More than 2,000 people have died in Moroccos’s deadliest earthquake in more than 60 years.

The magnitude 6.8 earthquake struck at about 11:11pm (22:11 GMT) at a depth of nearly 26km (16 miles), according to the US Geological Survey. The depth was initially reported at about 18km.

Authorities have declared three days of national mourning, with many left homeless following the country’s deadliest earthquake in more than 60 years.

Which areas are affected?

The earthquake’s epicentre was located in Al Haouz province in the High Atlas of the mountains – an area usually not associated with earthquakes – about 75km (44 miles) from Marrakesh, Morocco’s fourth largest city. Marrakesh’s old town, a UNESCO World Heritage Site, is reported to have been badly affected, with images emerging of collapsed buildings.

Al Haouz was the hardest-hit province, as Ouarzazate, Azilal, Chichaoua, and Taroudant provinces were also severely affected.

How are earthquakes measured?

The magnitude 6.8 quake is classified as “strong” on the Richter scale, which measures the strength of earthquakes.

Magnitudes are based on a logarithmic scale, meaning for each whole-number increase on the scale. So it was a 10 times stronger earthquake than the last strongest 5.8 magnitude quake of 1960.

To put that in context, earthquakes registering a magnitude of 4 or less are considered to be light, but may still cause some damage.

A magnitude 5 earthquake is, by definition, 10 times more intense than a magnitude 4 and can cause moderate damage to buildings.

Magnitude 6 quakes are typically considered strong and are 100 times more intense than a magnitude 4 quake.

Magnitude 7 earthquakes are considered severe, with the potential to cause significant loss of life and damage to built spaces.

Earthquakes registering a magnitude 8, 9 or higher can result in much more loss of life and near-total destruction of the affected area.

The magnitude, depth, proximity to inhabited areas, soil conditions, and chances of triggering secondary disasters such as tsunamis and landslides are just some of the many factors that determine how destructive an earthquake can be.