In the news we regularly hear about earthquakes and some also cause tsunamis. It seems like they occur more and more often. Recently, many people lost their lives with a magnitude 7.5 earthquake in Palu, Sulawesi (Indonesia), which was followed by a tsunami. Only a week later a deadly earthquake hit Port-de-Paix in Haiti – this time there was no tsunami, however. Why do we see so many earthquakes, when do they happen and when are they followed by tsunamis? How does it all work?
The cause: Earth’s tectonic plates on the move
In the map above the Earth’s surface is divided by red lines. Each of these lines highlight the boundaries between tectonic plates. Tectonic plates are pieces of the Earth’s outer layer that geologists call the lithosphere (the crust is only a thin part of this lithosphere).
These plates of lithosphere slowly move and shuffle around. They rub alongside each other (transform boundaries), or they dive underneath one another (convergent boundaries or subduction zones), or they are stretched and break apart to form new material (divergent boundaries). This all happens very slowly at the same rate that your fingernails grow: a few centimeters per year.
As these plates are made up of solid rock (lithosphere means “rock layer“), you can imagine that they aren’t all that smooth. As they move and slide alongside each other, they don’t all move that well. At times they are jammed and pressure builds up. Each time the pressure becomes so high that they become unstuck and move again, we feel that as an earthquake.
Each time the pressure between jamming tectonic plates becomes so high that they become unstuck and move again, we feel that as an earthquake. – click to Tweet this
Places where the Earth shakes
Earthquakes most often occur along plate boundaries. They especially occur along the convergent boundaries where plates move towards each other to form mountain belts like the Andes or the Himalayas, or transform boundaries like the famous San Andreas Fault.
Indonesia and Haiti, the ones most recently struck by deadly earthquakes, are both located along such plate boundaries. That is why these places are frequently shaken up by earthquakes.
Sometimes earthquakes also occur in the middle of tectonic plates, far away from any boundaries. People in south-eastern Australia will be familiar with these, but also those in China or some places in Europe. This is caused by the pressure buildup along different margins of these plates, that activates older faults and fractures in rocks in the middle of these plates.
South-eastern Australia doesn’t just have intraplate earthquakes. There are also volcanoes. Read more here.
Like a sandwich with multiple layers separated by weak contacts (two pieces of bread and a layer of cheese weakly attached with butter) the Earth’s upper layer contains lots of weak contacts between rocks where they can move alongside each other. Geologists call these weak contacts faults.
When you press a sandwich from all four sides, the cheese will slide between the pieces of bread. The sandwich itself will look differently after pressing it in than before. The same thing happens with intraplate earthquakes. Pressure from all sides will cause movement along faults; each movement is an earthquake.
When you press a sandwich from all four sides, the cheese will slide between the pieces of bread.The same thing happens with intraplate earthquakes. Pressure from all sides will cause movement along faults; each movement is an earthquake.
When tsunamis arise
As a very large part of the Earth’s surface is covered in water, many earthquakes actually occur under ocean or seafloors. Some of these “seaquakes” cause what the Japanese call “harbour waves” or “tsunamis”.
Not every earthquake underneath a body of water causes a tsunami. A tsunami is essentially a big wave caused by quickly moving a water layer up or down. So, only when there is an upward or downward shift of one plate compared to the other will an earthquake result in a tsunami.
Sometimes, earthquakes are indirectly related to tsunamis. Other times tsunamis are not formed by earthquakes at all. This is the case when a big landslide occurs where a whole cliff or mountainside collapses and slides into the sea. This happened in Hawaii last about 13,000 years ago. Some have suggested that these collapses cause mega-tsunamis hundreds of meters high, but this is not likely.
Earthquakes are important for life one Earth
Without earthquakes and the causing factor, plate tectonics, life on Earth would not be possible. Thanks to our moving plates water and gases are constantly released from the Earth’s interior. They replenish the Earth’s oceans and atmosphere.
If there was no movement of these plates, we would not have such a thick atmosphere, or such abundance of water. We would have slowly lost it to space.
Look at Mars. This red, dusty planet has no moving plates, but also no big water reservoirs. Its atmosphere is also very thin. It is not a place that is friendly to life as we have it on Earth.
Do earthquakes and tsunamis occur more often?
No they don’t. Of course the Earth sometimes experiences more movement in the plates and therefore more earthquakes, but this goes in cycles. Geologists still don’t really know what the timescale of these cycles is, or what causes it.
But why do we hear more and more about deadly earthquakes? Earthquakes themselves aren’t deadly. It’s the collapsing buildings, or landslides, or rising groundwater mixing with soft mud and sand in swampy areas that causes the casualties we see at earthquakes.
Unfortunately, more and more people move into areas with lots of earthquakes, but don’t always build the right buildings, or develop the right infrastructure. Then it is of no surprise that we hear more reports of people being badly affected by earthquakes. The best thing we can do to prevent such bad outcomes is to design and build better buildings and develop structures that can withstand the movement after pressure in the Earth’s lithosphere is released.
Unfortunately, more and more people move into areas with lots of earthquakes, but don’t always build the right buildings, or develop the right infrastructure. – Click to Tweet this