Causes and Measurement of Earthquake Hazards

How slow plate movement produces sudden shaking

An earthquake occurs when rocks suddenly slip along a fault, releasing stored energy. A fault is a fracture in rock along which movement occurs. Most earthquakes happen near plate boundaries, where moving plates deform the rocks.

Although plates move slowly, usually only a few centimetres each year, movement along a fault is not necessarily smooth. Friction can hold the two sides together while plate movement continues. Stress builds and the rocks gradually deform, storing energy. Eventually, the stress overcomes the friction holding the fault locked. The rocks slip suddenly and release energy as seismic waves, which travel through the Earth and make the ground shake.

The focus is the point underground where the rupture starts. The epicentre is the point on the surface directly above it. Seismic waves spread out from the earthquake: the epicentre is not where the earthquake begins.

Why earthquake depth matters

An earthquake’s depth means the depth of its focus below the surface. A shallow-focus earthquake begins relatively close to the surface; a deep-focus earthquake begins much further underground.

For earthquakes of similar magnitude, shallow earthquakes generally produce stronger shaking near the epicentre. Their seismic waves have a shorter distance to travel before reaching the surface. Waves from a deeper earthquake travel further, spreading their energy over a larger area before reaching the surface. Deep earthquakes can still be felt over wide areas, but usually cause less intense shaking immediately above their focus than comparable shallow earthquakes.

Depth is therefore one reason why two earthquakes with the same magnitude can present different hazards. It does not determine damage on its own: distance from the earthquake, local geology and building construction also matter.

Contrasting earthquakes at plate boundaries

Earthquakes occur at all three main boundary types, but their characteristics differ.

At divergent boundaries, plates move apart and faults develop as the rocks are stretched. Earthquakes tend to be shallow and relatively small. The Mid-Atlantic Ridge is an example.

At conservative boundaries, plates slide past each other. Friction can lock sections of the boundary while stress builds, followed by sudden slip. These boundaries can produce large, shallow-focus earthquakes, making strong surface shaking a major hazard. The San Andreas Fault in California is an example.

At convergent boundaries involving subduction, earthquakes occur both where the plates meet and within the descending oceanic plate. This produces earthquakes at a range of depths, including the deepest earthquakes. These boundaries also produce many large-magnitude events. For example, the Nazca plate descends beneath the South American plate along the Peru–Chile trench.

Convergent continental collision zones, such as the Himalayas, also experience earthquakes as compression deforms rocks and causes movement along faults. These can be powerful even though there is no oceanic plate sinking beneath a continent.

Measuring and comparing magnitude

A seismometer records ground movement caused by seismic waves. Its record is called a seismogram. Scientists use these recordings to estimate an earthquake’s magnitude, a measure of its size related to the energy released.

The Richter scale is logarithmic rather than a simple scale of equal increases. Each increase of one magnitude unit represents ten times the recorded wave amplitude, after allowing for distance. Amplitude means the size of the recorded ground-motion vibration.

For example, a magnitude 6 earthquake has ten times the recorded amplitude of a magnitude 5 earthquake. A magnitude 7 earthquake has $10 \times 10 = 100$ times the amplitude of a magnitude 5 earthquake. These are comparisons of earthquake size, not predictions of damage.

Energy increases more rapidly than amplitude: a one-unit magnitude increase corresponds to approximately 32 times as much energy released. It does not mean 32 times as many buildings collapse.

Today, scientists usually use the moment magnitude scale for large earthquakes. This considers the area of the fault that ruptured, the amount of slip and the strength of the rocks. Richter’s original scale can underestimate large events.

Magnitude must also be distinguished from intensity. An earthquake has an overall magnitude, but its intensity varies from place to place. Intensity describes the shaking experienced and its effects on people, objects and buildings; it generally decreases with distance from the epicentre.

How an earthquake can generate a tsunami

A tsunami is a series of waves caused by sudden displacement of a large volume of water. Earthquake-generated tsunamis are especially associated with undersea subduction zones.

During a large earthquake, part of the seabed may suddenly rise or fall. This displaces the water above it, and waves spread out across the ocean. The important link is movement of the seabed that displaces water: not every undersea earthquake does this sufficiently to generate a tsunami.

In deep water, tsunami waves can have a long wavelength and a relatively low height. As they enter shallower coastal water, they slow down and their wavelength decreases. Water builds up into taller waves, which can flood low-lying coastal land. Tsunami height varies, so there is no single height that applies to every event or coastline.

A tsunami is a secondary earthquake hazard because it follows the initial earthquake and seabed movement. Landslides entering water and underwater volcanic eruptions can also produce tsunamis by displacing water.

Earthquake formation

Plate movement → friction locks a fault → stress and stored energy build → sudden slip → seismic waves and ground shaking.

• Focus: underground starting point.
• Epicentre: surface point directly above the focus.

Contrasting hazards

• Shallow focus: generally stronger shaking near the epicentre than a similar-magnitude deep earthquake.
• Divergent: usually shallow, relatively small earthquakes.
• Conservative: can produce large, shallow earthquakes.
• Subduction: shallow to deep earthquakes; many large events.
• Continental collision: compression and fault movement can produce powerful earthquakes.

Magnitude and intensity

• Seismometers record ground motion.
• Richter: each one-unit increase means 10 times the wave amplitude and approximately 32 times the energy.
• A two-unit increase means 100 times the amplitude.
• Moment magnitude: generally used for large earthquakes.
• Magnitude describes earthquake size; intensity describes shaking and effects at a particular place.

Tsunami formation

Undersea earthquake → sudden vertical seabed movement → seawater displaced → waves spread → waves slow in shallow water → wavelength decreases and height increases → coastal flooding.

Not all undersea earthquakes generate tsunamis. Landslides and underwater eruptions can also displace water.