Violent, ultra-audible earthquakes are much more common than previously thought

Photo of a building after an earthquake.

According to a study by UCLA geophysicists in Los Angeles, strong, super-hearing earthquakes, previously thought to be rare, are significantly more common than previously thought. The study was recently published in the journal Nature Geoscience.

The researchers examined all earthquakes of magnitude 6.7 or greater globally since 2000, and identified 12 supershear, or about 14%. (Four of the earthquakes have not been previously reported.)

This number is more than double what experts expected; Before, less than 6% of slope earthquakes were classified as super-quakes.

Turbulent slip earthquakes occur when the edges of two tectonic plates push one another aside. Superquakes are a subtype of that group that occur when faults rupture beneath the surface faster than shear waves—the seismic waves that shake the earth—can travel through rock. Similar to a sonic boom, the impact traps energy that is subsequently violently released.

As a result, compared to other earthquakes of similar magnitude, supernovae tend to cause more shaking and can be more destructive.

“When an aircraft flies faster than sound can travel through the air, a cone of suppressed sound waves forms in front of the aircraft and when they catch up with it, we hear them all at once,” said Lingsen Meng, Lyon and Joanne VC of UCLA Knopov Professor of Physics and Geophysics, and corresponding author of the paper. “Superhearing earthquakes are potentially more destructive than other types of earthquakes because they are more efficient at generating seismic waves, with more vibrations, which can cause more damage.”

The research also found that super-quakes occur as commonly under oceans as they do on land and that they are just as likely to occur along slip faults, such as the San Andreas fault in California.

The findings suggest that disaster planning efforts should consider whether nearby faults are capable of triggering super-earthquakes, and if so, take measures to prepare for a higher level of shaking and potential damage than non-super-earthquakes might cause.

The reason why there are relatively few super-earthquakes, Meng said, is that researchers mainly study earthquakes on Earth.

The paper’s co-authors are Han Bao and Liwei Xu of UCLA, and Jean-Paul Amberou, senior researcher at the University of Côte d’Azur in Nice, France.

The scientists used a method called a backproject to determine the direction in which the seismic waves arrived to infer how fast the earthquake was moving along the fault. This technique applies an algorithm to analyze the short time delay between seismic waves as they are detected by a set of sensors. This method is similar to the way a person can be located by tracking the signals their smartphone sends to cell towers.

The data revealed that super-earthquakes tend to occur on mature slip faults, where the edges of the two continental plates rub laterally against each other. In a mature fault, this action has been happening long enough to create a region of damaged rock that acts like a dam around the fault, slowing or preventing the propagation of seismic waves and concentrating their energy.

Ampero said the findings could help scientists better understand what it takes for a fault to produce the types of ruptures that lead to super-quakes.

In the past century, at least one major earthquake has occurred in California: In 1979, a 6.5-magnitude earthquake in southern California’s Imperial Valley injured people as far away as Mexico and caused severe damage to irrigation systems. And although it preceded scientific observation, the 1906 earthquake that caused massive damage in San Francisco also fell into the supershear category.

Not all infrasound earthquakes are catastrophic. The shape of the fault, surrounding rocks, and other factors can affect the propagation of seismic waves and limit the accumulation of energy. Faults that bend tend to slow, deflect, or absorb seismic waves, while straight faults allow them to flow freely.

In a previous study, Meng’s research group identified the catastrophic 7.5-magnitude earthquake that struck the Indonesian island of Sulawesi in 2018 as a paranormal event. The earthquake and subsequent tsunami killed at least 4,000 people. Despite the bending in the Indonesian earthquake’s fault, Meng said, the horrific damage was done because the fault moved faster than any previously recorded and it’s likely that energy from previous earthquakes was stored in the rocks, waiting for the moment to explode.

Fortunately, Meng said, super-oceanic earthquakes are less likely than earthquakes that cause the sea floor to move vertically to produce tsunamis.

On the other hand, the San Andreas Fault is mostly straight and could experience a more explosive rupture than the Sulawesi earthquake.

References: “Global Frequency of Oceanic and Super-Continental Earthquakes” By Han Bao, Liwei Xu, Lingsen Meng, Jean-Paul Ampero, Li Gao, and Haijiang Zhang, Oct. 31, 2022, Nature Geoscience.DOI: 10.1038/s41561-022-01055-5

“Early and persistent hearing rupture of the 2018 magnitude 7.5 Palu earthquake” By Han Bao, Jean-Paul Amberou, Lingxin Meng, Eric J. Fielding, Conren Liang, Christopher W.D. Millener, Tian Feng, Hui Huang, Feb. 4, 2019, Nature Earth Sciences. DOI: 10.1038/s41561-018-0297-z

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