Earthquakes

Types of Plate Margins.

• Subduction Zones:
• Oceanic plate converges with a continental plate, the denser oceanic plate is forced under the lighter continental plate and sinks into the mantle.
• Heat in mantle melts crust to form magma.
• Magma rises because it is less dense and hotter than the surrounding rock.
• Pressure decreases, gases dissolved in magma build up to eventually form Volcanoes that violently erupt.
• Collision Zones/Margin:
• When two continental plates collide, none of them cannot sink because they are both light.
• Land buckles upwards to form fold mountains.
• Earthquakes can form at collision margins.
• Constructive Plate Margins:
• Occurs when two oceanic plates move away from each other.
• Crust fractures, Magma rises from mantle and creates new sea floor along the mid-oceanic ridge. (This is called Sea-floor spreading)
• As more magma rises from mantle, Underwater volcanoes form, if they grow enough, they can form volcanic islands above sea-level.
• Conservative plate margins:
• Occurs when two plates move past each other in opposite directions.
• Pressure builds up if the plates do not move past each other smoothly which makes them stick together.
• Eventual release in this pressure is earthquakes.

Earthquakes

• Earthquakes are a sudden or violent movement within the Earths crust, caused by the release of built-up pressure in conservative or destructive plate margins.
• Epicentre - where the earthquake is mostly strongly felt.
• Point where shockwave on earth surface happens is called focus.
• 90% of earthquakes occur where plates are moving together at destructive plate margins or slide past each other at conservative plate margins.
• Limited pressure builds up and earthquakes are a low magnitude.
• Movement of one point isn't always smooth and sometimes sticks as they move past each other.
• Intense pressure builds and rocks deform under stress.
• As pressure is released there is a sudden movement, creating seismic waves which radiate outwards from the focus to earths crust.

Measuring Earthquakes

• Richter Scale - Measures the magnitude or energy released, on a scale of 1 - 10.
• Scale is Logarithmic, so an earthquake measuring 7/10 is ten times more powerful than 6/10.
• Mercalli Scale - Describes the intensity and likely effects of an earthquake.
• Resulting damage is compared with descriptors on a scale of 1-12 where 1 = (weak) and 12 (Complete destruction)

Impacts of Earthquakes on People

• The size of the earthquake on the Richter scale - the higher it is on the scale, the more destruction it can cause.
• Level of development - whether it occurs in a rich or a poor country. Richer countries will be more likely to be able to predict, protect and prepare themselves from the effects of an earthquake.
• The depth of the focus - if it's shallow, it can be more destructive.
• Distance from epicentre - the effects of an earthquake are more severe at its centre.
• Population density - the more people living in an area, the more likely that more deaths and casualties may arise.
• The time of day - whether people are in their homes, work or travelling.
• Many Buildings can be destroyed
• As a consequence of an Earthquakes happening. There may be a lot of economical drawbacks. In Haiti, 295,000 Buildings were destroyed. Leaving 1.5 Million people homeless.
• Population density - Highly vunerable if they live in earthquake zones in densely populated areas. In China, when an earthquake hit sichuan, 15 Million people lived close to the epicentre. The earthquake killed 87,150 people.
• Secondary Effects:
• Fires: Gas pipes can break and fires take hold in urban areas. Over 300 fires burnt for days in Kobe, Japan, after an earthquake struck in 1995.
• Diseases: 2010 earthquake in Haiti led to shortages of fresh water and damaged sanitation systems resulting in the spread of chloera to 740,000 leaving 9000 killed.

How can Earthquake risk be reduced?

• Major earthquakes happen relatively infrequently so many generations of a family can take the risk that a major earthquake will not occur in their lifetime (Example = San Francisco Earthquake. There was a violent earthquake in 1906 but did not have another one until 1989.)
• Many of the Earths's natural resources of oil, minerals and geothermal energy are concentrated near plate margins. (Economic benefits can outweigh the risks)
• People can be prepared to evade their situations of being disastrous. Reducing risk can be by, educating people, or improving building design and planning through emergency evacuation and aid plans.

Case Study (Tohuko) Earthquake

• Causes:
• Pacific plate is forced under the Okhotsk plate at a subduction zone.
• Earthquake was shallow-focused at a depth of approximately 30km.
• Japan's coastal area is flat, low lying and the soft soil amplifies shockwaves.
• Impacts of the Earthquake:
• Oil refineries in Ichihara and Sendai were ablaze. (Economic drawbacks)
• A school in Miyagi lost 84 of 121 students and teachers.
• 4.4 Million households in north-east Japan were left without electricity and 1.5 Million without running water.
• Secondary Effects:
• Earthquake triggered a tsunami that took 10 minutes to reach the coast with heights of 38 m.
• Earthquake knocked out the Fukushima Nuclear power plant.
• Tsunami triggered nuclear meltdown in three reactors.
• 127,000 buildings were completely destroyed and 1,000,000 destroyed across north-east Japan.