Earthquakes – how do they occur? Here are the major disasters

Earthquakes are the kind of natural phenomena that continue to inspire fear. Unfortunately, they are often tragic in their consequences – people die as a result. Why are earthquakes so unpredictable and how do they occur? Which ones have been particularly notable in history?

Why do earthquakes scare us so much?

Why does the earth shake? Earthquakes are caused by a sudden release of stress in the earth’s crust and are associated with the movement of rocks along tectonic faults. The strength of the phenomenon is important – it can be associated with gentle rippling or it can be very violent. Primary shaking is often followed by aftershocks

How long does an earthquake last?

This devastating phenomenon is very short-lived. It lasts from a few seconds to a few minutes. The problem, however, is that the force of these tremors can be several thousand times greater than that of the bombs dropped on Hiroshima and Nagasaki

What is the structure of an earthquake?

A characteristic of these phenomena is that they do not proceed with the same force over their entire area. The place where the waves originate is called the focus of the earthquake. The epicenter is the point on the earth’s surface above the earthquake center. From this point, seismic waves propagate in different directions:

• shallow (source at a depth of up to 70 km),
• intermediate (up to 300 kilometers),
• deep (up to 700 km).

What are the types of earthquakes?

Common earthquakes include

• tectonic – very often occur at the boundary of two tectonic plates. However, they sometimes occur away from faults. It is estimated that up to 90% of all earthquakes occur on the planet;
• volcanic – volcanic activity can also contribute to earthquakes;
• collapse – rare situations where a cave-in or other void in the ground collapses;
• anthropogenic – paradoxically, human activity can also contribute to the ground shaking. They can be related to mining activities, but in the past also to nuclear testing.

Where do earthquakes occur?

Not all areas on the planet are haunted by large magnitude earthquakes. There are seismic zones with high risk of earthquakes, panseismic zones with rare strong earthquakes, and aseismic zones where even moderate earthquakes are extremely rare. Poland belongs to the last group

How are earthquakes measured?

A seismograph is used to measure them. Degrees of magnitude are used to compare the strength of earthquakes. The strongest quakes registered so far had the magnitude of 9.5, but it is worth remembering that the measurements are made in such a way that the next degree is ten times stronger than the previous one. In Poland there were maximum tremors of magnitude 6.0.

The biggest and the most tragic – these earthquakes made history

The most famous earthquakes in history are not only those with the highest magnitude, but also the most deadly. In the past, earthquakes have destroyed entire cities, traumatized generations, and even been reflected in literature. An example is the earthquake that destroyed Lisbon in 1755. Voltaire alluded to it in “The Candide” and spun his philosophical visions based on it

Highest magnitude

Among the strongest earthquakes in the world were those recorded:

• in Chile in 1962. – registered shocks of magnitude 9.5;
• alaska (USA) in 1964. – magnitude 9.2;
• sumatra (Indonesia) and the Andaman and Nicobar Islands (India) – magnitude 9.1.

Highest number of human casualties

Not always the strongest tremors are the most tragic ones. Sometimes relatively weaker earthquakes in densely built-up areas have the most tragic toll:

• Shaanxi (China) in 1556. – magnitude 8.0, about 830,000 deaths;
• Haiti in 2010. – magnitude 7.0, over 316,000 fatalities;
• Tangshan (China) in 1976. – magnitude 7.5, 242,419 fatalities.