Violent supershear earthquakes are more common than previously thought

Powerful supershear earthquakes, once considered rare, are much more common than previously thought, according to a study led by UCLA geophysicists and published today in Natural Geosciences.

The scientists analyzed all 6.7-or-greater strike-slip earthquakes worldwide since 2000 – there were 87 in total – and identified 12 of the supershear type, or about 14%. (Four of these earthquakes were previously unreported.)

That percentage is more than double what scientists expected; So far, less than 6% of earthquakes that strike have been identified as supershear.

Strike-slip earthquakes occur when the edges of two tectonic plates rub side by side against each other. Supershear quakes are a subtype of that group, which are caused when faults below the surface break faster than shear waves — the seismic waves that shake the ground back and forth — can move through rock. The effect corrals 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 travels through the air, a cone of raised sound waves is created in front of the plane and when it picks up, we hear everything at once,” said Lingsen Meng, UCLA’s Leon and Joanne VC. Knopoff Professor of Physics and Geophysics, and the corresponding author of the paper. “Supershear earthquakes are potentially more destructive than other types of earthquakes because they are more effective at generating seismic waves, with more shaking, which could cause more damage.”

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

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

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

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

Scientists used a method called back projection to determine the direction in which seismic waves arrived to determine how fast an earthquake moves along the fault. The technique applies an algorithm to analyze short time delays between seismic waves as 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 the cell tower.

The data revealed that supershear earthquakes tend to occur on mature strike-slip faults, in which the edges of two continental plates rub against each other laterally. In a mature fault, this action has occurred long enough to create a zone of damaged rocks that act like a dam around the fault, slowing or blocking seismic wave propagation and concentrating their energy.

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

In the past century, at least one large supershear earthquake has occurred in California: In 1979, a 6.5-magnitude earthquake in southern California’s Imperial Valley region injured people as far away as Mexico and caused extensive damage to irrigation systems. And, although there was scientific monitoring beforehand, the 1906 earthquake that caused extensive damage in San Francisco probably also fell into the supershear category.

Not all supershear earthquakes are this catastrophic. The shape of the fault, the surrounding rocks and other factors can affect the propagation of seismic waves and limit the accumulation of energy. Faults that curve tend to slow down, deflect or absorb seismic waves, while straight faults let them 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 supershear event. The tsunami and tsunami killed at least 4,000 people. Despite the curve in the Indonesian earthquake fault, the terrible damage occurred because the fault moved faster than any previously recorded and energy from previous showers is likely stored in the rocks, waiting for a moment to burst, Meng said.

Fortunately, Meng said supershear earthquakes in the ocean are less likely than earthquakes that cause the sea floor to produce vertical tsunamis.

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