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.