Skip to 0 minutes and 7 secondsSo whether you've experienced an earthquake or not, I hope we've got you thinking about the size of earthquakes and where they occur. If you've never felt an earthquake, don't worry. I'm a seismologist, and I've looked at probably tens of thousands of earthquakes on my computer, but I've only ever felt one really, really small one. And actually, the fact that you've not experienced an earthquake can tell us something important about where they strike. For many people, the most distinctive feature of earthquakes is where they occur in the world. You'll notice that they tend to strike in very specific areas, and there are large parts of the Earth that experience few or even no earthquakes at all.

Skip to 0 minutes and 48 secondsNow, I'm based in Cardiff in the UK, which is a long way from any of these actively seismic regions. That's why I've only ever felt one small earthquake. But I'm sure some of you live much closer to one of these regions, and have probably felt some significant earthquakes. The reason why earthquakes occur in some parts of the world and not others is down to one very important concept. And that is plate tectonics. Plate tectonics is the theory that the Earth's surface is made up of a series of rigid blocks or plates.

Skip to 1 minute and 23 secondsNow, these blocks collide at so-called plate boundaries, and it's here where the Earth's crust folds, breaks, crumples, and slides against each other, creating changes to our Earth such as the formation of the largest mountain ranges or the deepest sea trenches beneath the oceans. At some of these plate boundaries, the plates are colliding head-on, which is why they experience some of the largest, most powerful earthquakes. One such boundary is the Ring of Fire at the edge of the Pacific Ocean. It's so-called because, as well as experiencing earthquakes, it's where some of the most explosive volcanic eruptions occur. Large earthquakes can also strike at boundaries where plates move against each other side by side.

Skip to 2 minutes and 8 secondsOne example of this is the San Andreas fault in California in the United States. So plate movement of this type occurs over hundreds of miles. And this area has experienced many large and significant earthquakes. Earthquakes are a dangerous and destructive force, and pose a serious threat to cities across the globe. As well as the obvious hazard they present to human life, they also have the potential to cause other catastrophic events, such as building fires or the flooding and tsunamis we consider in this course, as well. Earthquakes also presents us with an intriguing contradiction because we know that they're hazardous, but what is less well-known is the crucial role that they play in maintaining the habitability of our planet.

Skip to 2 minutes and 56 secondsEarlier in the course, we spoke about the cycle of carbon dioxide being consumed and released into the atmosphere, and how this regulates our climate. Well, plate tectonics, and therefore earthquakes, play a crucial role in this process because, as tectonic plates move, they transport rocks and minerals that actually contain the carbon. When one plate moves under another at a so-called subduction zone, they allow these materials to be buried deep within the Earth for thousands, even millions of years, only later to be released back into the atmosphere via volcanoes. So as you can see, earthquakes are fascinating and have been the cause of some of the most extreme events on our planet.

Skip to 3 minutes and 39 secondsAnd in the next few steps, we'll continue our exploration by looking at some of history's most important earthquakes. We'll think about what constitutes the very largest kind of earthquake, a mega quake, and we'll look into the future and think about how we deal with the constant risk presented by earthquakes.

Where do earthquakes strike?

In this video, Dr David Thompson explains where earthquakes occur in the world and the reasons for this.

In Week 1, we looked at the differentiation or internal layering of the Earth. The top layer is the atmosphere and the thin top solid layer on which we walk is called the crust.

a cross section showing the subsurface layers of the Earth from inner core, outer core, mantle, crust and the atmosphere A cross-section of the Earth

Plate tectonics

The crust is made up of rigid sections, known as tectonic plates. Plates are continuously moving over the mantle (middle part of Earth) at different speeds and in different directions.

There are seven major plates on Earth and earthquakes typically occur at the boundaries where plates meet. There are different types of boundaries characterised by how plates interact at them.

World map showing the position of the seven major tectonic plates and what direction they move towards each other at plate boundaries. The Eurasian plate covers most of Europe and Asia. The Australian plate covers the continent of Australia, and portions of New Guinea, New Zealand, and the Indian Ocean. The North American Plate covers most of North America, Greenland, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. The South American Plate covers the continent of South America as well as a region of the Atlantic Ocean seabed. The African Plate straddles and includes much of the continent of Africa. The Antarctic Plate covers the continent of Antarctica. The Indian plate covers most of the Indian subcontinent and parts of South China and western Indonesia Plate tectonics retrieved from USGS, Public Domain.

Plate boundaries

There are three types of boundaries; divergent, convergent and transform boundaries and it is here where earthquakes often occur.

A table showing how at divergent plate boundaries, plates are moving apart. At convergent plate boundaries plates are moving towards one another destroying Earth’s crust as they collide. At transform plate boundaries plates slide horizontally past one another Three main Tectonic Plates boundary types retrieved from Age of the Sage

Divergent plate boundary

These boundaries occur along spreading centres where plates are moving apart and new crust is created by magma pushing up from the mantle.

Picture two giant conveyor belts, facing each other but slowly moving in opposite directions as they transport newly formed oceanic crust away from the ridge crest.

Convergent plate boundary

At these boundaries, plates are moving towards one another destroying Earth’s crust as they collide.

At some of these boundaries one plate sinks under the other as they collide and the area where the plate sinks is referred to as a subduction zone.

At other boundaries the plates collide head-on and neither plate sinks under the other, both are forced upwards forming mountains.

Transform plate boundary

At these boundaries, two plates slide horizontally past one another.

Summary

As we discovered in Week 1, the movement of plates at Earth’s crust is a unique process on our planet. It drives change and causes some of the extreme events we experience. This movement is key to understanding why earthquakes occur.


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This video is from the free online course:

Extreme Geological Events

Cardiff University