geology - geohazards ⛏️👷🏽🏔️

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Geohazard

Geohazard

a geological condition that is dangerous or potentially dangerous to the environment and the people who live within it

Geohazards can be…

• natural or artificial

• long term or short term

• large scale or small scale

• frequent or infrequent

Examples of geohazards

• landslides/mudslide/avalanche

• sinkhole

• flooding/tsunami

• volcanoes/earthquakes

• drilling/fracking in over-pressurized land

Focus

• The point within the Earth where the earthquake originates

• It occurs alongside a fault plane as one section of crust moves alongside another

• Earthquakes do not actually originate from a single point but it is more convenient to think of it that way

• Seismic waves radiate out from the focus

Depth of focus

Earthquakes are classified by their depth of focus…

• Shallow focus  0 – 70 km

• Intermediate focus  70 – 300 km

• Deep focus  300 – 700 km

Shallow focus

0 - 70km

Intermediate focus

70 - 300km

Deep focus

300 - 700km

Why do earthquakes not occur deeper than 700km?

Earthquakes do not originate at greater depths because the greater temperatures lead to rocks behaving in a plastic rather than brittle manner

Epicentre

• The point on the Earths surface directly above the focus

• The epicentre is marked on surface maps to represent the position of the focus

• The epicentre is where the greatest amount of damage is likely to occur

How do earthquakes occur?

1. Earthquakes can be dangerous because of the physical effects produced by ground acceleration

2. Relative movement of the bedrock on either side of a fault applies stress to the rock in the fault zone which undergoes strain.

3. This process transfers energy and increases the elastic strain energy stored in the rock

4. When the fault suddenly ruptures, elastic strain energy is reduced and the energy is released as movement on either side of the fault, as heat and as seismic waves

5. The energy transferred by P, S and L waves is around 1% of the work done by the earthquake

6. The greater the energy, the greater the amplitude of the earthquake waves

Wave terminology

Amplitude

The maximum extent of an oscillation, measured from the position of rest (how high a peak measures)

Wave-length

How long a wave measures from peak-to-peak

Attenuation

The loss of energy experienced by a wave shown as a reduction in amplitude as it propagates through a material

Geometric Dispersion

What happens when pebble is tossed into a pond of water? Ripples spread out in circles

As the ripples increase in diameter, they spread out along a greater length

What happens to the amplitude of the waves? They get smaller

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The amplitude also dissipates by attenuation

As the wave propagates through the rock, some elastic energy is transferred, near grain boundaries by friction and is transferred to thermal energy

Scattering of the waves by the grains accounts for some dissipation

The wave eventually disappears altogether

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Competent rocks such as granite or limestone allow these vibrations to pass through them easily and attenuation is negligible

Weaker rocks such as clay or poorly cemented sandstone absorb some energy

Unconsolidated sands and gravels vibrate and move easily and attenuation may be ten times greater than in competent rocks

What do we use to measure seismic waves?

• The amplitude of the wave (and therefore the amplitude shown on the seismogram) depends on the energy

• Seismologists assess the amount of energy (E) released by the earthquake by measuring the maximum amplitude recorded on the seismogram

• This is converted to a magnitude (M) scale to indicate the size of the earthquake

Energy

• Energy is measured in joules (the energy transferred when a force of one newton acts through a distance of one metre)

• For earthquakes this is a very small unit - a very small earthquake may transfer 10⁹ joules of energy

• On a logarithmic scale, this value for E is shown as 9, a much easier number to deal with

Now a number on a scale with no units…

• Early magnitude scales were empirical, based on taking many readings from many earthquakes

• A general relationship was established and used to define the term magnitude

Fault geometry and seismic moment (Mo)

Magnitude scales are based on observation rather than maths. They do not measure a particular property of the earthquake. In 1979 a new scale, moment magnitude (Mw) was introduced

= Gives the most reliable magnitude for very large earthquakes because it is derived from factors that can be measured:

• Size of fault rupture

• Amount of displacement

• Energy released

Seismic moment