The causes and main characteristics of earthquakes Flashcards
What is an earthquake?
Stress within rocks - pressure builds up.
Pressure is released - parts of the surface experience an intense shaking motion.
Where do earthquakes tend to be located? 4
- 90% at destructive margins - oceanic plate subducts under the continental plate.
- Conservative e.g. San Andreas Fault line.
North American plate and Pacific plate snag. - Constructive - magma movement between plates.
- Collisional margins (C/C) - pressures generated by collision.
50,000 occurring every year around the globe.
Highly unpredictable/occur suddenly without warning.
What is the focus?
The point of pressure release within the crust.
What is the epicentre?
The point of pressure release immediately above the Earth’s surface.
How are earthquakes caused?
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.
What are the 3 broad categories of earthquakes?
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).
What are the different types of seismic waves?
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.
What are the characteristics of P waves?
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.
What are the characteristics of S/transverse waves?
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.
What are the characteristics of L waves?
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.
How could human activity cause earthquakes?
Large reservoirs - water puts pressure on surface rocks.
Subsidence (caving in) of deep mine workings.
What is the Mercalli scale?
Measures earthquake effects.
Runs from 1 to 12.
The higher up the scale - more damage.
What is the Richter scale?
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.
What were the largest recorded earthquakes?
The largest ever recorded was in Valvidia in Chile in 1960 and recorded 9.5 on the scale.
UK, Dudley - 4.8.
What is the primary effect of earthquakes?
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.
What are the secondary effects of earthquakes? 4
- Soil liquefaction - when violently shaken, soils with high water content lose strength and behave like a fluid.
- Landslides/avalanches - slope failure.
- Effects on people/built environment e.g. collapsing buildings.
- Tsunamis.
Why does earthquake damage vary? 5
- Population density.
- Earthquake depth - deeper the waves, the less damage is caused (loss of energy deeper down).
- Building design - building modification limits impacts.
- Geology - solid rock limits impacts. Sands/clay result in bigger impacts - liquefaction of clay results in sinking.
- Earthquake strength.
What are the characteristics of a tsunami?
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.
What are the warning signs of a tsunami?
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.
What causes tsunamis?
Landslides, underwater volcanoes, underwater earthquakes.
E.g. The Boxing Day tsunami of 2004, Indian Ocean - created by an underwater earthquake.
What factors affect the impact of tsunamis? 5
- The height/wave distance.
- The length of the causal event.
- The extent of warnings.
- Coastal physical geography.
- Coastal land use/population density.
How are tsunamis created?
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.
What are the general impacts of tsunamis?
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.
What is a famous tsunami (small) example?
Tsunamis from the Krakatoa in 1883.
Drowned 35,000.
Produced waves that traveled around the world - highest 40m.
