Seismicty- occurance / frequency of earthquakes in a region.
Seismic waves- shock waves released by the rupture of rock strata at the focus of an earthquake. Travel through rock and are recorded on a seismograph.
Focus- The point at which pressure released occurs within the crust,
Epicentre- Point immediately above the focus on the earth's surface.
Causes of earthquake:
- Movement of plate tectonics (occurs at all margins). Build up of pressure at margin, earthquake caused when they suddenly jerk past each other, releasing pressure = sends out seismic waves.
3 depths of earthquake:
1) Shallow- 0-70km deep, lots of damage, amount for 75% of all earthquake energy
2) Intermediate- 70-300km deep
3)Deep- 300-700km deep.
The nearer the focus is to the surface of the earth the more damage it does.
- Reactivation of old fault lines that have not been active for a long time.
- Subsidence as a result of deep mining
- Pressure of surface rocks from water in large resevoirs.
3 Types of seismic waves
Primary (P) Waves:
- Fastest wave, compressional (compress and bump into each othe, sending wave further forward in movement direction)
- Push and pull earth in same direction they are travelling
- Can travel through solids and liquids - fastest
Secondary (S) Wave:
- Travel at 1/2 speed of P waves
- Vibrates at right angles (90o) creating a shearing motion, causing high amount of damage.
- Travel through solids due to right angle movement, unable to move through liquids because the particles are not chemically bound together so when one is moved the others do not necessarily move.
P and S waves are known as BODY WAVES, can only travel through interior of earth, arrive between surface waves. Higher frequencies than surface waves.
- Only travel near the earth's surface
- Slowest, positined near ground surface, travel through crust.
- Split into LOVE and RAYLEIGH waves
- Side to side motion creating a shearing effect.
- Can go through solids.
- Rolling motion
- Travels through liquids and solids.
Seismic waves are measured on a seismograph.
Magnitude and frequency of earthquakes
- Magnitude refers to the strength of the earthquake / amount of energy released.
- Measured by using a seismometer- measure magnitude, duration and direction of vibrations.
- 2 Scales on which earthquakes are measure:
1- Richter scale:
- Measures the magnitude, calculated using info from seismograph (records motion of ground during earthquake)
- Logarithmic scale, tenfold increase between each scale, each vaue represents approximately 30x more energy released than previous.
- Value is decided upon striaght away.
- Limitations- magnitude over 8.0 cannot be measured. There is also no upper limit, measurement cannot be precise.
2- Mercalli Scale:
- Intensity of event & impacts measure- using observations of event and photos.
- On scale of 1 to 12. On scale 8 is heavy structural damage. 3 is felt by people indoors
Primary effects of earthquakes:
- Initially causes groundshaking.
- This is dependent on:
1- distance from epicentre
2- surface material type
4- depth of focus
Secondary effects of earthquakes:
- Soil liquifaction- when shaken violently at high water content, soil will lose mechanical strength and behaves like liquid. Happened in Mexico City.
- Landslides / avalanches- Slope failure as a result of groundshaking.
- People and environment - collased buildings, road systems destroyed, communications and services destroyed (gas, water, electricty) disease, food shortages, disrupts local economy.
- Tsunami- great sea waves created by shallow focu underwater earthquakes.