The causes and main characteristics of earthquakes

Question 1

What is an earthquake?

Answer 1

Stress within rocks - pressure builds up.

Pressure is released - parts of the surface experience an intense shaking motion.

Question 2

Where do earthquakes tend to be located? 4

Answer 2

1. 90% at destructive margins - oceanic plate subducts under the continental plate.
2. Conservative e.g. San Andreas Fault line.
   North American plate and Pacific plate snag.
3. Constructive - magma movement between plates.
4. Collisional margins (C/C) - pressures generated by collision.
   50,000 occurring every year around the globe.
   Highly unpredictable/occur suddenly without warning.

Question 3

What is the focus?

Answer 3

The point of pressure release within the crust.

Question 4

What is the epicentre?

Answer 4

The point of pressure release immediately above the Earth’s surface.

Question 5

How are earthquakes caused?

Answer 5

Stresses build up between the plates as one plate passes another.
Grinding plates - energy build up.
When the plates eventually move again - energy released as seismic waves.

Question 6

What are the 3 broad categories of earthquakes?

Answer 6

Shallow-focus (0-70km deep) - greatest damage.
Accounts for 75% of all earthquake energy released.
Intermediate-focus (70-300km deep).
Deep-focus (300-700km deep).

Question 7

What are the different types of seismic waves?

Answer 7

Primary (P).
Secondary (S).
Surface (L).
P and S waves can be referred to as BODY WAVES.
P and S waves travel through the interior of the Earth and are recorded on a seismograph.
Studying them has helped built up a picture of the interior of the Earth.

Question 8

What are the characteristics of P waves?

Answer 8

Fastest/compressional - vibrates in the direction in which they are travelling.
Will shake the ground UP, as well as causing back and forth movement.
Weakest waves.

Question 9

What are the characteristics of S/transverse waves?

Answer 9

Travel at half the speed of P waves - slower.
Crust movement from side to side at right angles to the outward motion of the main wave.
Causes the most damage.

Question 10

What are the characteristics of L waves?

Answer 10

Love waves and Rayleigh waves travel along the surface only - travel the slowest.
Some surface waves shake the ground at right angles to the direction of wave movement.
Some have a rolling motion - vertical ground movement.

Question 11

How could human activity cause earthquakes?

Answer 11

Large reservoirs - water puts pressure on surface rocks.

Subsidence (caving in) of deep mine workings.

Question 12

What is the Mercalli scale?

Answer 12

Measures earthquake effects.
Runs from 1 to 12.
The higher up the scale - more damage.

Question 13

What is the Richter scale?

Answer 13

Logarithmic scale - x10 every step up the scale.
Runs from 1 to 10.
The higher the magnitude of an earthquake the less frequent its occurrence.
Earthquake monitoring only dates back to 1848.

Question 14

What were the largest recorded earthquakes?

Answer 14

The largest ever recorded was in Valvidia in Chile in 1960 and recorded 9.5 on the scale.
UK, Dudley - 4.8.

Question 15

What is the primary effect of earthquakes?

Answer 15

Ground shaking.
Severity = depends on the magnitude, the distance from the epicentre and local geological conditions.
E.g. Mexico City 1985 - seismic waves were amplified by the lake sediments on which the city was built.

Question 16

What are the secondary effects of earthquakes? 4

Answer 16

1. Soil liquefaction - when violently shaken, soils with high water content lose strength and behave like a fluid.
2. Landslides/avalanches - slope failure.
3. Effects on people/built environment e.g. collapsing buildings.
4. Tsunamis.

Question 17

Why does earthquake damage vary? 5

Answer 17

1. Population density.
2. Earthquake depth - deeper the waves, the less damage is caused (loss of energy deeper down).
3. Building design - building modification limits impacts.
4. Geology - solid rock limits impacts. Sands/clay result in bigger impacts - liquefaction of clay results in sinking.
5. Earthquake strength.

Question 18

What are the characteristics of a tsunami?

Answer 18

Open ocean - very long wavelength - 100km.
Low wave height (under 1m).
Travel very quickly - 700kmh-1.
Shallow water - increase in height.
If the trough (bottom wave bit) arrives before the wave, there may be a drawdown of the coastline - exposes the ocean floor.
Wave amplitude.
5-10 mins before the actual tsunami arrives.

Question 19

What are the warning signs of a tsunami?

Answer 19

Wave trough in front of the tsunami - reduction in sea level - drawdown.
Behind this comes the actual tsunami - can reach 25m+.
Consists of a number of waves, the largest not necessarily being the first.

Question 20

What causes tsunamis?

Answer 20

Landslides, underwater volcanoes, underwater earthquakes.

E.g. The Boxing Day tsunami of 2004, Indian Ocean - created by an underwater earthquake.

Question 21

What factors affect the impact of tsunamis? 5

Answer 21

1. The height/wave distance.
2. The length of the causal event.
3. The extent of warnings.
4. Coastal physical geography.
5. Coastal land use/population density.

Question 22

How are tsunamis created?

Answer 22

EARTHQUAKE EXPLANATION
Pressure becomes too much - upper plate snaps back – causing the earthquake and tsunami.
Most plate deformation/movement - horizontal.
Shockwaves through the ocean water - initiation.
Upward sea bed movement displaces billions of tonnes of water above it.
Some sea floor falls - water rushes to replace it.
Uplifted water collapses - rushes outwards at a thousand km per hour.

Question 23

What are the general impacts of tsunamis?

Answer 23

Boats washed inland - backwash carry them out to sea.
Drowning/injuries from water/debris.
Most effects are felt 500/600m inland.
Buildings, roads, bridges, trees and soil are washed away.

Question 24

What is a famous tsunami (small) example?

Answer 24

Tsunamis from the Krakatoa in 1883.
Drowned 35,000.
Produced waves that traveled around the world - highest 40m.

Question 25

Where do most tsunamis occur?

Answer 25

90% are generated within the Pacific basin.
Associated with the tectonic activity around the edges.
Most are at destructive plate boundaries - subduction.
E.g. Japan-Taiwan island arc (25% of occurrences).