How strong is a magnitude-7.8 quake?

A quake that struck a densely-populated part of southeastern Turkey and that was felt as far away as Israel and Cyprus was strong and shallow enough to be lethal on a devastating scale, seismologists fear. Authorities in Syria and Turkey said at least 200 people have died and expect the toll to rise.

The earthquake, which hit at 4:17 a.m., according to the United States Geological Survey, measured at magnitude 7.8. Quakes can be far stronger, experts said, but what matters more than the numerical magnitude is relative strength combined with location — whether many people live nearby — and depth, or whether a quake is shallow enough to impact a wide area.

In a report issued about 30 minutes after the earthquake, experts at the U.S.G.S said there was a 34 percent chance of between 100 and 1,000 fatalities, and a 31 percent chance of between 1,000 and 10,000 fatalities.

“Extensive damage is probable and the disaster is likely widespread,” the report said. It estimated economic losses of as much as 1 percent of Turkey’s gross domestic product.

Januka Attanayake, a seismologist at the University of Melbourne in Australia, said that the energy released by the earthquake was equal to about 32 petajoules, enough to power New York City for more than four days.

“In terms of energy, the magnitude 7.8 that occurred is 708 times stronger than a magnitude 5.9,” he said, citing as an example an earthquake in Melbourne, Australia, in 2021, in which some minor damage was sustained to the city.

The strength of earthquakes is measured on a scale known as the local magnitude scale. An earlier version was known as the Richter scale. It is a logarithmic scale: For each whole-number it rises, the amount of energy released by an earthquake increases by about 32 times.

But the potential damage of an earthquake depends on far more than its magnitude, with the population density of a given area as well as the shallowness of the epicenter both contributing to the level of devastation, with a shallower quake holding the potential for more damage. This one was about 10 miles deep.

Another important factor is the quality of construction of buildings in the area. “The population in this region resides in structures that are extremely vulnerable to earthquake shaking, though some resistant structures exist,” the U.S.G.S report noted, adding: “The predominant vulnerable building types are unreinforced brick masonry and low-rise nonductile concrete frame with infill construction.”

In a post on Twitter, the seismologist Susan Hough of the U.S.G.S. said that the quake, while far from the strongest the world has seen in recent decades, risked being particularly dangerous because of its location and shallow depth.

At the lowest end of the scale, a magnitude 1 quake would be a micro-earthquake that is all but imperceptible to humans. A magnitude 7 earthquake has been described by seismologists as having “an energy equivalent to around 32 Hiroshima atomic bombs,” as Renato Solidum, the director of the Philippine Institute of Volcanology and Seismology, told The Times in 2013.

At magnitude 7.8, the earthquake in Turkey is classified as a “major” earthquake. Other quakes of a similar magnitude have included a 2013 earthquake in Pakistan, in which about 825 people died, and the April 2015 earthquake in Nepal, when nearly 9,000 people were killed.

This earthquake appeared to be one in a series, said Dr. Attanayake, the seismologist in Melbourne. A long fault line of roughly 1,500 kilometers, or 930 miles, dividing the Eurasian plate to the north from the Anatolian plate to the south, had produced multiple earthquakes of magnitude 6.7 or greater since 1939.