Seismic hazards and Tohuko Flashcards
What is an earthquake and why do they happen?
The Earth’s crust is mobile leading to a slow build-up of stress within the rocks. When the pressure is suddenly released, parts of the earth’s surface experience an intense shaking motion
What is a shockwave?
Energy released as pressure within the earth’s rocks is released. They have the highest level of energy at the focus and the energy decreases as the waves spread outwards
What are the focus and epicentre?
focus = the point at which the pressure release occurs within the crust, the shallower the focus, the worse the earthquake
epicentre = the point immediately above the focus on the earth’s surface
What are P and S waves?
primary waves are longitudinal and arrive at the detector first, secondary waves are transverse and arrive second
How can humans cause earthquakes?
- building large reservoirs - this puts pressure on underlying rocks
- fracking
What factors might the intensity of the effects of an earthquake depend on?
- magnitude of earthquake
- distance from epicentre
- population density
- preparedness and education
- design and strength of buildings
- secondary hazards
What is liquefaction?
- the ground is shaken so violently by seismic waves that the ground becomes liquid
- this is because there is increased pore water pressure which reduces the strength of the soil, which when saturated, causes it to flow
- this results in huge damage to infrastructure and the sinking of buildings, hard to build back after this
How are tsunamis formed?
- earthquakes occur underwater due to subduction (friction between the continental and oceanic plate)
- this releases huge amounts of energy which displaces vast volumes of water, forming large waves
- the waves radiate out from the epicentre and travel very quickly in deeper water
- on approaching the coastline, the sea bed starts to interfere with the waves motion due to friction, which slows it down
- as the wave slows, the water piles up as a wall before breaking as a tsunami
What factors effect the intensity of the impacts of a tsunami?
- height of the tsunami wave and the distance the wave has travelled
- extent to which warning are given
- coastal physical geography
- coastal land-use and population density
How does level of development influence damage caused by earthquakes?
although HIC’s will likely have more strategies put in place to mitigate eg. stronger buildings, warning systems, educated population, they will also have a very dense, built up area with high population density which makes them more vulnerable and at risk. Residents will also likely have the perception that they are safe due to high levels of wealth which puts them more at risk
What are the factors that influence the effectiveness of responses to tectonic hazards?
- wealth and level of development - less damage to local economy
- globalisation = greater access to aid
- topography - steeper land is less accessible so harder to respond
- past experiences - if there have been recent hazards that the area is still recovering from, the response will be worse
What are the physical and human factors that cause some seismic hazards to be more disastrous than others?
physical:
- proximity to coastline
- proximity to plate boundary/epicentre
- topography eg. funnel-shaped bays direct flow of tsunami water making it more concentrated in one area
- geology - liquefaction
human:
- built environment
- wealth
- urban/rural
- prediction, prevention, protection
How effective is earthquake prediction?
prediction is extremely difficult - regions at risk can be identified and hazard zones mapped out through plate tectonics, however earthquakes occur suddenly and randomly therefore the population can only be warned a few mins before
- some attempts to measure groundwater levels or unusual animal behaviour but very limited
Can earthquakes be prevented?
prevention is thought to be impossible, therefore the only way to manage the impacts is through mitigation
What are the richter and mercalli scale?
richter scale measures magnitude alone whereas mercalli scale also accounts for the observed effects, so is more subjective but also more accurate
What are some methods that countries use to protect against seismic hazards?
Modifying the human and built environments to decrease vulnerability
- hazard resistant structures eg. large concrete weights on top of buildings which will move in the opposite direction to the force of an earthquake/ rubber shock absorbers in the building foundations
- education - earthquake kits with emergency supplies, earthquake drills
What plate boundary is Tahuko on and how was the earthquake caused?
on a destructive boundary
- happened due to the build up in strain energy as the pacific plate moves west towards the eurasian plate and is subducted at a rate of 8cm/year. The strain energy of this subduction eventually overcame the frictional force holding the plates in place, and was released as seismic waves
- this resulted in 480km of the pacific plate surging under Japan, and the eurasian plate lifting by 9m resulting in a 9.0 magnitude earthquake (megathrust)
How did the Tahuko tsunami form and why was it so disastrous?
- as the eurasian plate popped upwards, a large volume of water above in the pacific ocean was displaced, the epicentre was also located under the ocean floor, so all of the water above this point was suddenly pushed vertically upwards
- the wave travelled at 500mph and took 10 mins to reach Japan’s coastline leaving little time to prepare/evacuate
- the shallow coastline in Japan resulted in friction which slowed the lower part of the wave, this caused the wave to build up to 40m high
- water flowed 10km inland - very widespread impacts
- funnel shaped bays meant that water was concentrated in one area leading to devastating impacts
Why is Japan’s population especially vulnerable?
- frequent earthquakes = recovery efforts more difficult because as soon as the area is rebuilt it may be at risk of another hazard, so Japan can never fully recover
- majority of Japan’s population live on the coast due to Japan’s physical geography, having a mountainous spine and only a narrow strip of flat land along the coastline
- upper 10m of soil in the zone that was hit was unconsolidated resulting in liquefaction - very hard to mitigate against/recover from
- had sea walls but they were only 15m high - easily overcome by 40m high tsunami wave
- 66% victims were age 60+
- 3 7+ magnitude aftershocks within the first 7 days - recovery efforts and aid almost impossible
Why did Japan have a high capacity to cope?
- highly developed so most buildings designed to be earthquake proof, however 700,000 buildings damaged so clearly not very effective
- highly educated population
- huge GDP, was $34,000 in 2011 so has huge leeway for planning, but also makes the economy more vulnerable eg. 10% value of stock market lost
- network of around 200 seismographs, which didn’t predict the earthquake, only sent out warnings 8 seconds after the first wave was detected leaving little time to respond
What were the immediate responses and how effective were they?
- warning 8 seconds after first wave and sent messages to 52 million phones - widespread warning but too late leaving little time to prepare or evacuate
- very widespread and global aid eg. 230 emergency service teams sent from red cross, short-term was effective but didn’t combat long-term effects, 1235 aftershocks also made this less effective
- early tsunami warning 3 mins after earthquake, but nothing to mitigate against it so not very effective, sea wall was too short
- 116 countries helped with assistance, totalling 520 billion yen
- access to the affected areas was restricted because many were covered in debris and mud following the tsunami, so it was difficult to provide immediate support in some areas.
What were the social impacts of the Tohuko earthquake?
- Over 340,000 displaced people in the region
- 15,854 deaths, 90% of the deaths were caused by drowning
- Almost 4,000 roads, 78 bridges and 29 railways were also affected
- 130,000 buildings totally collapsed and another near 700,000 buildings partially damaged
- 1.5 million without water
What were the economic impacts of the Tohuko earthquake?
- The World Bank estimated cost was US$235 billion, making it the most expensive natural disaster in world history
- 4.4 million businesses and households lost
- All of Japan’s ports had to close at least temporarily during the disaster
- Japan’s economy wiped 5–10% off the value of stock markets post-earthquake
What were the environmental impacts of the Tohuko earthquake?
- 3 nuclear meltdowns at the Fukushima power plant, residents within a 20km radius evacuated
- Agriculture was affected as salt water contaminated soil and made it impossible to grow crops
- Oil industry as two refineries were set on fire during the earthquake - secondary hazard of fires
Why do people continue to live in Japan?
- They have lived there all their lives, are close to family and friends and have an attachment to the area.
- The northeast has fertile farmland and rich fishing waters.
- There are good services, schools and hospitals.
- They are confident about their safety due to the protective measures that have been taken, such as the construction of tsunami walls.
What were the long-term responses of the earthquake?
- The government set up a Reconstruction Design Council who had a budget of over 23 trillion Yen to rebuild houses. These aim to provide incentives to attract investment, both in terms of business and reconstruction,
- The runway at Sendai Airport had been badly damaged. However, it was restored and reusable by the 29th of March
- A decade later, the area is still rebuilding.
- As of March 1, 2021, only 1,585 people, or around a tenth of evacuees, have returned to the communities near the Fukushima Daiichi Nuclear Power Station.
- As of the end of March 2021, the government will have spent an estimated ¥38.9 trillion on recovery over the last 10 years
What were the factors that contributed to making the Tohuko earthquake a disaster?
- shallow 30km deep focus - very strong seismic waves 9.0 magnitude
- very vulnerable ageing population so difficult to evacuate
- topography - funnel-shaped bays
- geology - liquefaction
- lack of infrastructure to protect - sea wall was easily breached and ‘earthquake proof’ buildings destroyed
- perceptions - people thought they were protected and over reliant on technology
How has Japan mitigated since the earthquake?
- The original tsunami barriers were replaced with ones that were 18m high. However, some scientists have noted that this would still not protect property if a similar earthquake was to take place.
- The central government decided on a coastal protection policy, such as seawalls and breakwaters which would be designed to ensure their performance to a potential tsunami level of up to the approximately 150-year recurrence interval
