Explained: Why Was Morocco's Earthquake So Deadly?

The earthquake occurred at a shallow level, thereby causing more destruction. But what does that mean?

People comfort each other while digging graves for victims of the earthquake, in Ouargane village, near Marrakech, Morocco, on Saturday, 9 September. 

(Photo: AP/PTI)

On Friday, 8 September, a deadly earthquake struck Morocco, killing over 2,100 people and flattening homes and villages. The earthquake's epicentre was the Ighil area, about 70 km south of Marrakech (or Marrakesh).

The US Geological Survey (USGS) said the quake had a preliminary magnitude of 6.8 when it hit at 11:11 pm. The agency reported an aftershock of 4.9 magnitude 19 minutes later.

According to the Associated Press, it was the county's strongest in over a century. It was so strong that tremors were felt as far away as Portugal and Algeria, according to the Portuguese Institute for Sea and Atmosphere and Algeria's Civil Defense agency, which oversees emergency response.

But what made it so deadly? The Quint explains.

What Exactly Happened in Morocco?

To understand what happened, it is essential to go back to the basics of geography. The Earth's surface is constituted of tectonic plates, which are nothing but large segments of the planet's outer layer that move against each other. This movement is responsible among others for geological phenomena, including volcanoes, earthquakes, and the formation of mountains.

In this case, there was a collision of the African and the Eurasian tectonic plates at a relatively shallow depth. But how shallow was it – and how did it have a bearing on the destruction it unleashed thereafter?

While the USGS pegged the epicentre at roughly 18.5 km below the Earth's surface, Morocco's own seismic agency put the depth at 11 km. Nevertheless, it is safe it say that it was a shallow earthquake.

What Is the Significance of Depth in a Quake?

According to the USGS, an earthquake's destructive power depends on three factors: strength, location, and distance from the epicentre.

The strength of shaking from an earthquake reduces with increasing distance from the earthquake's source, a US government website said. This explains why the strength of shaking at the surface from an earthquake that occurs at 500 km deep is considerably less than an earthquake which was to occur at, say, 20 km depth, it added.

"Shaking is more intense from quakes that hit close to the surface like setting off a bomb directly under a city," explained USGS seismologist Susan Hough, as quoted in a Business Insider report.

For waves that originate deep in the earth's surface, the seismic waves have to travel farther to the surface, losing energy along the way.

John Cassidy, another seismologist, talking to CBS News, echoed Susan Hough. "At 6.8, the magnitude of the earthquake was large – and it occurred at a shallow depth. Tremors originating from shallow depths cause more destruction," he said.

How Faulty Infrastructure Proved Fatal?

One of the reasons cited behind the earthquake in Morocco being so deadly is, as Bill McGuire, professor emeritus of geophysical and climate hazards at University College London, told Time magazine, is "faulty and inadequate infrastructure."

Families sit outside their destroyed homes after an earthquake in Moulay Ibrahim village, near Marrakech, Morocco, on 9 September.

(Photo: AP/PTI)

"The problem is that where destructive earthquakes are rare, buildings are simply not constructed robustly enough to cope with strong ground shaking, so many collapse, resulting in high casualties," the magazine quoted him as saying.

Lanchen Haddad, a Moroccan senator and former minister, told Al Jazeera the area was "not known for being active in terms of earthquakes."

Further explaining the scale of the damage and the high death toll, Cassidy told CBS News, that the area was densely populated, too.

"The fatality was because it occurred in an area close to where the population is high at about 2-3 million people. And most of the houses were damaged due to faulty construction codes," he added.

How Does this Compare to Other Earthquakes ?

Friday's earthquake may have been the country's strongest in over a century. "Though such powerful earthquakes are rare in the region, it is not the country’s deadliest," according to Associated Press.

Over 60 years ago, a 5.8 magnitude earthquake struck the country and claimed the lives of over 12,000 people on its western coast in the city of Agadir.

There had not been any earthquakes stronger than magnitude 6.0 within 500 km of Friday's tremor in at least a century, according to the USGS.

Earlier this year, a magnitude 7.8 hit Syria and Turkey killing more than 21,600 people.

A destroyed building is seen in Aslanli, southeastern Turkey, on 9 February.

(Photo: AP/PTI)

The most devastating earthquakes in recent history have been above magnitude 7.0, including a 2015 tremor in Nepal that claimed the lives of over 8,800 people, and a 2008 quake that killed 87,500 in China.