Understanding the Science Behind Earthquakes: What Causes Them?

What Is An Earthquake? The Science That Underlies Earthquakes

The earthquake is one of the most powerful natural events on earth. They alter environments, trigger secondary threats and unleash massive amounts of energy in seconds. To understand the source of earthquakes, we have to consider the structure of the interior of the planet, the gradual motion of tectonic plates, and the fast release of stress accumulated along faults. Their science includes geology, physics and careful observation of ground movements.

Introduction to the Layers of the Earth and Moving Plates

The Earth is made of a thin outer crust, a thick mantle and a core. The lithosphere consists of the crust and the upper mantle. The lithosphere is divided into huge, rigid pieces known as tectonic plates. The plates float on the weaker, partly molten asthenosphere and they are always moving. Very slowly though, usually only a few centimetres a year.

The plates move because of mantle convection, slab pull and ridge push. At their edges the plates push against each other and the rocks become strained. If the stress exceeds the rock’s strength, the rock will fracture or slip along a pre-existing fracture. That sudden slip is an earthquake.

Elastic Rebound Theory and Earthquakes

Most earthquakes occur along faults, which are fractures in the crust where blocks of rock on either side can move relative to each other. As the plates attempt to move past, toward or away from each other, stress builds slowly. Rocks are elastic, they can store energy like a spring that is compressed. When friction can no longer retain pebbles they slip suddenly. Seismic waves travel, releasing the stored elastic energy. This is known as elastic rebound.

The hypocenter (or focus) is the point inside the ground where the slide initiates. The epicentre is the point on the surface right above. Seismic waves radiate from the hypocenter in all directions. That is what causes the ground to shake in an earthquake.

Earthquakes and Plate Boundaries – What’s the Connection?

Distinct earthquake patterns at the various plate boundaries:

- Convergent boundaries – Plates collide and either crash into each other or one plate slides under the other (subduction). Many of the world’s largest and deepest earthquakes occur in these zones. Many of the subduction zones in the Pacific Ring of Fire result from tremendous phenomena.

- Divergent boundaries – Plates move away from each other, often along mid-ocean ridges. Earthquakes here likely to be milder and of lesser size. New crust is born.

- Transform boundaries – Plates grind past one another. A good example is the San Andreas Fault system. These faults produce frequent shallow earthquakes.

Intraplate earthquakes are less frequent in the interior of plates, along ancient fault zones or zones of crustal weakness.

Further natural factors

Most large earthquakes are triggered by the movement of tectonic plates but other natural processes may also be involved.

- Volcanic activities - When magma moves under volcanoes it breaks the rock and creates volcanic earthquakes. These earthquakes are often precursors to eruptions.

- Crustal adjustment - Seismic activity can sometimes be a result of postglacial rebound, as land rises up after the melting of heavy ice sheets.

- Land Slides or Collapse - Large underground collapses in mines or caverns or large landslides can produce localised seismic signals called collapse earthquakes.

Man-Made Earthquakes

Earthquakes can be triggered by human activity as well. These include oil and natural gas production with wastewater injection, reservoir impoundment behind big dams, geothermal energy extraction, and underground mining. The magnitude of produced seismicity is usually less than that of large tectonic events . In highly populated areas damage might occur when incidents occur near the surface.

Earthquake Waves and the Shaking of the Earth

When rocks slip they create many types of seismic waves:

Primary (P) waves - Compressional waves are the fastest and are the first arrivals.

Secondary (S) waves - Shear waves that travel slower and cannot pass through liquids.

- Surface waves - Love and Rayleigh waves that travel along the surface of the Earth and are typically responsible for the most catastrophic shaking and destruction.

The interaction of these waves with local geology - soft soils, sedimentary basins or ridge effects - can amplify ground motion and cause more damage.

Measuring and Studying Earthquakes

Modern seismometers and seismographs are very accurate measures of ground motion. Networks of instruments help scientists to find hypocenters, fault mechanism and magnitudes. For larger events, the magnitude scale has largely replaced the previous Richter scale, as it more accurately shows the total energy released.

Earthquake early warning systems use the fact that the faster P waves come first, followed by slower, more damaging waves, to provide seconds to tens of seconds of warning before the commencement of strong shaking. Long-term investigations include mapping of faults, paleoseismology, GPS measurements of plate motion, and statistical forecasts of aftershock sequences.

Sequences of Earthquakes: Foreshocks and Aftershocks

After the large earthquakes come aftershocks, smaller earthquakes as the crust around the fault adjusts to the new stress pattern. Sometimes the mainshock is preceded by a foreshock of lower magnitude. Some of the earthquake swarms have no identifiable mainshocks. Understanding these sequences is critical for emergency responders and for improving hazard models.

The Importance of Science

Knowing the causes of earthquakes is useful for building codes, land-use planning, early warning and public preparedness. While we cannot currently predict when and where future large catastrophes will occur, communities can reduce risk by identifying active faults, quantifying strain accumulation and modelling ground-motion scenarios.

The basic motor is the slow but relentless motion of the tectonic plates and the sudden release of elastic strain along faults. Volcanic, gravitational and human induced events, secondary mechanisms, increase the complexity yet plate tectonics provides the essential foundation for understanding why and where most earthquakes happen.

This information is further improved by further advances in seismology, geodesy and computational modelling, which also contribute to developing more resilient societies in earthquake-prone areas.

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