The largest earthquakes in nearly 25 years rocked Taiwan on Wednesday morning, killing at least nine people and injuring hundreds more. The magnitude 7.4 quake led to a series of aftershocks, the largest of which reached magnitude 6.4.

Residents posted stunning scenes of swaying buildings and rocking bridges on social media as the earthquakes rocked their morning routines.

Taiwan has a long history of tremors and has invested a great deal in building codes to ensure structures can withstand shaking and early warning systems to alert residents via cellphones to seek shelter. That’s part of why despite Taiwan’s dense, urban population, there were relatively few casualties. But that wasn’t always the case — a magnitude 7.6 earthquake shook the island in 1999, killing close to 2,400 people.

Around the world, quakes remain a potent, deadly threat. A series of earthquakes reaching magnitude 6.3 struck Afghanistan last year, leading to more than 2,400 deaths. A magnitude 6.8 earthquake rocked Morocco last September, the largest earthquake to hit the country in at least 120 years. Officials reported that it killed more than 2,900 people.

The Mediterranean region is also seismically active, according to the US Geological Survey, though such strong earthquakes are rare in North Africa. Quakes are more common in places like Turkey, where two major fault lines cross and trigger shocks on a regular basis. A huge magnitude 7.8 earthquake rattled across Turkey and Syria in 2023 and another quake with a magnitude of 7.5 rocked the region a few hours later. The quakes killed more than 50,000 people and toppled more than 5,600 buildings in the region.

While scientists have drastically improved their understanding of where earthquakes are likely to occur, forecasting when one will occur is still impractical. The rumbling earth can easily catch people off-guard, worsening the ensuing death and destruction.

In light of the recent disasters, here’s a refresher on earthquakes, along with some of the latest science on measuring and predicting them.

1) What causes earthquakes

An earthquake occurs when massive blocks of the earth’s crust suddenly move past each other. These blocks, called tectonic plates, lie on top of the earth’s mantle, a layer that behaves like a very slow-moving liquid over millions of years.

That means tectonic plates jostle each other over time. They can also slide on top of each other, a phenomenon called subduction. The places on the planet where one plate meets another are the most prone to earthquakes. The specific surfaces where parcels of earth slip past each other are called faults. As plates move, pressure builds up across their boundaries, while friction holds them in place. When the former overwhelms the latter, the earth shakes as the pent-up energy dissipates.

Scientists understand these kinds of earthquakes well, which include those stemming from the San Andreas Fault in California and the East Anatolian Fault in Turkey. However, earthquakes can also occur within tectonic plates, as pressure along their edges cause deformations in the middle. These risks are harder to detect and measure.

“Our understanding of these within-plate earthquakes is not as good,” said Stanford University geophysics professor Greg Beroza. An earthquake within a tectonic plate has fewer telltale signs than those that occur at fault lines, he added.

2) The Richter scale isn’t the only measurement game in town anymore

The Richter scale, developed by Charles Richter in 1935 to measure quakes in Southern California, has fallen out of fashion.

It uses a logarithmic scale, rather than a linear scale, to account for the fact that there is such a huge difference between the tiniest tremors and tower-toppling temblors. On a logarithmic scale, a magnitude 7 earthquake is 10 times more intense than a magnitude 6 and 100 times more intense than a magnitude 5.

The Richter scale is actually measuring the peak amplitude of seismic waves, making it an indirect estimate of the earthquake itself. So if an earthquake is like a rock dropped in a pond, the Richter scale is measuring the height of the largest wave, not the size of the rock nor the extent of the ripples.

And in the case of an earthquake, the ripples........

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