Enlarge (credit: Jason Edwards via Getty)

Earthquake scientists detected an unusual signal on monitoring stations used to detect seismic activity during September 2023. We saw it on sensors everywhere, from the Arctic to Antarctica.

We were baffled—the signal was unlike any previously recorded. Instead of the frequency-rich rumble typical of earthquakes, this was a monotonous hum, containing only a single vibration frequency. Even more puzzling was that the signal kept going for nine days.

Initially classified as a “USO”—an unidentified seismic object—the source of the signal was eventually traced back to a massive landslide in Greenland’s remote Dickson Fjord. A staggering volume of rock and ice, enough to fill 10,000 Olympic-sized swimming pools, plunged into the fjord, triggering a 200-meter-high mega-tsunami and a phenomenon known as a seiche: a wave in the icy fjord that continued to slosh back and forth, some 10,000 times over nine days.

Enlarge / Two skeletons found in the ruins of a Pompeii building may have been crushed by wall collapses triggered by earthquakes. (credit: Pompeii Archaeological Park)

Archaeologists have hotly debated the precise cause of death of those who perished in Pompeii when Mount Vesuvius erupted in 79 CE. Did they die of asphyxiation, from the extreme heat, or from a combination of factors? A new paper published in the journal Frontiers in Earth Science examines the complicating effects of earthquakes that occurred just prior to and concurrently with the eruption. Of most interest was the discovery of two skeletons of people who likely died when their shelter collapsed around them, weakened by the seismic tremors.

As previously reported, the eruption of Mount Vesuvius released thermal energy roughly equivalent to 100,000 times the atomic bombs dropped on Hiroshima and Nagasaki at the end of World War II, spewing molten rock, pumice, and hot ash over the cities of Pompeii and Herculaneum in particular. Archaeologists believed that the vast majority of the victims died of asphyxiation, choking to death on the thick clouds of noxious gas and ash.

However, a 2001 study in Nature, co-authored by University of Naples archaeologist Pierpaolo Petrone, estimated a temperature of 500° Celsius (932° Fahrenheit) for the pyroclastic surge that destroyed Pompeii, sufficient to kill inhabitants in fractions of a second. In 2018, we reported on Petrone's conclusion that inhabitants of Herculaneum may have suffered a similar fate. He observed fracturing in the bones of some 100 excavated skeletons, as well as "cracking and explosion" of the skullcaps, consistent with forensic cases where skulls burst from extreme heat.

Enlarge / One of the craters identified seismically, then confirmed through orbital images. (credit: NASA/JPL-Caltech/University of Arizona)

Mars trembles with marsquakes, but not all of them are driven by phenomena that occur beneath the surface—many are the aftermath of meteorite strikes.

Meteorites crash down to Mars every day. After analyzing data from NASA’s InSight lander, an international team of researchers noticed that its seismometer, SEIS, detected six nearby seismic events. These were linked to the same acoustic atmospheric signal that meteorites generate when whizzing through the atmosphere of Mars. Further investigation identified all six as part of an entirely new class of quakes known as VF (very high frequency) events.

The collisions that generate VF marsquakes occur in fractions of a second, much less time than the few seconds it takes tectonic processes to cause quakes similar in size. This is some of the key seismological data that has helped us understand the occurrence of earthquakes caused by meteoric impacts on Mars. This is also the first time seismic data was used to determine how frequently impact craters are formed.

Enlarge / Top: A view of the downtown Memphis skyline, including the Hernando De Soto bridge which has been retrofitted for earthquakes. Memphis is located around 40 miles from a fault line in the quake-prone New Madrid system. (credit: iStock via Getty Images)

The first earthquake struck while the town was still asleep. Around 2:00 am on Dec. 16, 1811, New Madrid—a small frontier settlement of 400 people on land now located in Missouri—was jolted awake. Panicked townsfolk fled their homes as buildings collapsed and the smell of sulfur filled the air.

The episode didn’t last long. But the worst was yet to come. Nearly two months later, after dozens of aftershocks and another massive quake, the fault line running directly under the town ruptured. Thirty-one-year-old resident Eliza Bryan watched in horror as the Mississippi River receded and swept away boats full of people. In nearby fields, geysers of sand erupted, and a rumble filled the air.

In the end, the town had dropped at least 15 feet. Bryan and others spent a year and a half living in makeshift camps while they waited for the aftershocks to end. Four years later, the shocks had become less common. At last, the rattled townspeople began “to hope that ere long they will entirely cease,” Bryan wrote in a letter.