Seismic Hazards and Magnitude (AQA A-Level Geography): Revision Notes

Seismic Hazards and Magnitude

What are earthquakes?

Earthquakes are one of the most powerful natural hazards on Earth. They occur because the Earth's crust is constantly in motion, which creates a gradual build-up of pressure between masses of rock moving in different directions. When this accumulated stress is suddenly released, parts of the surface experience violent ground movement that typically lasts from just a few seconds up to around 30 seconds. However, in exceptional cases like the Boxing Day earthquake of 2004, the shaking continued for nearly six minutes.

Nature and distribution of earthquakes

Understanding focus and epicentre

When pressure is released within the Earth's crust, this release occurs at a specific point beneath the surface. This point of origin is called the focus (sometimes referred to as the hypocentre). The point on the Earth's surface directly above the focus is called the epicentre. Understanding this distinction is crucial because the depth of the focus significantly affects the intensity of damage experienced at the surface.

Types of earthquake focus

Earthquakes are categorised into three types based on the depth of their focus:

• Shallow focus (0-70 km deep): These earthquakes tend to cause the greatest damage and are responsible for approximately 75% of all earthquake energy released globally. Because they occur closer to the surface, the seismic waves have less distance to travel before reaching populated areas.

• Intermediate focus (70-300 km deep): These occur at moderate depths and typically cause less surface damage than shallow focus events.

• Deep focus (300-700 km deep): These are the deepest earthquakes and generally cause the least surface damage due to the greater distance seismic waves must travel.

Where earthquakes occur

The overwhelming majority of earthquakes occur along plate boundaries. The most powerful earthquakes are typically associated with destructive margins, where plates collide and one is forced beneath the other. At conservative margins, where plates slide past each other horizontally, the boundary is characterised by transform faults. These faults experience sudden differential movement that produces earthquakes.

Perhaps one of the most famous examples of this is the San Andreas Fault in California. This represents the boundary between the North American and Pacific plates. In reality, the San Andreas system is actually a broad, complex zone containing numerous fractures within the crust rather than a single fault line.

Earthquakes away from plate boundaries

Some earthquakes occur away from major plate boundaries. For instance, in February 2018, an earthquake with an intensity of 4.6 on the Mercalli Scale was felt across much of Wales and south-west Britain. An earthquake of this magnitude occurs somewhere on mainland Britain approximately every four years.

Human-induced earthquakes

It has been suggested that certain human activities could trigger minor earthquakes. Building large reservoirs can put pressure on underlying rocks. Similarly, in recent times, the process of fracking (hydraulic fracturing of rock to release gas) has raised concerns. A series of earthquakes, the largest measuring 2.9 on the moment magnitude scale, were recorded near a fracking site close to Blackpool, Lancashire. This prompted authorities to stop drilling operations pending a thorough review of safety procedures.

Magnitude and frequency of earthquakes

The moment magnitude scale

The magnitude of earthquakes can be measured in various ways. The most widely used measurement today is the moment magnitude scale, which measures the size of an earthquake in terms of the total energy released, expressed as Mw.

This logarithmic nature is crucial to understand. It means that the difference between a magnitude 6 and magnitude 7 earthquake is not simply "one unit more" - it represents 32 times more energy. Similarly, a magnitude 8 earthquake releases 1,000 times more energy than a magnitude 6 event.

The calculation of moment magnitude involves several factors including the rigidity of the rock affected, the distance the rock moved, and the total area where movement occurred.

Intensity vs magnitude

Whilst magnitude provides a quantitative measure of energy released at the earthquake's source, intensity offers a qualitative assessment of the strength of ground shaking and its effects at a specific location.

The Mercalli scale

The Mercalli scale measures earthquake intensity and its observable impacts on people, buildings, and the environment. Unlike magnitude, which has a single value for each earthquake, intensity varies depending on location - areas closer to the epicentre typically experience higher intensity.

The Mercalli scale ranges from I (Not felt) through to XIII (Extreme). It describes the severity of shaking in terms that are observable and measurable, such as whether people feel the tremor, whether objects fall, and the extent of structural damage to buildings.

Seismic records enable earthquake frequency to be observed, but these records only extend back to 1848 when instruments capable of recording shock waves were first developed. For earlier events, we must rely on historical records to understand the date and effects of earthquakes.

The impacts of seismic events

Very few people are actually killed directly by the ground shaking itself if they are standing in an open area. Instead, people are harmed by what earthquakes do to their surroundings - collapsing buildings and infrastructure (roof collapse), the sea (tsunamis), and slopes (landslides). Some effects occur immediately, whilst others develop over a longer period and depend heavily on the area's capacity to recover.

Primary effects

The initial, or primary, hazard of an earthquake consists of the immediate impacts caused by the seismic event itself.

Ground shaking is the most obvious primary effect. This is caused by shock waves travelling through the crust from the focus up to the surface, then radiating outwards in all directions. In extreme cases, these waves behave more like sea waves, with a clear up-and-down motion observable at the surface. The severity of shaking depends upon several factors:

• The magnitude of the earthquake
• Its depth (distance from the epicentre)
• The distance from the epicentre
• Local geological conditions

Ground rupture represents another significant primary hazard. This is the visible breaking and displacement of the Earth's surface, typically occurring along the line of a fault. Ground rupture poses a major risk to large engineered structures such as dams, bridges, and nuclear power stations, which can suffer catastrophic damage if the ground beneath them suddenly shifts or breaks apart.

Secondary effects

Secondary effects are those that occur as a consequence of the primary earthquake impacts.

Soil liquefaction occurs when soils with a high water content are violently shaken, causing them to lose their mechanical strength and behave like a fluid rather than a solid.

Landslides and avalanches result from slope failure triggered by ground shaking.

Tsunamis are giant sea waves generated by shallow-focus underwater earthquakes involving vertical movements of the sea bed. They can also be triggered by landslides entering the sea. These waves can travel vast distances across ocean basins at high speeds, causing devastating impacts on coastal communities far from the earthquake's epicentre.

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