Strong Supershear Earthquakes Are More Common Than Thought

According to a study conducted by UCLA geophysicists and published today in nature geoscience.

Scientists analyzed all the strike-slip earthquakes of magnitude 6.7 or greater around the world since 2000 – there were 87 in all – and identified 12 of the supershear type, or about 14%. (Four of these earthquakes had not previously been reported.)

This percentage is more than double what scientists expected; so far less than 6% of strike-slip earthquakes have been identified as super shear.

Strike-slip earthquakes occur when the edges of two tectonic plates rub against each other laterally. Super shear earthquakes are a subtype of this group that occur when faults beneath the surface rupture faster than shear waves — seismic waves that shake the ground back and forth — can move through the rock. The effect captures the energy which is then violently released; the effect can be compared to a sonic boom.

As a result, supershear earthquakes tend to cause more shaking and are potentially more destructive than other earthquakes of the same magnitude.

“When a plane flies faster than sound can travel through air, a cone of pent-up sound waves forms in front of the plane and when it catches up to it, we hear it all of a sudden,” said Lingsen Meng, Leon and Joanne VC of UCLA. Knopoff Professor of Physics and Geophysics, and corresponding author of the paper. “Super shear earthquakes are potentially more destructive than other types of earthquakes because they are more efficient at generating seismic waves, with more shaking, which could cause more damage.”

The research also found that supershear earthquakes occur just as often under the oceans as they do on land, and are more likely to occur along strike-slip faults, such as the San Andreas Fault in California.

The results suggest that disaster planning efforts should consider whether nearby faults are capable of producing super shear earthquakes and, if so, take action to prepare for a higher level. more shaking and potential damage than could be caused by non-supershear earthquakes.

Meng said the reason why relatively few supershear earthquakes have been found is that researchers mainly study earthquakes on land.

The co-authors of the article are UCLA doctoral students Han Bao and Liuwei Xu of UCLA and Jean-Paul Ampuero, senior researcher at the Université Côte d’Azur in Nice, France.

The scientists used a method called backprojection to determine the direction from which the seismic waves arrived to infer how fast an earthquake is moving along the fault. The technique applies an algorithm to analyze the short delays between seismic waves when they are detected by a group of sensors. The method is similar to how one can locate a person by tracking the signals their smartphone sends to cell towers.

The data revealed that supershear earthquakes tend to occur on mature strike-slip faults, in which the edges of two continental plates rub laterally against each other. In a mature fault, this action has been occurring long enough to create an area of ​​damaged rock that acts as a barrier around the fault, slowing or blocking the propagation of seismic waves and focusing their energy.

Ampuero said the findings could help scientists better understand what it takes for a fault to produce the types of ruptures that lead to super shear earthquakes.

In the past century, at least one large supershear earthquake has occurred in California: in 1979, a magnitude 6.5 earthquake in the Imperial Valley region of Southern California injured people as far away as Mexico and caused extensive damage to irrigation systems. And, although it predates scientific monitoring, the 1906 earthquake that caused extensive damage in San Francisco likely also fell into the supershear category.

Not all supershear earthquakes are equally disastrous. The shape of the fault, the rocks surrounding it, and other factors can affect the propagation of seismic waves and limit energy buildup. Bending faults 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 hit the Indonesian island of Sulawesi in 2018 as a super shear event. The quake and subsequent tsunami killed at least 4,000 people. Despite the curve of the Indonesian earthquake fault, the horrific damage occurred because the fault moved faster than anything previously recorded and energy from earlier tremors was likely stored in the rocks, waiting for a moment to burst, Meng said.

Fortunately, Meng said super shear earthquakes in the ocean are less likely than earthquakes that cause the seafloor to shift vertically to produce tsunamis.

The San Andreas Fault, on the other hand, is mostly straight and could rupture even more explosively than the Sulawesi earthquake.

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