It’s not just magnitude: These are the factors that make earthquakes deadly


Currently, more than 2,800 people have been confirmed dead after the earthquake that hit Morocco on September 8.

The measured magnitude of the earthquake is 6.8, which is smaller than the earthquake that hit the US state of California in 2019.

And yet no one was killed in that 7.1-magnitude earthquake – nor did it cause major damage to infrastructure.

There are many factors that affect how severe an earthquake is in terms of death toll and destruction – and these are the most important:

Magnitude and duration

Earthquakes are measured using a scale called Earthquake Magnitude (Mw).

It replaced the better-known Richter scale, which today is considered outdated and less accurate.

The number assigned to an earthquake is a combination of the distance the fault line has moved and the force that moved it.

An earthquake of magnitude 2.5 or less cannot usually be felt, but can be detected by instruments.

Earthquakes up to magnitude 5 are felt and cause less damage.

An earthquake in Morocco with a magnitude of 6.8 would be defined as moderate to strong, and an earthquake in southern Turkey from February of this year with a magnitude of 7.8 is classified as large.

Anything above 8 is classified as “major” and causes catastrophic damage that can completely destroy communities at its core.

Like magnitude, how long the tremors caused by an earthquake last can greatly affect its destructive power.

“While tremors from smaller earthquakes typically last only a few seconds, strong shaking during moderate to large earthquakes, such as the 2004 Sumatra earthquake, can last for several minutes,” says the Pacific Northwest Seismic Network.


But the sheer size of an earthquake is not the only significant factor: the location on Earth where the earthquake started is also important.

In the case of the earthquake in Morocco, this focal point was about 18 kilometers below the Earth’s surface.

That’s about twice the height of Mount Everest, but it’s not considered deep by geological standards.

“This earthquake was relatively shallow. This means there is less ground on top to dissipate the energy and force of the impact. The vibrations and shaking must have been very energetic,” Dr Carmen Solana, a volcanologist and geologist at the University of Portsmouth, told the BBC.

By comparison, the magnitude 6.2 earthquake that struck the landlocked Indonesian province of North Maluku (September 11) had a depth of 168 kilometers.

There were no reported deaths.

Time of day

The earthquake that struck central Morocco occurred at 11:11 p.m. local time – a factor that could have had a significant impact on the earthquake’s mortality.

“Many buildings were destroyed while people were sleeping,” says Dr. Solana.

Many of those who die during earthquakes are killed by building collapses – and indeed, seismologists have a saying that “earthquakes don’t kill people, they kill buildings”.

And so earthquakes that strike during the day when fewer people are inside tend to have fewer deaths.

Structure of buildings

It is possible to build houses that will withstand all but the most powerful earthquakes.

To achieve this, buildings must absorb as much seismic energy as possible.

Japan, which according to the US Geological Survey (USGS) is the country most prone to earthquakes in the world, is a pioneer in the construction of resistant buildings.

“When a building can absorb all the energy from an earthquake, it won’t collapse,” says Jun Sato, a structural engineer and associate professor at the University of Tokyo.

This mostly happens in a process called seismic isolation.

Buildings or structures are placed on some type of bed or shock absorber – sometimes as simple as blocks of rubber about 30-50 centimeters thick – to withstand earthquake movements.

But this type of foundation insulation is expensive, and construction budgets—and so do materials—vary wildly around the world.

In the remote parts of Morocco where the most damage occurred, many buildings were made of mud bricks or unbaked bricks, which can hardly withstand a strong earthquake.

Taller buildings with more occupants can lead to a devastating death toll if they collapse in an earthquake.

Many Turks have criticized poor construction standards after the collapse of numerous buildings in the February earthquake, particularly the failure to comply with building regulations.

Although these earthquakes were strong, experts say that properly constructed buildings should have remained upright.

“The peak intensity of this earthquake was strong, but not necessarily enough to bring down well-constructed buildings,” said Professor David Alexander, an emergency planning expert at University College London.

“In most places the degree of shaking was less than the maximum, so we can conclude that of the thousands of buildings that collapsed, almost none of them met the most basic building codes for earthquake protection.”

Population density

A massive 8.2 magnitude earthquake struck near the Alaskan Peninsula in July 2021, but you may not remember it.

The Čignik earthquake is considered the seventh largest in American history, but it did not kill or injure anyone.


It was a relatively deep earthquake far from the main population centers.

In contrast, the magnitude 7.0 earthquake in Haiti in January 2010 resulted in a massive death toll and massive destruction – more than 250,000 people are estimated to have died, around 300,000 were injured, and more than 1.5 million were displaced. is homeless.

The capital Port-au-Prince took the brunt of the impact, and its high population density – more than 27,000 people per square kilometer – partly explains the high death toll.

Land type

Our chances of surviving an earthquake depend greatly on how solid the ground is under our feet.

If the soil is loosely compacted and with wet sediments at or near the soil surface, the surface is less able to withstand strong ground shaking, the USGS says, causing an effect called liquefaction.

Then the otherwise solid material begins to behave as if it were liquid, greatly increasing the destruction caused by earthquakes such as the 1964 Nigara earthquake in Japan.

By comparison, the recent earthquake in Turkey killed more than 50,000 people, but the city of Erzin – only about 80 kilometers from the epicenter – managed to escape virtually unscathed.

No one from the city was killed and not a single building collapsed, while the surrounding towns were razed to the ground.

Geologists claim that Erzin was saved due to its location on a protective layer of rocky soil and firmer earth, which managed to absorb the shock waves.

The reaction of the rescuers

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Preparedness for natural disasters can be a significant factor in saving lives.

In Japan, schools have to hold earthquake drills twice a year, and children are taught how to react whether they are at home, outside or in a vehicle.

Taiwan is conducting national earthquake drills to test the capabilities of its rescue teams.

However, this is not the case in many other countries that rarely experience such disasters.

The speed and scale of the rescue response is also critical – although many people are still being pulled from the rubble alive more than ten days after the earthquake in Turkey, many injured and trapped people will not survive that long.

For this reason, transport infrastructure and the ability to recover it are key to the response.

In a remote part of Morocco hit by the recent earthquake, many of the few roads that exist there have been blocked by landslides and debris, and various villages have complained that there has been little or no response from rescuers.

There was also criticism of the Moroccan government, which was slow to accept some international offers of help.

Secondary effects

Collapsing buildings are not the only cause of death after an earthquake.

Coastal populations are at risk from seafloor earthquakes that can trigger deadly tsunamis.

The 2004 Asian tsunami was triggered by a massive 9.1 magnitude earthquake under the Indian Ocean, near Banda Aceh in northwest Sumatra.

The earthquake and massive waves that followed killed an estimated 230,000 people in more than ten countries.

The waves were so powerful that they claimed lives on the other side of the Indian Ocean in faraway Africa.

In hilly terrain, earthquakes trigger landslides, which bury homes and hamper rescue efforts.

In 2015, a powerful earthquake in Nepal killed nearly 9,000 people.

Geologists say the earthquake triggered more than 3,000 landslides in the affected area.

The 1906 San Francisco earthquake produced violent ground shaking lasting only 20 to 25 seconds, but the waves were enough to burst the city’s gas and water mains.

The gas that leaked caught fire in several places, and the lack of water made it difficult to fight the fire, which ultimately killed a total of 3,000 people.

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