Seismic

Question 1

What is an earthquake?

Answer 1

The shaking caused by the rupture and subsequent displacement of rocks beneath the Earth’s surface

Question 2

What is the distribution of earthquakes?

Answer 2

• Generally concentrated in linear clusters along plate boundaries
• 95% occur close to or at a plate margin
• 90% occur at the Pacific Ring of Fire

Question 3

What is the focus of an earthquake?

Answer 3

The point at which the earthquake starts below the Earth’s surface

Shallow focus - 0-70km
Intermediate focus - 70-300km
Deep focus - 300-700km

Question 4

What is the epicentre of an earthquake?

Answer 4

The point on the Earth’s surface where seismic waves reach first - usually above the focus

Question 5

How do earthquakes form?

Answer 5

• Tectonic plates move due to convection currents and processes like slab pull and ridge push
• Plates become stuck due to friction between plates
• This causes rocks to deform which leads to a build up of stress and pressure
• When the rock can no longer stand the deformation, it fractures and the two sides slide past each other
• This releases energy in a process known as elastic rebound
• Seismic waves travel outward from the focus towards the Earth’s surface, causing the ground to shake

Question 6

What is an aftershock?

Answer 6

An earthquake triggered by stress transfer from a preceding earthquake

Question 7

What are the different types of shockwaves?

Answer 7

Body - P waves, S waves
Surface - L waves, R waves

Question 8

What are P (primary) waves?

Answer 8

• Fastest, reach surface first
• Travel through solids and liquids (mantle and core)
• Longitudinal
• Least damaging

Question 9

What are S (secondary) waves?

Answer 9

• Half as fast as P waves
• Reach surface 2nd
• Only travel through solids (mantle)
• Transverse
• More damaging than P waves

Question 10

What are L (love) waves?

Answer 10

• Slowest
• Cause most damage
• Transverse

Question 11

What are R (Rayleigh) waves?

Answer 11

• Longitudinal
• Radiate from the epicentre in a complicated low frequency rolling motion

Question 12

How is magnitude measured?

Answer 12

From the epicentre using seismographs

Question 13

What scales are there to measure earthquakes?

Answer 13

• Richter scale
• Moment magnitude scale
• Modified mercalli scale

Question 14

What is classified as a major earthquake?

Answer 14

Magnitude greater than 7

Question 15

Outline the frequency and regularity of earthquakes

Answer 15

• Low frequency EQs occur much more frequently than high magnitude
• 20,000 every year
• 55 per day

Question 16

Can earthquakes be predicted?

Answer 16

No, but plate tectonics show areas where EQs are most likely to occur

Question 17

What are signs before an earthquake?

Answer 17

• Microquakes, bulging of ground
• Decreasing radon gas concentration in groundwater
• Raise groundwater levels
• Electric and magnetic charges within local rocks
• Curious animal behaviour

Question 18

Outline liquefaction

Answer 18

Liquefaction is when saturated, loose soil temporarily loses its strength and behaves like a liquid

If soil is saturated, shaking causes the liquid to move into gaps between the soil grains - pore spaces. This causes the soil particles to lose contact with each other, decreasing the overall strength of the ground.At the same time, the water is responding to the increase pressure by trying to move up towards the surface. The sediments lose their structure and therefore strength, acting as a liquid when the ground begins to shake. Can cause buildings and infrastructure to collapse.

Question 19

What is a landslide (with example)?

Answer 19

The downward movement of soil and rock on a slope. Occurs due to intense shaking cause the collapse of material downhill.

eg. 2008 Sichaun Earthquake caused 60,000 landslides

Question 20

What is a tsunami?

Answer 20

Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea

Question 21

How are tsunamis formed?

Answer 21

• When oceanic crust is jolted during an earthquake, all of the water above the plate is displaced and forced upwards, creating a wave
• This water travels fast at a low amplitude
• As it approaches the coast, the sea level decreases so there is friction between the sea bed and waves
• This causes the waves to slow down and gain height

Question 22

Outline the 2011 Tohoku Earthquake and Tsunami

Answer 22

• Magnitude of 9.0
• Depth of 30km
• Occurred 400km off the North-East coast of Japan
• Tsunami waves reached up to 40m
• At destructive boundary where the Pacific Plate subducts the North American Plate

Question 23

What were the primary impacts of the 2011 Tohoku Earthquake and Tsunami?

Answer 23

• Over 15,000 deaths, over 90% due to drowning
• Over 1 million buildings damaged
• Sendai City was completed flooded, leaving many without electricity, water and communication systems
• $235 billion in economic losses -> most expensive natural disaster in history
• Saltwater damaged crops and soils

Question 24

What were the secondary impacts of the 2011 Tohoku Earthquake and Tsunami?

Answer 24

• 240,000 still living in temporary housing 3 years later
• 600 factories shut, causing Japan’s industrial production to fall by 15%
• Disabled cooling systems of 3 nuclear reactors, leading to core meltdowns -> 520km^2 of land around Fukushima was contaminated by radioactive fallout
• Liquefaction destroyed over 1000 buildings in Tokyo

Question 25

What were the immediate responses to the 2011 Tohoku Earthquake and Tsunami?

Answer 25

- 1.5 million food rations delivered in the first few days by the Japanese Red Cross society - 1 million households had access to water restored after 2 weeks - 470,000 evacuated in 2500 evacuation centres - 100,000 of the Japanese Self-Defence Forces deployed for search and rescue - Over 25 search and rescue teams from 20 countries - Over 380 medical teams

Question 26

What were the long-term responses to the 2011 Tohoku Earthquake and Tsunami?

Answer 26

- New housing developments constructed on higher ground to reduce future risk - Government launched a reconstruction programme worth over £200 billion -> over 400km of roads and railways rebuilt, 15 major ports reconstructed - New tsunami sea walls up to 15m high built along the northeast coast

Question 27

What is the potential mega-tsunami?

Answer 27

- Scientists said that an eruption of the Cumbre Vieja volcano in the Canary Islands cold cause a huge chunk of the volcano's western flank to collapse into the Atlantic - Could cause waves to reach Europe, Africa and the Carribean - However, scientists now believe this is unlikely

Question 28

Outline monitoring during the 2011 Tohoku Earthquake and Tsunami

Answer 28

- 11-12 GPS buoys which allowed authorities to update warnings quickly - 50 DART (Deep-ocean Assessment and Reporting of Tsunamis) Buoys helped track the tsunami across the Pacific - 1000+ seismometers nationwide gave immediate estimation of location, depth and magnitude

Question 29

Outline prediction during the 2011 Tohoku Earthquake and Tsunami

Answer 29

- JMA used a database of 100,000+ tsunami simulations but wave height was severely underestimated (5 vs 40m) - Long term hazard mapping identified the Japan Trench as high risk

Question 30

Outline protection during the 2011 Tohoku Earthquake and Tsunami

Answer 30

- Strict seismic building codes including flexible steel frames - 80% of urban residential buildings are now earthquake resistant - Sea walls overtopped

Question 31

Outline planning during the 2011 Tohoku Earthquake and Tsunami

Answer 31

- 80-90% of coastal municipalities had tsunami hazard maps - Regular drills on Disaster Prevention Day (1st September)