All About Earthquakes - Understanding Earthquake Activity and Its Impact

Origin of Earthquakes

Earthquakes mainly occur along geological faults, where tectonic plates meet. These faults can be:

• Divergent Faults: When two plates move apart, often creating new oceanic crust.
• Convergent Faults: When two plates move toward each other, with one sliding under the other (subduction) or overlapping.
• Transform Faults: When two plates slide horizontally against each other (e.g. the San Andreas Fault).

Measurement Scale

Historically, people referred to the 'Richter scale', but today, the moment magnitude scale (Mw) is more commonly used. It more accurately reflects the actual energy released by the earthquake. It is logarithmic: an earthquake of magnitude 5 releases about 30 times less energy than an earthquake of magnitude 6.

Example of Classification by Magnitude

• Less than 2 : Micro-earthquakes, generally not felt.
• 2 to 3 : Very lightly felt, no damage.
• 3 to 4 : Rumbles and mild vibrations, rare damage.
• 4 to 5 : Can shake furniture, crack fragile walls.
• 5 to 6 : Light to moderate damage, falling objects.
• 6 to 7 : Serious damage over several kilometers, cracked buildings.
• 7 to 8 : Major damage over large areas, infrastructure damaged.
• 8 to 9 : Catastrophic earthquakes, massive destruction.
• More than 9 : Very rare and extremely powerful earthquakes (e.g. 9.5 in Chile in 1960).

Risks and Consequences

The extent of damage depends on many factors:

• Distance to the epicenter and the depth of the focus.
• Soil type (clay, rock, fill, etc.).
• Population density and the seismic resistance of buildings.
• The presence of slopes that can trigger landslides.
• The possibility of tsunamis if the earthquake occurs in an underwater zone.

Prevention and Protection

In areas with high seismic activity, constructing buildings according to earthquake-resistant standards is essential. Regular evacuation drills (e.g. drop, cover and hold on) also help reduce the human impact of an earthquake.

Different Types of Earthquakes

• Tectonic earthquakes: The most common, caused by plate movements.
• Volcanic earthquakes: Related to magma movement and volcanic activity.
• Collapse earthquakes: Collapse of underground cavities (mines, karsts, etc.).
• Induced earthquakes: Triggered or worsened by human activity (e.g. hydraulic fracturing).

History of Major Earthquakes

Some earthquakes have left their mark on history:

• Chile (1960): Magnitude 9.5, the most powerful ever recorded.
• Sumatra-Andaman (2004): Magnitude 9.1, triggering a devastating tsunami.
• Japan (2011, Tōhoku): Magnitude 9.0, causing a tsunami and the Fukushima nuclear accident.

Understanding and monitoring seismic activity is therefore crucial, not only for prevention but also to improve building standards and emergency plans.

Learn More

For further information, feel free to consult the websites of your country's geological and seismological institutes, such as the USGS (United States) or the BRGM (France), which regularly publish real-time data and information bulletins on earthquakes.