Geohazards 1 - earthquakes

Geohazard

A geological condition that is dangerous or potentially so to people and the ennvironment

Epicentre

Point on the surface directly above the focus

Focus

The point of dislocation of the rocks

Fault plane

The surface along which the motion occurs

Fault scarp

Cliff created by movement along a fault

Subduction zone boundary

Reverse faults, denser oceanic plate subducted

Why do most earthquakes occur nearer the surface? (Subduction Benioff zone)

Energy dissipates at deeper depths so doesn't reach the surface

Continental collision boundary

thrust faults, earthquakes occur shallower where plates meet

Divergent boundary

Shallow focus where plates move apart

Transform fault boundary

Eg. San Andreas, shallow focus, occur near surface

At which depth do earthquakes cause the most damage?

Shallow and intermediate since there is less rock to travel through so maintains energy

Why doesn't the Mariana trench have any deep focus earthquakes?

The plate isn't fully solid at this depth so there isn't much friction between plates

P waves

Longitudinal, travel fastest, least damage

S waves

Transverse, medium speed, moderate damage

Surface waves

Love and Rayleigh, roll, slowest speed, most damage

Moment magnitude

Combine seismograph recordings and physical displacement, force needed to generate these sizes of waves

Moment magnitude equation

Mo \= rock rigidity x displacement x area ruptured

Attenuation

Loss of energy in a wave as it travels from the focus to the surface

Competent rocks

Deform only under great stresses

Incompetent rocks

Deform under moderate to low stresses

More displacement

Incompetent rocks so less attenuation so more movement

Less displacement

Competent rocks so more attenuation so less movement

Relationship between moment magnitude, energy released and frequency

As moment magnitude increases, the energy released increases but the frequency decreases

Why are large earthquakes infrequent?

Partial melting occurs at deep depths which causes slow dissipation of the energy rather than releasing all of it at once

Earthquake size

Larger magnitude, more damage

Focus depth

Deeper focus, less damage due to increased attenuation

Distance from epicentre

Further away, less damage due to increased attenuation

Local ground conditions

Saturated ground, more damage due to less attenuation

Topography

Low lying area, more damage and risk, eg. tsunami

Time of day/week/year

More damage - rush hour, night time, where people are congregated, winter

building standards

Are buildings earthquake proof or made of mud?

Population and building density

Higher density, more damage

Liquefaction

Occurs when wet soil acts more like a liquid during an earthquake

When is liquefaction destructive?

When accompanied with ground displacement or failure

Frequency of a wave

The number of complete waves that pass a point per second

Period of a wave

The time taken for one wavelength to pass a given point

Natural resonance of a building

The time taken for a building to rock back and forth

How do tsunamis form?

Can be triggered by earthquakes, landslides and volcanoes as energy and debris is displaced, disrupting the water

Why are most deaths close to the source of a tsunami?

The wave has the most energy, higher amplitude and there is little warning time

Why is it difficult to detect tsunamis in open water?

The wave has a long wavelength and small amplitude