The number of fatalities and people injured in the recent Taiwan earthquake continues to rise. Taiwan’s Central Weather Administration said the 7.2 magnitude earthquake hit around 25 kilometers (15.5 miles) southeast of Hualien county, a mountainous region on the country’s east coast, where dozens of buildings collapsed.

News agencies report that strict building codes prevented an even larger catastrophe. The last time a similarly strong earthquake hit Taiwan was in 1999. Back then, more than 2,400 people died.

Earthquakes aren’t rare in Taiwan. The island lies on a fault line, where two of Earth’s tectonic plates collide.

How do earthquakes happen?

Earthquakes result from movement along these tectonic plates, big rocky slabs that make up the Earth’s outer crust. Some of the plates are giant, while others are relatively small. Researchers still debate exactly how many pieces make up the Earth’s jigsaw puzzle of a crust.

The plates making up the crust “float” on the layer below, which is called the mantle. At fracture points (where the puzzle pieces fit together), heat causes molten rock in the mantle to swell, pushing the plates to shift and migrate a few centimeters each year, either away from or towards each other. These movements, which have been happening for billions of years, are called plate tectonics.

If tectonics cause the edges of plates to catch on one another, strain builds up. When it grows too high, the plates can break loose suddenly, causing waves of pressure to spread to the Earth’s surface in the form of an earthquake.

Regions that lie above fault lines — where these movements take place — are particularly at risk.

When earthquakes hit in the ocean, tsunamis can occur, causing huge waves that spread at high speed and cause devastating floods when they strike land.

How are earthquakes measured?

The strength of an earthquake is most commonly measured on the Richter scale, which classifies its magnitude using a scale of 1-10 with the help of a device called a seismograph. Earthquakes with a magnitude of 1 are very common and generally can’t be felt, while earthquakes with a magnitude of 10 occur extremely rarely and can cause catastrophic damage.

In general, seismologists say earthquakes with a magnitude over 5.0 can cause notable destruction.

Another scale used to measure quakes is the “moment magnitude scale,” which is less common but becoming more preferred among seismologists because it can more accurately measure large earthquakes (over the magnitude of 8.0) than the Richter scale.

What are aftershocks?

Powerful earthquakes are nearly always followed by smaller aftershock quakes. These occur because tectonic plates at the epicenter of an earthquake don’t simply stop moving after the quake occurs — they continue shifting as they settle.

Aftershocks can also cause serious destruction. They can cause buildings damaged during an original quake to collapse, leading to more deaths, injuries and displacement. Aftershocks are generally strongest in the two days following an initial quake, but they can continue to occur for years.

In general, the magnitude of aftershocks is lower than that of the initial earthquake. So if an initial quake had a magnitude of 5, its aftershocks might have a magnitude of 4 or less. But this isn’t always the case.

“Sometimes you get an aftershock that’s actually larger than the main shock. So as a seismologist, you always have to be prepared to be surprised by what the Earth throws at you,” seismologist Roger Musson, an honorary research associate with the British Geological Survey, told DW after the 2023 Turkey-Syria earthquakes. That event saw aftershocks almost as large as the initial quake.

An earthquake is considered an aftershock and not a separate quake when it occurs between one and two fault lines away from a preceding earthquake. Generally, aftershocks are the result of the Earth’s tectonic plates trying to shift back into place along a fault line.

What is the difference between shallow and deep earthquakes?

When you read about earthquakes in the news, you might notice that quakes are often described as “deep” or “shallow”. In general, seismologists say shallow earthquakes cause more destruction.

An earthquake is considered “shallow” if it strikes less than 50 kilometers from the earth’s surface. These quakes travel fast, making their impact on the surface much more violent than if they had occurred deeper underground and taken longer to reach the surface. Quakes that occur more than 300 kilometers beneath the Earth’s surface are considered “deep.”

Mehdi Kashani, Associate Professor of Structural and Earthquake Engineering at the University of Southampton in the United Kingdom, told DW that the difference between shallow and deep quakes can be illustrated by thinking about waves in the ocean.

Particularly massive waves will immediately knock a nearby surfer off his board, he said. But once those waves have traveled through the ocean far enough, they will have little impact on even the most rookie surfers.

Can you predict earthquakes?

Although earthquake early-warning systems exist, they are only able to detect the primary waves released by an earthquake seconds before it hits.

No technology is currently able to predict quakes in the longer term, experts say.

“What’s extremely clear is we have no means to predict earthquakes. This is totally different from volcanoes, which can be predicted to some extent within a few days,” Patricia Martinez-Garzon, a seismologist at the German Research Centre for Geosciences, told DW in February 2023.

Why are some earthquakes so destructive while others of the same magnitude aren’t?

The 7.8-magnitude quake that hit Turkey and Syria in February 2023 killed over 50,000 people, while quakes of 8.2 and 8.3 that hit Chile in 2014 and 2015 killed fewer than two dozen. How is this possible?

According to experts, the explanation comes down to two main factors: depth and design.

Of course, if an earthquake hits a sparsely populated area, it will cause less destruction than a quake in a major urban center.

And if an earthquake is especially deep, it will likely cause less destruction than a shallow quake of the same magnitude.

But a lot of it also comes down to building design, Kashani told DW. He said the basic principles that govern seismic design were only developed in the second half of the 20th century, so many countries are still catching up.

Seismic codes are regulations governing how to build in areas close to fault lines. Although some countries — like earthquake-prone Japan and Chile — have made enormous progress in updating their buildings to comply with these codes, that’s not the case in countries where the money and political will to support these massive infrastructure projects is sparse.

Many of the buildings that crumbled in the Turkey-Syria earthquakes hadn’t been updated to meet seismic codes, which contributed to the mass destruction.

Seismic design doesn’t require buildings to be torn down and built again from the ground up. Kashani told DW it’s also possible to retrofit old buildings to provide them with better protection during earthquakes.

Edited by: Derrick Williams

Editor’s note: This article was originally published on September 9, 2023. It has been updated with information regarding the earthquake in Taiwan on April 3, 2024.