slip/jolt
How do earthquakes form at destructive boundaries (Subductive)
O + C plate collide
As the Oceanic plate subducts, it grinds on the Continental crust. Huge friction and pressure builds and is eventually released
Continental plate can ‘ping’ upwards to form a tsunami
How do earthquakes occur at conservative plate margins
Plates move in opposite direction or in the same direction at different speeds
Build up of pressure as the plates get stuck as they grind against each other, high levels fo friction
Plates suddenly slip along the strike slip fault Lin e
Energy is released which causes an earthquake
How do smaller earthquakes form at constructive plate margins
As the plates move apart, they will vibrate and move which causes minor earthquakes
Little energy or pressure is built up so minor vibrations
Why is the depth of the focus significant
Shockwaves have more distance to travel at deep focus earthquakes, therefore energy is disputed and reduced by the time it reaches the surface (epicentre)
Shallow focus - shockwaves have less distance to travel, so the earthquake is more powerful at surface, damage increased
Give the depth of a shallow focus earthquake
0-70 km
Give ht depth of a deep-focus earthquake
300-700km
Give the two types of Seismic waves/shockwaves
Body waves (travel through the earth). Can be subdivided into Primary (P) and Secondary (S) waves
Surface waves (travel along the Earth’s surface. Can be subdivided into Love and Rayleigh waves
Which type of seismic wave is more deadly
Surface waves cited to be most hazardous as they tend to cause the moist damage to buildings which collapse and kill people
What is ground rupture
Movement in a. fault breaks through to the surface. Ruptures may occur suddenly during an earthquake of gradually in the form of fault creep
Fault ruptures almost always follow pre-exisiting faults which are zones of weakness
Describe Primary Waves
AKA Compression/pressure waves
Fastest and reach the surface first (first to eb decade by seismometers)
High frequency and travel through both solid and liquids (mantle and core) to the opposite side of the earth at speeds of 5.5km/s
Rocks move back and forth and cause the ground to break up (slinky diagram)
Describe Secondary Waves
AKA Shear/transverse waves
Half as fast as P waves and reach surface next
High frequency and can travel through solids but not liquids
Travel at speeds of 3km/s
Cause material to shake at right angles to the direction of trade travel. The greater the displacement the greater the amplitude of the wave.
Cause ground to shake up and down
Describe love waves
Slowest of the seismic waves and cause the most damage
Shaking is at right angles to the direction of movement
Moves ground side to side, damaging infrastructure and buildings
Describe Rayleigh waves
Waves radiate from the epicentre in complicated low frequency rolling motions
Produce both horizontal nd vertical ground movements (like at sea)
Rocks move in an ellipsis as the wave passes and breaks up the surface
What might affect the way in which surface waves travel
Type of material they re travelling through
Unconsolidated sediment (sand/silt) can cause seismic waves to be amplified, less so than shaking of solid bedrock
How are tsunami’s formed (5)
Earth’s crust separated into masses of tectonic plates that spread apart ad collide, creating earthquakes (submarine)
The energy released from an earthquake can be so powerful that it lifts millions of tonnes of water above it, casing tsunami waves
Tsunami waves move in big flat waves across the open ocean - in the deep ocean these waves are barley noticeable at the surface. Travels 550mph+, with a time period of 10-60 mins between each wave
As they approach shallower water, they slow down, become compressed and grow in hight
Sudden retreat/drawback of water from shore
What are some human impacts of a tsunami
2004 Indonesia
Widespread homelessness (500,000 faced int refugee camps in Achech province, Indonesia)
Economies devastated - fishing, agriculture and tourism. 44% in Achech province lost their livelihoods
Loss of deep sea trawlers resulted in fewer catches. The few fish that are caught are less likely to reach market because of damaged infrastructure
Water supplies and soil contaminated by seawater
Gap between rich and poor increases
Effects of tsunami’s vary accordingly to its……and…………….but are also influenced by the population……………, costal relief and…….use of the costal region
Size
Displacement
Density
Land
Describe liquefaction
Occurs in unconsolidated sediments that are saturated with water.
Shaking moves the sediment material apart, soil stability declines and soil behaves like a liquid due to an increase in water pressure
Describe the key features of landslides
Occurs as a result of the ground shaking, causing particles and sediment in the soil to become loose, causing slope failure - even on gentle slopes
Involves a block of rock moving very rapidly downhill along a planar surface.
Unlike a mudflow where the moving material becomes mixed, the block of material remains largely intact
What are the impacts of landslides
Further deaths and injuries
Flooding from blocked rivers creating quake lakes
Permanent disruption to natural drainage patterns
Loss of farmland for food production
Infrastructure and equipment destroyed
What is the richer scale
Formula based on the amplitude of the largest recorded seismic wave and the distance between the earthquake focus and the seismometer
Richter scale is logarithmic, so magnitude 7 is 10x larger than magnitude 6
Energy released is proportional to the magnitude
What is the Mercalli scale
Qualitative descriptor scale of earthquakes, based on obersvatiosn by humans on the amount of damage that has been caused nearest to the epicentre
Give some pros of the Mercalli scale
Reflects the impacts, easily comparable
Clear parameters for each level
Internationally used, easily accessible, doesn’t require specific skills/specialised equipment
Tells us where aid needs to be directed
Give some cons of the Mercalli scale
Subjective - may not be a standardised scale that can be compared easily
Outdated (last updated 1956), technology and development has since increased in durability + tech
Doesn’t tell us the magnitude
Only works in urban areas, with witnesses and infrastructure
Upper limit
Give some Pros of the Richter scale
Useful form small scale earthquakes
Doesn’t have an upper limit
Non subjective, easy to classify and compare.
Give some cons of the Richter scale
Less useful for larger earthquakes over mag 8
Doesn’t show the amount of energy released
Doesn’t measure the impacts/damage caused
Requires skilled workers and expensive scientific equipment - not accessible to everyone
What is the Moment Magnitude Scale
Most commonly used scale globally - adaptation of the Richter scale
Combines the amount of movement on the fault, rock strength, and size of the rupture area
Measures the total moment release of the earthquake where moment is a products of the distance the fault has moved, and the force required to move it
Gove some Pros of the Moment Magnitude Scale
For events greater than magnitude 8, significantly more accurate
Most commonly used - sea easily comparable world-wide
Enhances understanding of the characteristics of the earthquake (future area etc)
Accurately calculates the time, location, energy released and magnitude
Describe the pattern of the largest earthquakes in th eWorld
Nearly all major earthquakes occur along plate boundaries (Notably the Pacific Ring of Fire)
Mostly at destructive plate boundaries (subjective
Some clusters around mountain ranges (Andes, Himalayas)
Large concentration of earthquakes on the Alpide Bet
Frequency and regularity of earthquakes shows no……………….
An understanding of plate tectonics allows seismologists to know which areas are at risk, however it is…………….to predict when or……….a seismic disaster will strike
Predictability
Impossible
Where
Is earthquake activity increasing
NO
On average there are about 15 earthquakes every year with a. magnitude of 7+
As with any random phenomena, the number of earthquakes every year varies slightly, but in general there are no dramatic variations
Why might it seem like there a more earthquakes recently (5)
Technology - Recording and prediction constantly being improved - more earthquakes are now being recorded
Earthquake clustering - although long term averages ae fairly constant, in any quasi-random process, you get clustering in time. People notice the clusters, they don’t notice the gaps in between
Population increases mean that more people than ever live in earthquake prone regions. So although number of earthquakes remain the same, the impact increases
Earthquakes in populated areas, far more apparent than those in rural areas
Global communication - now have near instant pictures of eventuating earthquakes worldwide. More people are aware of the earthquake and it impact
Can we predict earthquakes (yes)
Geoscientists able to identify particular areas at risk and make probabilistic forecasts about the likelihood of earthquakes happening in a specified area over a specified period
These forecasts are based on date gathered through local seismic monitoring networks, local monitoring, geological fieldwork and historical records
Forcsts are improved as our theoretical understanding of earthquakes grow
Long term forecasts (years to decades) are currently much more reliable than short term forecasts
Give some main short-term predictors of earthquakes and the issue with this
Micro-quakes before main tremor
Bulging of the ground
Rising levels of radon gas in concentrations of water
Electric + magnetic changes within local rocks
irrational animal behaviour
However none have found to be a good indicator as hey can all occur without being followed by an earthquake
Can we predict earthquakes (no)
Not currently possible to make deterministic predictions of when and where earthquakes will happen
Predictions have a long timescale and do not allow for movements of minor/unknown faults
For this to be possible it would be necessary to identify a diagnostic precursor
However, so far the search for diagnostic precursors have been unsuccessful and most geoscientists don’t believe there is a realistic prospect of accurate prediction in the foreseeable future
Why is it difficult to predict earthquakes
The crust’s response to changing stress is not linear, (making prediction of behaviour more difficult) and is dependant on the crust’s complex and highly variable geography
As a result, very difficult t build accurate simulations which predict tectonic events. Laboratory experiments cannot accurately reflect the complexities of real-world geological settings
Earthquakes originate deep beneath the ground - so data gathering depends on remote observation techniques
How can earthquake hazards be mitigated
Seismic monitoring research provides the basis for forecast monitoring and hazard mapping.
Geoscientists can engage with local authorities and communities to ensure that this information s used effectively
Failing buildings are the greatest causes casualties so essential that new buildings are constructed to withstand their effects
Local populations should be aware of the risks and how two act during an earthquake
Why is evacuation not an effective strategy for earthquakes
Not possible to predict precise timing, location, and size of an earthquake to a sufficiently high degree of accuracy
How common are earthquakes in the UK
Fairly common though most not noticed by people
20/30 earthquakes felt by people each year, with a few hundred smaller ones only recorded by sensitive instruments
What doesn’t the UK experience large earthquakes
Most happen at plate boundaries, but the UK is located in the middle of the Eurasian plate so not subject to significant tectonic activity
Largest possible earthquake in UK = 6.5 (British Geological Society)
What does the UK experience earthquakes
Eurasioan plate is riddles with fault lines, often slipping slightly casing tremors detectable only by specialist equipment
Fracking has the potential to cause mini earthquakes
Glacial rebound - uplift resulting from melting ie sheets that used to cover UK
North Sea oil extraction
The UK is only likely to get a magnitude 4 earthquake every….years, and a magnitude 5 earthquake every…….years
2+
10-20
