Unit 1, Chapter 1: Locations at risk from tectonic hazards

The global distribution of tectonic hazards

Earthquakes

Volcanoes

It is far from random. Earthquakes are found in clusters along plate boundaries.

FACT: About 70% of all earthquakes are found in the 'Ring of Fire' in the Pacific Ocean.

The most powerful earthquakes are associated with convergent and transform (conservative) plate boundaries, although rare intra-plate earthquakes can occur.

Pattern of tectonic activity

Continental Fracture Zone (CFZ) - activity following the mountain ranges from Spain, via the alps etc.

Scattered earthquakes in continental interiors - activity along old fault lines, associated with the reactivation of this weakness.

Oceanic Fracture Zone (OFZ) - activity along mid-ocean ridges.

The majority of volcanoes are found along both convergent and divergent plate boundaries and there is some intra-plate activity in hotspot places such as Hawaii.

Plate boundary types and their distribution

Divergent (constructive) margins - most clearly displaced at mid-ocean ridges. Large numbers of shallow focus and low magnitude. Most are submarine (under the sea).

Convergent - actively deforming collision locations with plate material melting in the mantle, causing frequent earthquakes and volcanoes.

Conservative (transform) - lithosphere is neither created nor subducted. Does not result in volcanic activity, however,they are the sites of extensive shallow focus earthquakes.

Plate movement and earthquake type

Divergent - earthquakes tend to be frequent, small and typically a low hazard risk due to their geographical position.

Transform - can present more risk when earthquakes occur.

Convergent - most damaging earthquakes. As strain builds over time in the subduction zone, the friction between the two masses of rock is overcome.

Plate movement and volcanic activity

Destructive plate boundaries - form either a subduction zone or a continental collision.

Divergent boundaries - create rift volcanoes where plates diverge from one another at the site of a thermally buoyant mid-ocean ridge. Less explosive, more effusive.

Hotspot volcanoes - found in the middle of tectonic plates. Thought to be fed by underlying mantle plumes that are unusually hot compared with the surrounding mantle.

Hotspot volcanoes and mantle plumes

The presence of a hotspot is inferred by anomalous volcanism, such as the Hawaiian volcanoes within the Pacific Plate.

Volcanic hotspot: area in the mantle from which heat rises as a hot thermal plume from deep in the Earth.

As tectonic plates move over the stationary hotspot, the volcanoes are rafted away and new ones form in their places.

As oceanic volcanoes move away from the hotspot, they cool and subside, producing other islands.

What causes tectonic plates to move?

Convection current - heat is produced by the decay of radioactive elements in the Earth's core heats the lower mantle. As the magma is heated, it rises and this creates convection currents which move in a cellular motion in the asthenosphere, causing the plates above them to move.

Seafloor spreading - huge mid-ocean ridges formed when hot magma escapes from the mantle and hardens. New crust pushes tectonic plates apart.

Subduction - two plates collide, the denser plate subducts into the mantle where it melts in a subduction zone. This creates the mechanism of slab pull.

Slab pull - denser oceanic plate subducts under mantle. Slab pull is the pull of gravity on a plate that is being subducted.

Paleomagnetism - results from the zone of magma 'locking in' or 'striking' the Earth's magnetic polarity when it cools.

Different plate margins

Convergent (also known as destructive/collision) - where two plates collide/are moving towards each other.

Divergent (also known as constructive) - where two plates are moving apart.

Transform (also known as conservative) - where two plates and sliding past each other, they can either be going in different directions or the same direction but different speeds.

The Benioff Zone and subduction processes

The Benioff Zone is an area of seismicity corresponding with the slab being thrust downwards in a subduction zone. The different speeds and movements of rock at this point produce numerous earthquakes.

This theoretical framework is an important factor in determining earthquake magnitude, since it determines the position and depth of the hypocentre.

Earthquakes, crustal fracturing and ground shaking

Faults are zones of pre-existing weakness in the Earth's crust.

A sequence of events occurs in the generation of an earthquake...

  1. When the pressure exceeds the strength of the fault, the rock fractures.
  1. This produces the sudden release of energy, creating seismic waves that radiate away from the point of fracture.
  1. The movements are preceded by a gradual build-up of tectonic strain, which stores elastic energy in crustal rocks.
  1. The brittle crust rebounds either side of the fracture, which is the ground shaking.

Seismic waves

A seismometer measures the amount of ground shaking during an earthquake, recording both the vertical and horizontal movements of the ground. Analysis of the data shows that an earthquake produces different seismic waves.

Primary (P) waves - vibration by compression. Spread quickly from the fault at around 8 km/sec.

Secondary (S) waves - move slower, however, at around 4 km/sec. Vibrations are at right angles to the direction of travel and cannot travel through liquids.

Love (L) waves or Q waves - surface waves with the vibration occurring in the horizontal plain. High amplitude.

Secondary hazards of earthquakes: liquefaction and landslides

Liquefaction can cause buildings to settle, tilt and eventually collapse in the most serious of events. In some earthquakes, tilts of up to 60 degrees have been recorded.

Landslides can be major secondary impacts.They rarely occur when the earthquake magnitude is less than 4, however, a high magnitude means landslides which are hazardous to people and property for miles from their source.