Global distribution of earthquakes
- The global distribution of tectonic hazards appears to be random, however mapping clearly shows that the main earthquake zones are found (often in clusters) along plate boundaries.
- About 70% of all earthquakes are found in the Pacific Ocean in the ‘Ring of Fire’.
- The most powerful earthquakes are associated with convergent or conservative boundaries, although rare intra-plate earthquakes can occur.
Global distribution of volcanoes
- Main volcanic zones are found on or near to plate boundaries, depending on the margin type
- Rare intra-plate volcanoes (hotspots) can occur near the middle of plates. These are associated with upwelling or mantle plumes
The three types of plate boundaries
Divergent (constructive): moving apart
Convergent (destructive): moving towards
Conservative: moving past
Plate-tectonic theory
The plate tectonic theory refers to the study of the lithosphere that has been broken up into parts known as tectonic plates. Their movement is driven by a number of different processes:
1) mantle convection: In mantle convection, heat produced by the decay of radioactive elements in the Earth’s core heats the lower mantle, creating convection currents. These hot, liquid magma currents are thought to move in circles in the asthenosphere- thus, causing the plates to move.
2) Slab pull: Newly formed oceanic crust at mid-ocean ridges becomes dense and thicker as it cools. This causes it to sink into the mantle under its own weight, pulling the rest of the plate further down with it. Sinking in one place leads to plates moving apart in other places.
3) Seafloor spreading: In the middle of many oceans are huge mid-ocean ridges, or underwater mountain ranges. These are formed when hot magma is forced up from the asthenosphere and hardens, forming new oceanic crust. This new crust pushes the tectonic plates apart in a process called seafloor spreading.
The process of an earthquake occurring
When plates move together, they stick and cause huge amounts of pressure to build up. When the pressure becomes too much, the rock fractures along cracks called faults and energy is released as seismic waves which causes the ground to shake.
Subduction zone
- At a convergent plate boundary where on plate moves down beneath another plate.
- It is common for the leading edge to lock under friction
Locked fault
This is a fault that is stuck. The frictional resistance is greater than the stress across the fault.
When the frictional resistance is overcome, the stores stress is eventually released as a large magnitude earthquake.
Benioff Zone
- This 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
Seismic waves
P waves (primary waves): These are vibrations caused by compression. They spread quickly from the fault. Only damaging in powerful earthquakes
S waves (secondary waves): These move more slowly. They vibrate at right angles to the direction of travel and cannot travel through liquids (unlike P waves). They do more damage than P waves
L waves (love waves): These are surface waves with the vibration occurring in the horizontal plain. They have a high amplitude
Primary hazards from earthquakes
- Ground shaking: This causes buildings, bridges, roads and infrastructure to collapse, killing and injuring people
- Crustal fracturing: When energy released during an earthquake is released causes the earths crust to crack leaving gaps
Secondary hazards from earthquakes
- Liquefaction: When loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking. Liquefaction occurring beneath buildings and other structures can cause major damage during earthquakes.
- Landslides: When an earthquake occurs, the transmission of seismic waves can cause shaking and vibration of ground surface. This often trigger the collapse of potential landslide areas.
- Avalanches: The forces induced by an earthquake can cause an increase in the load down the slope and can also decrease the shear strength and both effects can cause the release of an avalanche.
Liquefaction example- Loma Prieta earthquake (1989)
- A magnitude 6.9 earthquake struck near San Francisco. The city’s Marina district suffered some of the worst damage.
- The marina was built on man-made landfill, meaning the area’s soft, Sandy soils amplified the ground shaking, increasing the damage experience by buildings and other structures. In addition to this, the Sandy soil liquefied, causing buildings to collapse.
Landslide example- El Salvador (2001)
- In 2001, an earthquake occurred in El Salvador
- As a result, a landslide occurred in the town of Santa Telca in which 500 houses were buried, and 800 people were reported missing.
- In total, there were 10,000 landslides caused from this earthquake, resulting in damage to around 200,000 houses
Avalanche example- Ancash earthquake, Peru (1970)
- The deadliest avalanche in Recorded history was triggered by the Ancash earthquake, Peru.
- The rock-ice avalanche resulted in the deaths of 18,000 to 20,000 people.
Hotspots
The vast majority of volcanic eruptions occur near plate boundaries, but there are some exceptions: hotspot volcanoes.
- A hotspot is a constant source of heat known as a mantle plume.
- The plume rises from the hotspot through the mantle.
- As it reaches the upper mantle, the asthenosphere and base of the lithosphere melt.
- Magma then rises through weaknesses in the crust and erupts to form active volcanoes
- As the tectonic plate moves over the stationary hotspot, the volcanoes are rafted away and new ones are formed in their place above the hotspot
- As oceanic volcanoes move away from the hotspot, they cool and produce an island or seamount.
Over time, this can also create chains of volcanoes, such as the Hawaiian Islands.
There are theoretical frameworks that attempt to explain plate movements (continental drift evidence)
1) Jigsaw fit: the continents, notable South America and South Africa visually appear to have a similar pattern and ability to fit together
2) Biological evidence: such as the remains of the reptile Mesosaurus being found in Southern Africa and South America- it would not have evolved identically in two different locations, as well it not having the ability to swim across
3) Coal deposits: these require warm and wet climates, however deposits were found in Antarctica, therefore it must have been closer to the equator at some point
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