Edexcel GCSE Geography B (1GB0) — Hazardous Earth
Understanding why a smaller earthquake can completely destroy a city while a larger one leaves it barely shaken comes down to one crucial measurement: depth. Earthquakes originate at a specific point within the Earth's crust known as the focus (hypocentre). The point on the Earth's surface directly above this break is the epicentre, which is typically where the most intense ground shaking is felt.
The focal depth is the distance below the surface where the earthquake occurs, and it drastically alters the resulting hazard. Shallow-focus earthquakes occur between 0–70 km below ground. These are highly destructive because the seismic waves travel a much shorter distance to the surface, meaning they lose very little energy and retain a high amplitude when they hit buildings and infrastructure.
In contrast, deep-focus earthquakes occur between 70–700 km deep, often within subduction regions known as the Wadati-Benioff zone. These events are generally less damaging. As the seismic waves travel through a massive volume of rock to reach the surface, their energy dissipates, resulting in weaker ground shaking.
It is easy to assume that a magnitude 7 earthquake is simply one step stronger than a magnitude 6, much like turning up a volume dial. However, the scientific reality of earthquake measurement is far more extreme. Magnitude is a quantitative measure of the energy released by an earthquake, recorded using a highly sensitive instrument called a seismometer.
The Richter Scale is used to compare the magnitude of different earthquakes. It is a logarithmic scale (base-10), meaning that it is not linear. Each whole number increase on the scale represents a 10-fold increase in the amplitude (the physical height of the ground shaking) and approximately a 32-fold increase in the total energy released at the focus.
While the Richter Scale is commonly referenced, scientists now frequently use the Moment Magnitude Scale (MMS) for larger events, which calculates size based on the area of the fault that slipped. Both of these scientific scales differ entirely from Intensity (Mercalli Scale), which measures qualitative, observable damage rather than energy.
| Feature | Magnitude 5.0 | Magnitude 7.0 | Comparison Factor |
|---|---|---|---|
| Scale Value | 5.0 | 7.0 | + 2.0 whole units |
| Wave Amplitude | Baseline | 100 times greater ground shaking | |
| Energy Release | Baseline | 1,024 times more energy released |
Compare the difference in wave amplitude and total energy release between an earthquake measuring 4.0 on the Richter Scale and one measuring 6.0.
Step 1: Calculate the difference in magnitude units.
Step 2: Calculate the difference in wave amplitude (ground shaking).
Step 3: Calculate the difference in total energy release.
Every time an oceanic plate slips underwater, a massive wave does not automatically follow. Most destructive tsunamis require a specific causal mechanism triggered by large submarine earthquakes at convergent plate boundaries. As an oceanic plate is forced beneath another, friction causes the plates to lock together, building immense pressure over time.
Eventually, the plates suddenly jolt free, resulting in a shallow-focus earthquake. This causes a sudden vertical thrust or shift of the seafloor. This movement acts like a giant paddle, causing massive water displacement of the entire overlying water column. This displaced water forms a series of outward-radiating waves known as a wave-train.
In the deep, open ocean, tsunami waves travel at enormous speeds (700–800 km/h) but have a very long wavelength and low amplitude, often passing entirely unnoticed by ships. However, as the waves approach the coast and enter shallow water, shoaling occurs. Friction with the seabed slows the front of the wave down, causing the wavelength to compress and the water to pile up into a towering, destructive wave.
Just before the tsunami crest reaches the shore, the sea level may drop dramatically. This phenomenon, known as drawback, occurs as coastal water is rapidly sucked outward to feed the approaching wave crest.
Students often state that a magnitude 7 earthquake is 'one unit stronger' than a magnitude 6. To secure marks, you must explicitly state that there is a 10-fold increase in wave amplitude (ground shaking).
When explaining tsunami formation in a 4-mark or 6-mark question, examiners actively look for the specific phrase 'vertical displacement of the seafloor' — learn this exact wording.
Remember the 'extreme hazard' formula for Edexcel: a high magnitude earthquake combined with a shallow focal depth will produce the most extreme primary hazards.
Do not confuse the Richter Scale, which measures scientific energy release (magnitude), with the Mercalli Scale, which describes observed structural damage (intensity).
Focus (hypocentre)
The exact point within the Earth's crust where the rock breaks and the earthquake energy is first released.
Epicentre
The point on the Earth's surface directly vertically above the focus of an earthquake.
Focal depth
The distance below the Earth's surface at which an earthquake occurs.
Shallow-focus
An earthquake occurring between 0 and 70 km below the Earth's surface, typically causing the most severe ground shaking.
Deep-focus
An earthquake occurring between 70 and 700 km below the surface, where seismic energy largely dissipates before reaching the ground.
Wadati-Benioff zone
A deep, active seismic area within a subduction zone where deep-focus earthquakes frequently occur.
Magnitude
A quantitative, scientific measure of the total amount of energy released by an earthquake.
Seismometer
An instrument used to detect, measure, and record the amplitude of seismic waves generated by an earthquake.
Richter Scale
A logarithmic numerical scale used to measure earthquake magnitude based on the amplitude of seismic waves.
Moment Magnitude Scale (MMS)
The modern standard scale for measuring earthquakes, calculated using the area of the fault that slipped and total energy released.
Intensity (Mercalli Scale)
A qualitative measure of an earthquake's severity based on observed effects and damage, rather than scientific energy release.
Water displacement
The movement of a volume of water from its original position, often caused by a vertical shift in the seafloor.
Shoaling
The process where tsunami waves slow down and increase dramatically in height as friction with the seabed increases in shallow water.
Drawback
The sudden recession of water from the coastline just prior to the arrival of a tsunami wave crest.
Put your knowledge into practice — try past paper questions for Geography B
Focus (hypocentre)
The exact point within the Earth's crust where the rock breaks and the earthquake energy is first released.
Epicentre
The point on the Earth's surface directly vertically above the focus of an earthquake.
Focal depth
The distance below the Earth's surface at which an earthquake occurs.
Shallow-focus
An earthquake occurring between 0 and 70 km below the Earth's surface, typically causing the most severe ground shaking.
Deep-focus
An earthquake occurring between 70 and 700 km below the surface, where seismic energy largely dissipates before reaching the ground.
Wadati-Benioff zone
A deep, active seismic area within a subduction zone where deep-focus earthquakes frequently occur.
Magnitude
A quantitative, scientific measure of the total amount of energy released by an earthquake.
Seismometer
An instrument used to detect, measure, and record the amplitude of seismic waves generated by an earthquake.
Richter Scale
A logarithmic numerical scale used to measure earthquake magnitude based on the amplitude of seismic waves.
Moment Magnitude Scale (MMS)
The modern standard scale for measuring earthquakes, calculated using the area of the fault that slipped and total energy released.
Intensity (Mercalli Scale)
A qualitative measure of an earthquake's severity based on observed effects and damage, rather than scientific energy release.
Water displacement
The movement of a volume of water from its original position, often caused by a vertical shift in the seafloor.
Shoaling
The process where tsunami waves slow down and increase dramatically in height as friction with the seabed increases in shallow water.
Drawback
The sudden recession of water from the coastline just prior to the arrival of a tsunami wave crest.
