The massive earthquake in central Morocco that killed at least 2,800 people was unusual for that part of the country - and that's what made it so deadly.
"If you don't have earthquakes a lot, you don't build to withstand them," said Wendy Bohon, an earthquake geologist based in Maryland. "So the types of buildings that exist in Morocco are not built to withstand the strong shaking from their base. And so they are very likely to collapse and damage like we've seen."
According to data from the United States Geological Survey, earthquakes in the magnitude 6 range are more common in the northern part of Morocco, near the Mediterranean Sea. The 6.8-magnitude quake that occurred in the country's High Atlas Mountains region late Friday was the largest recorded in that area in more than a century.
How the earthquake happened
The surface of the Earth is made of brittle rock, Bohon said - like a hard-boiled egg. "And just like when you crack an egg, the crust, that brittle part, is broken into pieces. Those are the tectonic plates."
Plates move at different speeds. In the San Andreas fault in California, the relative motion between the plates is as much as 50 millimeters a year. Where the earthquake occurred in Morocco, the African plate is slowly slamming into the Eurasian plate, but at a pace of only about 3.6mm a year.
The disaster occurred along the High Atlas Mountains range, and mountains are a strong indicator of reverse faults.
Stress and strain are still building up, Bohon said, but "when you have plate motion that's that slow, you don't get earthquakes very often." This can be an issue for understanding earthquake hazards.
Earthquakes in populated areas
The disaster is the second devastating quake this year to cause massive destruction and leave thousands dead. A 7.8-magnitude earthquake in Syria and Turkey in February killed nearly 60,000 people.
However, scientists said the two incidents were not related. More than a dozen earthquakes with a magnitude of 7 or higher have occurred just this year, according to the USGS. Most them were in much more rural areas, or in the ocean.
Scientists also stressed that earthquakes are not becoming more frequent, nor does climate change have an effect on them.
"The one thing humans can do to cause earthquakes is increase fluid pressure in the rock," such as by building large dams or fracking, said Lucy Jones, a California-based seismologist and visiting associate in geophysics at the California Institute of Technology.
While humans have little control over when and where earthquakes happen, they can mitigate the effects.
When a 6.3-magnitude quake hit northern Morocco in 2004, killing hundreds of people, the country rebuilt the area with seismic shocks in mind, said Jess Galindo-Zaldvar, a professor of geodynamics at the University of Grenada in Spain. He noted that in Marrakesh, Friday's earthquake had leveled many of the city's older buildings, while newer constructions stayed standing.
"Earthquakes don't kill people," Bohon said. "Buildings kill people."