Hazards: Seismic Flashcards
Where do the most violent earthquakes usually occur?
Give an example
Subduction zones
Eg Nepal, 2015
Eg Sendai -> tsunami
Give the physical causes of earthquakes
- Plate boundaries - collision, conservative, constructive, destructive.
- Quakes that occur away from boundaries are associated with the reactivation of minor fault lines.
Do earthquakes always occur along major fault lines?
No.
Quakes that occur away from boundaries are associated with the reactivation of minor fault lines.
Give the human causes of earthquakes
Human activity:
- building of large reservoirs.
- deep mining.
- fracking.
Define focus
The point at which pressure is released within the earth’s crust
Define epicentre
The point on the earth’s surface immediately above the focus, where the most intense ground shaking is felt.
Does a shallow or deep focus cause the greatest damage?
Shallow focus
Give the depths that defines a shallow focus, intermediate focus and deep focus
Shallow focus = 0-70km deep.
Intermediate focus = 70-300km deep.
Deep focus = 300-700km deep.
How many deaths occurred during the Haiti earthquake, 2010?
> 300,000 deaths
When did the Haiti earthquake occur?
2010
Name the different types of shock waves
Primary / pressure waves (P waves)
Secondary / shear waves (S waves)
Surface love (L waves)
Rayleigh (R waves)
Where do shock waves originate from?
The focus or epicentre
Which type of shock wave is detected first? Why?
Primary waves - they travel fastest so are detected first by seismographs.
Which type of shock wave causes the least damage?
Primary waves
What type of wave are primary waves? Which direction does the ground move?
Longitudinal waves - ground pulled in same direction as waves.
Can primary waves travel through solids and liquids?
Yes
Which type of shock wave travels slowest?
Surface love (L waves) - come after P and S waves
Are P waves or S waves more dangerous?
Secondary waves
What type of wave are secondary waves?
Transverse wave
Can secondary waves travel through solids and liquids?
Only solids
What direction do surface love (L) waves travel?
Move back and forth horizontally
Which type of shock wave causes the majority of building damage?
Surface love (L)
Where are surface love (L) waves created?
Close to epicentre
Which direction of shaking is caused by Rayleigh waves?
Vertical and horizontal shaking
Which type of shock wave is the most destructive?
Rayleigh
Name the scales used to measure earthquakes
Richter scale.
Mercalli scale.
Moment magnitude scale (MMS).
What is the Richter scale? How does it work?
A logarithmic scale with 10 categories.
The magnitude is calculated based on the amplitude measured on seismographs.
Each number on the scale is 10X greater mag than the last.
Energy release is proportional to mag -> for each unit increase on the scale, energy release is 30X higher.
How many categories are on the Richter scale?
10
Has a Richter scale 10 earthquake ever been recorded?
No
What is the mercalli scale?
12 point scale.
Measures the intensity of an event and its impact.
1 = virtually imperceptible (equiv 2 Richter)
12 = total destruction (equiv 8.5 Richter)
What is the moment magnitude scale? How does it work?
Measures the size of events in terms of energy release.
By measuring the amount of movement by rock (along the fault) and the area of the fault / fracture surface.
For each whole number on the mag scale = 10X magnitude, 30X amplitude recorded on seismographs.
Small quakes can be negative numbers.
There is no upper limit.
What is the largest earthquake ever recorded?
Valdivia, Chile, 1960.
9.5 MMS
Give the characteristics of earthquakes that occur on conservative boundaries.
Give an example of where they occur.
Frequent.
Low magnitude.
Eg San Andreas
Give the characteristics of earthquakes that occur on destructive boundaries.
Give an example of where they occur.
Less frequent.
High magnitude.
Eg Japan
Name the primary impacts of earthquakes
Ground shaking.
Ground rupture.
What does the severity of primary impacts of an earthquake depend on?
Magnitude.
Depth.
Distance from epicentre.
Local geological conditions.
What is ground rupture?
Visible breaking / displacement of the earth’s surface, probably along the line of a fault.
What risks are caused by ground rupture?
Risks for large structures e.g. Dams, bridges, nuclear power stations.
Name the secondary impacts of earthquakes
Flooding. Tsunamis. Fire. Deaths - disease, famine. Landslides / avalanches. Soil liquefaction. Building collapse. Food shortages. Destruction of infrastructure / services.
What do secondary impacts of an earthquake lead to?
Disruption to local economy
What is soil liquefaction?
Occurs when vibrations / water pressure within a mass of soil -> soil particles lose contact with one another.
The soil behaves like a liquid -> inability to support weight, flows down gentle slopes.
Is soil liquefaction permanent?
No.
Usually temporary condition -> often caused by an earthquake vibrating water-saturated fill or unconsolidated soil.
Define tsunami
A giant sea wave generated by a small-focus underwater earthquake, volcanic eruption, underwater debris slide or large landslide into the sea.
Give the characteristics of tsunamis
Long wave length - over 100km
Low wave height - under 1m in open ocean
(Height rapidly increases when approaching land due to shallower water - over 25m)
High speeds - over 700km/hr
What is the first warning sign to coastal areas that a tsunami is approaching?
Drawdown - The wave trough in front of a tsunami -> rapid reduction in sea level
Does a tsunami event only consist of one wave?
No.
There are often a number of waves, the first one isn’t necessarily the largest.
How far in land to tsunamis often affect?
At least 500-600m inland
What do the effects of a tsunami depend on?
Height of waves. Distance waves have travelled. Length of the event (at source). Extent to which warnings could be given. Coastal physical geography - both offshore / on the coastal area. Coastal land use / population density.
Where are 90% of all tsunamis generated?
Pacific Basin -> 25% of which occur on western side of Pacific / bordering eastern side of Indian Ocean.
Why is it that 90% of all tsunamis are generated in the Pacific Basin?
Due to tectonic activity, mostly at destructive boundaries.
Name 5 earthquakes/volcanoes that resulted in tsunamis
Krakatoa eruption, 1883. Boxing Day earthquake, 2004. South Java Coast earthquake, 2006. Solomon Islands earthquake, 2007. Tohoku earthquake, Japan, 2011.
Give details of the Krakatoa eruption that resulted in a tsunami
Krakatoa, Indonesia, 1883.
Highest wave 40m.
35,000 deaths.
Give details of the Boxing Day earthquake that resulted in a tsunami
Boxing Day quake, Indian Ocean, 2004. Mag 9.0. Tsunami travelled >3000 miles. Hit over 15 countries. $5 billion damage. >220,000 deaths due to lack of warning. Since then, warning systems have been put in place in Indian Ocean.
Give details of the South Java Coast earthquake that resulted in a tsunami
South Java Coast, 2006.
Mag 7.7.
112 miles offshore earthquake.
>600 deaths.
Give details of the Solomon Islands earthquake that resulted in a tsunami
Solomon Islands, 2007.
Mag 8.0.
>15 deaths.
Give details of the Tohoku earthquake that resulted in a tsunami
Tohoku, Japan, 2011. Mag 9.0 - most powerful ever in Japan. 70km offshore. Tsunamis >40m high. Travelled >10km inland. 16,000 deaths. 6000 injured. 2500 missing. 300,000 homeless. $30 billion damage -> total $235 billion cost to Japan.
Name the ways in which we try to protect against seismic hazards
Tsunami protection. Hazard resistant structures. Land use planning. Education. Fire prevention. Emergency services. Insurance. Aid.
How can we try to protect against seismic hazards by tsunami protection?
- Tsunamis can’t be entirely predicted.
- Automated systems can be installed to give warnings -> most effective with pressure sensors attached to buoys which measure pressure of overlying water column.
- Warning systems esp in high risk regions, eg use klaxons.
- Built prevention walls up to 12m high.
Give an example of somewhere that uses a warning system
Pacific Warning System, Hawaii.
Monitors earthquake activity.
Issues warnings to countries around the Pacific edge if a tsunami is likely.
How can we try to protect against seismic hazards by using hazard resistant structures?
- Large concrete weight on top of building that moves in opposite direction to earthquake.
- Rubber shock absorbers in foundations -> allow some movement.
- Cross bracing -> hold structure together.
- Automatic window shutters -> prevent falling glass.
- Easy access roads for emergency services.
- Fire resistant building materials.
How can we try to protect against seismic hazards by education?
- Often main way to minimise deaths.
- Authorities advise how to prepare -> securing homes, appliances, heavy furniture, e.quake kits.
- School / office / factory drills.
Give an example of a country that uses earthquake drills
Japan - Disaster Prevention Day (1st Sep), on anniversary of Tokyo earthquake, 1923.
Give an example of an organisation that suggests supplies to have at hand in case of an earthquake
American Red Cross. Water Food Clothes Bedding First aid kit Torch batteries Radio Matches...
How can we try to protect against seismic hazards by fire prevention? Give an example
‘Smart meters’ cut off gas if an e.quake of sufficient magnitude occurs.
Eg Tokyo Gas Company - computer network informs employers where to switch off major pipelines to reduce fires.
How can we try to protect against seismic hazards by emergency services?
- Heavy lifting gear must be available.
* People must have first aid training.
Give an example of an area that carefully prepares emergency services for seismic hazards
California.
Computer programmes identify areas that e,regency services should be sent to first
How can we try to protect against seismic hazards by land use planning?
- Most hazardous areas can be identified -> regulated land use.
- Buildings eg schools, hospitals should be put in low risk areas.
- Important to have sufficient open space -> safe areas away from fires / aftershock damage to buildings.
How can we try to protect against seismic hazards by insurance? Give an example
In richer areas people are encouraged to take out insurance to cover losses -> this is expensive.
Eg Kobe earthquake, Japan, 1995.
Only 7% of people were covered by e.quake insurance.
How can we try to protect against seismic hazards by aid?
• Most aid to poorer countries is a few days after event. - medical services. - tents. - water purification equipment. - search and rescue equipment. • Long term aid is problematic. - reconstruction of env/economy.
Name the ways that earthquakes can be predicted
Unusual animal behaviour. Hydrochemical precursors. Temperature change. Water level. Radon gas. Oil wells. Foreshocks. Theory of seismic gap. Changes in seismic wave velocity.
How can earthquakes be predicted by animal behaviour?
It is thought that some animals behave unusually before an earthquake.
- Show restlessness a few minutes-several days in advance, which gets stronger the closer the earthquake is.
- Particularly in high intensity / epicentre region close to active faults.
- Abnormal behaviour is observed during e.quakes of >5.0 mag.
Give an example of how earthquakes can be predicted by animal behaviour
Bahai earthquake, 1969. At the Tientsin Zoo. Swans suddenly scrambled out of water. A tiger stopped pacing. Tibetan yak collapsed. Pandas held their heads in hands and moaned. Turtles were restless.
How can earthquakes be predicted by hydrochemical precursors?
There are anomalies in the concentration levels of dissolved minerals and gaseous components in underground water
Give an example of how earthquakes can be predicted by hydrochemical precursors
Chemical composition of underground water was regularly observed in seismically active regions of Tadzhik and Uzbekistan…
• During inactive periods, concentrations of minerals/gases are stable.
• 2-8 days before an earthquake the concentration levels of dissolved minerals significantly increased.
• There was variation in water level, temperature, discharge of water sources and pressure of artesian water.
• After the earthquake, anomalies disappeared.
How can earthquakes be predicted by temperature change? Give an example
Temperatures increase before an earthquake occurs.
Eg. Lunglin earthquakes, China, 1976:
10°-15°C increase in temperature before hand.
How can earthquakes be predicted by water level?
There are drastic changes in water level in several wells just before a major earthquake.
- Studies conducted in Kurile Islands use purpose built wells 410-670m depths.
- Results show that water levels begin to fall 3-10 days before.
- After a short period it starts rising when the e.quake strikes.
Give examples of how earthquakes can be predicted by water level.
Nankai earthquake, Japan, 1946 -> fall in water level a few days before.
Lunglin quake, china, 1976 -> 3cm rise.
Meckering, Australia, 1968 -> 3cm rise a few hours before.
How can earthquakes be predicted by radon gas?
Radon is a radioactive gas.
It is discharged from rock masses before an earthquake.
It is dissolved in the well water and its concentration in the water increases.
Give examples of how earthquakes can be predicted by radon gas
Tashkent, 1972 -> 15-200% increase in concentration 3-13 days before.
Luhou, China, 1973 -> 70% increase 6 days before.
Tangshan, China, 1976 -> 50% increase 18 days before.
How can earthquakes be predicted by oil wells?
There are large scale fluctuations of oil flow from oil wells before an earthquake.
It’s suggested that when the tectonic stress builds up to a certain level, the pore pressure within a deep oil bearing strata reaches its breaking strength -> causing oil to sprout along the oil wells.
Give examples of how earthquakes can be predicted by oil wells?
Israel, 1969.
Northern Caucasus, 1971.
China, 1972.
-> increased oil flow before.
What are foreshocks?
Minor shocks that come before a major earthquake
Give examples of how can earthquakes be predicted by foreshocks
Haichang earthquake, china, 1975…
Increased seismicity 2 months prior.
Uttarkashi earthquake, 1991…
Foreshocks 5 days before, >3.5 mag on Richter.
Can earthquakes be predicted by foreshocks alone?
No.
It is not a flawless method and has to be supplemented by other methods of earthquake prediction
How can earthquakes be predicted by the theory of seismic gap? With an example
12 large e.quakes hit Japan between 1904-1963.
Seismologists plotted the size of each tremor-struck area.
Found that each quake segment touched the next one in the sequence without overlapping.
Each large e.quake was in a segment that was quiet for the last 39ish years.
-> prediction that those segments that were quiet for some time will be hit by an e.quake sooner than later.
Give an example of how can earthquakes be predicted by the theory of seismic gaps
Geophysicists in Texas predicted a major quake in southern Mexico around Puerto Angel town, based on seismic gap theory.
This quake occurred in 1978, measuring 7.9 on Richter scale.
It’s epicentre was within a km of the predicted site.
What is a seismicity gap?
A region where earthquake activity is less compared with its neighbourhood along plate boundaries.
What is lead time?
The time lag between the arrival of Primary (P) waves and Secondary (S) waves.
How can earthquakes be predicted by changes in seismic wave velocity?
Russian seismologists found that lead time (between P and S waves) began to significantly decrease for days/weeks/months before the earthquake.
But just before the quake hit, the lead time was back to normal.
Longer period of abnormality in wave velocity -> larger e.quake
Define prediction of an earthquake
Forecasting the occurrence of a quake.
- intensity
- location
- time limit
How far in advance is long term prediction?
Years before the e.quake
How far in advance is medium term prediction?
Months/year before the e.quake
How far in advance is short term prediction?
Hours/days before the e.quake
Give the warning signs of an earthquake
Micro waves before main tremor.
Bulging of ground.
Decreasing radon gas concentrations in ground water.
Raised ground water levels.
Increased argon gas concentrations in soil.
Strange animal behaviour.
Define protection
Preparing for the event by modifying the human/built environments -> lower vulnerability.
Modify loss by insurance / aid.
Give the objectives of the Federal Emergency Management Agency, USA
Promote understanding of earthquakes / effects.
Work better to identify earthquake risk.
Improve earthquake resistant design / construction techniques.
Encourage use of earthquake safe policies / planning practices.
