A massive quake in Morocco killed more than a thousand people. Here are eight things to know about these seismic events.
A powerful magnitude 6.8 earthquake rocked Morocco Friday night, the largest tremor to hit the country in at least 120 years. Officials report more than 1,000 people have died and expect the death toll to rise as rescuers reach remote afflicted areas.
Such strong earthquakes are rare in North Africa, which means few structures are designed to withstand them and residents have little experience responding when a quake occurs.
But earthquakes can still be deadly in earthquake-prone regions. In February, a huge magnitude 7.8 earthquake rattled across Turkey and Syria early Monday morning. Another quake with a magnitude of 7.7 rocked the region a few hours later. The quakes killed more than 50,000 people and toppled more than 6,600 buildings in the region.
Two major fault lines cross the Turkey and trigger shocks on a regular basis. Larger quakes are less frequent, but still a regular occurrence. Last November, Turkey suffered a magnitude 5.9 quake. A magnitude 7.0 quake rocked the Aegean Sea between Turkey and Greece in 2020.
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 aren’t traveling through a homogenous medium like water, but through solid rock that comes in different shapes, sizes, densities, and arrangements. Solid rock also supports multiple kinds of waves. (Some geologic structures can dampen big earthquakes while others can amplify lesser tremors.)
While Richter’s scale, calibrated to Southern California, was useful to compare earthquakes at the time, it provides an incomplete picture of risks and loses accuracy for stronger events. It also misses some of the nuances of other earthquake-prone regions in the world, and it isn’t all that useful for people trying to build structures to withstand them.
“We can’t use that in our design calculations,” said Steven McCabe, leader of the earthquake engineering group at the National Institute of Standards and Technology. “We deal in displacements.”
Displacement, or how much the ground actually moves, is one alternative way to describe earthquakes. Another is the moment magnitude scale. It accounts for multiple types of seismic waves, drawing on more precise instruments and better computing to provide a reliable measuring stick to compare seismic events.
When you hear about an earthquake’s magnitude in the news — like Turkey’s recent magnitude 7.8 quake — moment magnitude is usually the scale being used.
But this is still a proxy for the size of the earthquake. And with only indirect measurements, it can take up to a year to decipher the scale of an event, like the 2004 Indian Ocean earthquake, said Marine Denolle, an earthquake researcher at Harvard University.
“We prefer to use peak ground acceleration,” she said.
