HAZARDS: EARTHQUAKE

What are earthquakes?

• Shaking or trembling of the ground caused by sudden release of energy.

• Usually associated with faulting/breaking of rocks.

• Continuing adjustment of position = aftershocks

What release of energy causes seismic shock waves?

Represent release of stress - built up within Earth’s crust, caused by tension, compression or shearing of rock.

What happens in 2 specific areas of the earthquake?

1. Series of seismic shock waves originate from earthquake focus - where stress suddenly released.

2. Epicentre = immediately above the focus at Earth’s surface that’s closest.

What can happen before an earthquake?

• Series of foreshocks.

• If foreshocks have same S and P wave profiles - may signal a large event, allow few hours to warn population.

What can happen after an earthquake?

• Series of aftershocks

• Follows main quake

• Gradually reduces in intensity.

4 locations earthquake activity tends to be concentrated?

1. Mid-Ocean ridges

2. Ocean trenches & Island Arcs

3. Collision Zones

4. Conservative Plate Margins

1. Mid-Ocean ridges

• Tensional forces associated with spreading processes.

  + subsequent faulting & rifting

2. Ocean trenches & Island Arcs

Compressive forces associated with subduction of one plate below another.

3. Collision Zones

Compressive forces associated with intermittent plates carrying continental crust.

4. Conservative Plate Margins

Shearing forces associated with intermittent movement of one plate past another.

What is the Elastic Rebound Theory?

Elastic Rebound Theory:

1. Explains how energy is stored in rocks.

2. Rocks bend until strength of rock is exceeded

3. Rupture & rocks quickly rebound to an under deformed shape.

4. Energy released in waves - radiate outwards from fault.

What are the 2 types of earthquake?

1. Shallow focus

2. Deep focus

Shallow focus?

• Surface down to about 70km.

• In cold, brittle rocks - from fracturing of rock due to stress with crust.

• Very common - most release only low levels of energy.

• Other high-energy shallow quakes, capable of severe impacts.

Deep Focus?

• 70-700km

• Poorly understood - increasing depth, pressure & temps increase to v high levels.

• Minerals change type & volume - contributes to release of energy.

What plays a significant role in deep focus earthquake events?

• Dehydration of water in subducting planes - plays important role in deep focus events.

• But scientists continue to evolve ideas about less frequent, but powerful quakes.

Where do most earthquakes occur?

80% in the circum-Pacific belt

What do most earthquake in the circum-Pacific belt result from?

Convergent margin activity

Where do some other earthquakes occur?

15% in Mediterranean-Asiatic belt

Where do few earthquakes occur?

5% in interiors of plates & on spreading ridge centres.

Figures for number of earthquakes a year?

Over 150,000 quakes - strong enough to be felt, are recorded every year.

2 types of seismic waves?

1. Body waves

2. Surface waves

2 types of body wave?

1. P (primary) waves

2. S (secondary) waves

P waves?

• Fasest

• Through solid, liquids & gases.

• Compressional wave.

• Material movement = same direction as wave movement.

• Longitudinal

S waves?

• Slower than P waves.

• Through solids ONLY.

• Shear movement - transverse waves.

• Material moved perpendicular to wave.

2 types of surface waves?

1. Rayleigh

2. Love

Rayleigh?

• Slow along the ground.

• Low-frequency

• Some: strong, rolling movement - moves surface vertically.

• Others: move ground at right angles to direction of movement.

• More destructive than love waves.

Love?

• Just below or along ground

• V damaging to buildings.

• Side-to-side movement - like transverse but amplitude is greater.

What secondary impacts does this have?

Serious implications for public water supplies & irrigation for agriculture.

Liquefaction?

• Earthquake strikes an area with surface materials of fine-grained sands, alluvium & landfill with high-water content.

• Vibrations cause materials to behave like liquids.

What happens as a result to these materials?

• Lose their strength

• Structures tilt & sink as their foundations give way.

Example of where liquefaction was a major issue?

• Kobe (1995)

• Much of port had been built on reclaimed land in Osaka Bay.

• Just under 200 berths in port were destroyed.

• Affected Japanese economy & trade globally.

How are landslides & avalanches caused?

Ground shaking & liquefaction can cause slope failure.

Which regions are particularly vulnerable?

• Steep slopes in mountainous regions

• E.g. Himalaya-Karakoram Range = unstable & vulnerable to landslides.

Example of earthquake that triggered large numbers of landslides & avalanches?

Nepal (2015) - caused by ground shaking.

Recent example of earthquake causing landslides?

• Taiwan (April 2024)

• 7.4 magnitude

• Strongest recorded earthquake in 25 years there.

• Tremors triggered landslides - 9 died.

How can landslides/avalanches affect water?

• Movement of soil & rock on slops can block rivers.

• Natural dams create temp. lakes.

• Can threaten areas downstream with catastrophic floods.

2 examples of where this happened?

1. Kashmir (2005)

2. Sichuan (2008)

Why are upland valleys often favoured sites for reservoirs?

• If an earthquake creates a landslide on slopes above reservoir

• Displacement of water & waves generated could weaken & overtop the dam.

Example of where this happened?

• Northern Italy (1963)

• Collapse of hillside above Vaiont reservoir.

• Generated 100m wave.

• Swept over dam & down valley of Piave River.

• Drowned nearly 3,000 ppl.

What is a tsunami?

• Underwater earthquake

• Causes seabed to rise vertically

• Displaces water above

• Produces powerful waves at surface - spreads out a high velocity from epicentre.

How can a tsunami wave pass under a ship out at sea unnoticed?

• Low wave height = <1m

• Very long wavelength = up to 200km

What happens as a tsunami wave approaches the shore?

Wave height increases greatly as they enter shallow water

What happens before the wave breaks?

• Water in front of wave is pulled back out to sea = drawdown.

• Wave rushes in as a wall of water - can exceed 25m in height.

Example of tsunami?

• Dec 2004: Aceh Province, Sumatra.

• Estimated 1000 tonnes of water, per metre of shoreline.

How is local height of a tsunami affected?

• By shape of seabed & coastline

• Depends on relief of coastal zone

• Tsunamis can spread variable distances inland.

What can be caused underwater?

• Underwater landslides

• Can displace water & create tsunami wave

• Caused by earthquakes

How does an underwater landslide occur?

• A large volume of rock is shaken & slides downslope.

• Water is dragged in behind it from all sides - collides in the centre.

• Can generate a tsunami wave - radiates outwards.

Example of devastating local effects?

• 1998: 2,000 villages in coastal communities in Papau New Guinea.

• Killed by local tsunami generated from underwater landslide - earthquake.

What makes these events so hazardous?

Events are local = short warning times