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Skip to 0 minutes and 8 seconds So far, we’ve discussed what influences the size of an earthquake and discovered that there are actually several hundred of earthquakes occurring every single day. They’re often too small to be felt, but from time to time, we do have to deal with large, destructive earthquakes, or megaquakes. Throughout the course, one of our big questions has been, what is regular activity? And what is an extreme event? So our answer to this has always been dependent on how we look at these different events and comparing them over longer timescales of hundreds, thousands, and even millions of years. Earthquakes are relatively unique in our course, as the largest earthquakes occur much more frequently than some of the other events that we’ve considered.

Skip to 0 minutes and 58 seconds So there have actually been nine measuring magnitude eight or above in the last 10 years alone. And two of the top five earthquakes ever recorded have occurred in the last 15 years. These are what we would call megaquakes, due to their exceptionally destructive power. And if one of these earthquakes occurred in a densely populated area, somewhere like Japan, Sumatra, or the West Coast of the United States, I think we would all be in agreement that this would be an extreme geological event. But what if one struck in the middle of nowhere, far away from any cities or infrastructure? If a large earthquake strikes in a remote location where its effects aren’t observed by humans, would we still call it extreme?

Skip to 1 minute and 45 seconds So should the size or magnitude of an earthquake be the only thing that we should consider? Or should we be more interested in the number of fatalities, casualties, and the amount of financial damage to our infrastructure? Now if we look at two of the largest earthquakes ever observed, we’ll see some significant differences that bring us back to two very important terms. And that is hazard and risk. Our first earthquake is currently the second largest earthquake ever recorded. Measuring a huge magnitude 9.2, it struck in Alaska, in the USA, in 1964. And whilst a large tsunami wave was created and observed as far away as Hawaii and Japan, amazingly, only 139 deaths were recorded.

Skip to 2 minutes and 35 seconds Our second earthquake is the devastating Indian Ocean earthquake of 2004, striking just off the coast of Indonesia. At magnitude 9.1, this is the third largest earthquake ever recorded. It also created a tsunami wave that affected coastal communities across the Indian Ocean. Now despite both earthquakes being larger than magnitude nine, and both creating significant tsunami waves, the human effects were drastically different. More than 230,000 people were killed in the 2004 quake, compared with 139 in 1964. We’ll continue now by discussing some of the reasons for this.

Risks from earthquakes and megaquakes

In this video, Dr David Thompson explores the risks of two of the largest earthquakes ever recorded.

1964 Alaska, USA

Of the two events we’ve considered, the magnitude 9.2 earthquake in Alaska in 1964 remains the 2nd largest earthquake in history. It triggered a tsunami that reached heights of over 60m, which was most destructively seen along a shallow waterway called Valdez Inlet, where landslides also contributed to the damage.

The effects of the tsunami were remarkably recorded as far away as the the Gulf of Mexico and Puerto Rico, far from the Pacific Ocean where the earthquake occurred.

As we’ve seen, despite its claim to being the 2nd largest earthquake in history, only 139 deaths were recorded. While it was devastating to the communities that did experience it, the death toll may appear small given the immense size of the earthquake.

A key reason for this was its remote location, Alaska is a sparsely populated region far from any major population centres. This meant that both the intensity of the shaking and the size of the tsunami wave had dissipated by the time it reached more densely populated areas.

2004 Indian Ocean

The Indian Ocean earthquake that struck in 2004 was only marginally smaller at a magnitude of 9.1, currently the 3rd largest ever recorded. It was located at a depth of 30km, and it ruptured a fault area of approximately 1200km x 100km.

It created a tsunami wave that in places exceeded 30 metres in height. The wave travelled in all directions from the earthquake and produced devastation and disruption to coastal areas across the Indian Ocean.

Comparing events

These events have some distinct similarities:

  • They were both “megathrust” earthquakes, occurring along regions of active subduction (this is typically where the largest earthquakes occur)
  • Both created a significant tsunami wave
  • They make up two of the largest three earthquakes ever observed.

The death toll from the Alaskan earthquake was just 139, but the Boxing Day earthquake and associated tsunami killed more than 230,000.

The ocean provides a livelihood for many across south-east Asia, leading to a significant population density in coastal regions. This contributed to the significant loss of life that resulted from this devastating earthquake.

So while both can be described as being extreme in terms of their size, there are other important factors that determine whether an earthquake produces a significant loss of life or damage to infrastructure.

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Extreme Geological Events

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