What makes an earthquake deadly? These are the things that matter

The largest earthquake to hit the United States since the 1960s was an 8.2 magnitude quake near the Alaska Peninsula on July 28, 2021.

If you’re having trouble remembering the horrifying details, that’s because there weren’t any. No one was killed or injured in the Chignik earthquake, the seventh largest in US history. Not a single building collapsed. A post-quake inspection in Perryville, the city closest to the epicenter, found nothing more worrisome than a few cracks in the drywall.

There will be no such relief in the areas devastated by the 7.8 magnitude earthquake that struck southern Turkey early Monday. The quake left more than 4,000 dead, scores more injured and tens of thousands homeless as buildings collapsed around them.

Magnitude alone does not determine the full extent of damage from an earthquake. The amount of death and destruction that a single tremor brings depends on several factors, each of which can make the difference between life and death for local people.

Location, location, location

Simply put, the closer an earthquake is to a human settlement, the more damage it causes.

“It’s kind of a real estate thing: location, location, location,” said Susan Hough, a seismologist with the US Geological Survey in Pasadena. “I mean, size matters. if it is a [magnitude] 3 against 8, that [makes] a difference. … But more often than not, the farther you get from the moving fault, the more the energy spreads, and it just loses its really strong punch.”

The Chignik earthquake erupted about 20 miles below the sea floor off the Alaska Peninsula. It was deep enough that its energy had mostly dissipated by the time it reached the nearest human settlement, Perryville, a village with a population of about 100 people about 65 miles away.

The earthquake in Turkey has no such geographical luck. Like the San Andreas Fault in California, the East Anatolian Fault – the seam along which this earthquake ruptured – runs beneath densely populated areas.

Worse, Monday’s quake struck relatively close to the surface, resulting in much stronger ground shaking. The main tremor erupted about 18 kilometers below the surface, and a large 7.5 magnitude aftershock was even shallower at 10 kilometers.

The earth

An earthquake hits differently depending on the nature of the ground you are standing on. Structures built on softer sedimentary soils – such as those found in the Los Angeles Basin and southern Turkey – are shaken more violently than those anchored on firmer ground.

“It’s basically tofu versus rock,” said An Yin, a professor of geology at UCLA. “If you’re building a house drilled into solid rock, or a house … drilled into tofu, which one will you trust?” Of course the rocks.”

If there is enough moisture in the soil, sedimentary soil also tends to liquefy. Then the combination of intense pressure and shaking causes sedimentary rock to lose its shape and behave more like a liquid than a solid. Mexico City’s soils are particularly prone to liquefaction, which is why earthquakes are so devastating there. This also applies to the region in which the earthquake in Turkey raged.

The built environment

“We have a saying in the industry that earthquakes don’t kill people – buildings do. We’re really seeing that here in Turkey and Syria, said William Ellsworth, a former chief scientist at the USGS Earthquake Hazards Program and now a professor of geophysics at Stanford University.

A magnitude 7.4 quake in eastern Turkey in 1999 that killed 17,000 people sparked a campaign for stricter earthquake building codes, which were followed by many new developments, particularly in major cities. But most buildings constructed before the new rules went into effect were not retrofitted to meet them, leaving many densely populated neighborhoods at risk of a disaster now unfolding.

An aerial view of debris from collapsed buildings in Hatay, Turkey, on Monday.

(Anadolu Agency via Getty Images)

“Turkey has very good earthquake codes. In many ways they are as strong as the ones we have in California. But if buildings aren’t built to modern codes, they’re vulnerable,” Ellsworth said. “When you look at the pictures on the internet, it’s just horrifying to see house after house that has just fallen into disrepair.”

timing is everything

Even the time of day or season that an earthquake occurs can make a big difference in whether someone survives the quake.

“Deep night is generally not a good time for earthquakes,” Hough said. The first shock in Turkey hit around 4:15 a.m. local time, when most residents of the affected areas were indoors and asleep. Early-morning tremors often result in higher death tolls as crowded buildings collapse.

And while there’s never a good time of year for a massive earthquake, mid-winter could be the worst. Heavy snowfall and rain slowed rescue workers attempting to travel to the hardest-hit province of Kahramanmaras, and those who were able to reach the worst-hit areas worked in the cold rain. Nighttime temperatures in the area are well below freezing, making both survival and rescue even more difficult.

The consequences

The largest losses in an earthquake are often the result of “secondary effects” or catastrophes triggered by the original tremor. In the 1906 San Francisco earthquake, it was the fires that ravaged the city, as gas and water lines burst. In the 2004 Indian Ocean earthquake, it was the resulting tsunami that swept ashore, killing nearly 228,000 people.

Fires were reported in the port of Iskenderun on the Mediterranean Sea and along a gas pipeline in the Turkey earthquake. The US Geological Survey said the region also faces a significant to significant risk of landslides.