OCR GCSE Geography B (J384) — Global Hazards
Have you ever tried to slide a heavy piece of furniture across a carpet, only for it to get stuck and then suddenly jerk forward? Tectonic plates experience a very similar process on a massive scale. Their movement is driven by mantle convection, Slab Pull, and Ridge Push, but the edges of these plates are rough and uneven.
As the plates attempt to slide past or beneath one another, they frequently become locked together due to immense friction. Because the driving forces keep pushing the plates, they cannot move, causing severe pressure and stress to build up within the rock over time.
Eventually, this accumulated stress exceeds the rock's strength or the frictional grip holding the plates together. The rock fractures and the plates suddenly slip or "jolt" free. This sudden movement instantly releases the stored potential energy as vibrations called seismic waves, which radiate outwards in all directions.
When an earthquake hits the news, reporters always mention the city where it struck, but the true origin of the quake is hidden deep underground. The exact point inside the Earth's crust where the rock first fractures and releases energy is called the focus (or hypocentre).
The point on the Earth's surface located directly above the focus is known as the epicentre. Because it is closest to the origin, the epicentre is usually where the shaking is most intense and where seismic waves reach the surface first.
As these seismic waves travel outward from the focus, they are detected by sensitive instruments called seismometers. The intensity of the shaking generally decreases as you move further away from the epicentre, because the waves lose energy as they travel.
Why do some moderate earthquakes flatten entire cities, while far more powerful ones barely rattle the teacups? The answer lies in the depth of the focus and a process called attenuation, which is the loss of wave energy over distance.
When an earthquake occurs deep underground, the seismic waves must travel through thick layers of rock and mantle to reach the surface, dissipating much of their energy along the way. Below is a comparison of the two main types of earthquakes:
| Feature | Shallow Focus Earthquakes | Deep Focus Earthquakes |
|---|---|---|
| Depth | Occur between 0 and 70 km below the surface. | Occur between 70 and 700 km below the surface. |
| Location | Found at all plate boundaries (constructive, convergent, conservative, collision). | Found almost exclusively at subduction zones (convergent boundaries). |
| Wave Path | Seismic waves tend to spread horizontally. | Seismic waves tend to travel more vertically toward the surface. |
| Surface Impact | Typically highly destructive because there is less distance for attenuation to occur. | Typically cause less damage at the epicentre due to high attenuation, but can be felt over a wider area. |
At convergent boundaries where oceanic crust sinks into the mantle, deep earthquakes occur along a specific feature known as the Benioff Zone. This is a slanted, inclined plane of earthquake foci that directly maps the downward path of the subducting tectonic plate.
Students often confuse the focus and the epicentre; remember that the focus is deep inside the Earth's crust, while the epicentre is directly above it on the surface.
In 4 or 6-mark 'Explain' questions about earthquake causes, you will not get full marks for simply stating 'plates move'. You must explicitly detail the build-up of pressure due to friction and the sudden slip/jolt that releases energy.
When explaining why a shallow focus earthquake is more hazardous than a deeper one of the same magnitude, always use the keyword 'attenuation' or explain that the seismic waves travel a shorter distance, so less energy is lost.
OCR examiners prefer the term 'convergent plate boundary' when describing plates moving towards one another, rather than 'destructive'.
Slab Pull
The force exerted by the weight of a subducting tectonic plate dragging the rest of the plate behind it.
Ridge Push
The force of new crust forming at constructive boundaries, pushing older tectonic plates apart.
Friction
The resistance encountered when one plate surface moves over another, causing plates to lock together and build up pressure.
Seismic waves
Pulses of energy that travel through the Earth's layers, radiating outwards from the focus of an earthquake.
Focus
The exact point inside the Earth's crust where the rock first fractures and seismic energy is released.
Epicentre
The point on the Earth's surface directly above the focus, usually experiencing the most intense shaking.
Seismometers
Scientific instruments used to detect, measure, and record seismic waves.
Attenuation
The reduction in the strength and energy of seismic waves as they travel further away from the focus through rock.
Subduction zones
Areas at convergent boundaries where a denser oceanic plate is forced downwards into the Earth's mantle.
Benioff Zone
A planar, inclined zone of earthquake foci that follows the path of a subducting oceanic plate into the mantle.
Put your knowledge into practice — try past paper questions for Geography B
Slab Pull
The force exerted by the weight of a subducting tectonic plate dragging the rest of the plate behind it.
Ridge Push
The force of new crust forming at constructive boundaries, pushing older tectonic plates apart.
Friction
The resistance encountered when one plate surface moves over another, causing plates to lock together and build up pressure.
Seismic waves
Pulses of energy that travel through the Earth's layers, radiating outwards from the focus of an earthquake.
Focus
The exact point inside the Earth's crust where the rock first fractures and seismic energy is released.
Epicentre
The point on the Earth's surface directly above the focus, usually experiencing the most intense shaking.
Seismometers
Scientific instruments used to detect, measure, and record seismic waves.
Attenuation
The reduction in the strength and energy of seismic waves as they travel further away from the focus through rock.
Subduction zones
Areas at convergent boundaries where a denser oceanic plate is forced downwards into the Earth's mantle.
Benioff Zone
A planar, inclined zone of earthquake foci that follows the path of a subducting oceanic plate into the mantle.
