Sonic Boom Effect Powerful Supershear Earthquakes Not so Rare, Study Proves

A study led by UCLA geophysicists found that strong supershear earthquakes with sonic boom effects are much more common than previously believed.

The researchers examined 87 strike-slip earthquakes with a magnitude of 6.7 or higher that occurred worldwide since 2000 and found 12 to be of the supershear type, or around 14%. Four of those powerful earthquakes had never been reported before.

Sonic Boom Effect and Supershear Earthquakes

Under 6% of strike-slip earthquakes had previously been classified as supershear, which means that this percentage is more than twice what scientists had anticipated.

Whenever the seams of two tectonic plates collide sideways, strike-slip earthquakes result.

As faults beneath the surface fracture more quickly than shear waves, the seismic waves that rattle the ground back and forth can travel through rock, and supershear earthquakes, a subtype of that group, are the result. The effect is similar to a sonic boom in that it gathers energy that is then violently released.

As a result, compared to other earthquakes of the same magnitude, supershear earthquakes typically cause more shaking and may be more destructive.

Lingsen Meng, the Leon and Joanne V.C. Knopoff Professor of Physics and Geophysics at UCLA, stated that Supershear earthquakes have the potential to be more destructive than other types of earthquakes because they are more effective at producing seismic waves. This can result in more shaking and damage. Meng is the corresponding author of the study.

Beneath Oceans

The study also discovered that strike-slip faults, like California's San Andreas Fault, are where supershear earthquakes are most likely to strike, and that these earthquakes happen just as frequently underneath the oceans as it occurs on land.

Considering whether faults near the area are capable of triggering supershear earthquakes and, if so, taking precautions to gear up for a higher degree of shaking and potential damage that might be caused by non-supershear earthquakes, are suggested as important considerations in disaster planning efforts.

Meng claimed that because researchers focus primarily on earthquakes on land, comparatively few supershear earthquakes were discovered.

Backprojection Method

Han Bao and Liuwei Xu, both doctoral candidates at UCLA, are co-authors of the study along with Jean-Paul Ampuero, a senior researcher from the Université Côte d'Azur in Nice, France.

To estimate how quickly an earthquake moves along the fault, the researchers used a technique called backprojection to pinpoint the direction along which seismic waves arrived. The method uses an algorithm to examine brief intervals of time between seismic waves whilst they are picked up by several sensors.

The process is comparable to finding someone by following the signals that a person's smartphone transmits to cell towers.

The data showed that mature strike-slip faults, where the seams of two continental plates press against each other laterally, are where supershear earthquakes typically occur. This process has been going on for long enough in a mature fault to produce a zone of damaged rocks that act as a dam surrounding the fault, decelerating or preventing seismic wave progression and concentrating their energy.

According to UCLA Newsroom, Ampuero said that the results might improve our understanding of what causes a fault to rupture in the ways that cause supershear earthquakes.

California's Supershear Earthquake

There has been at least one significant supershear earthquake in California over the past century. A 6.5-magnitude earthquake that struck Imperial Valley, Southern California, in 1979 left many irrigation systems severely damaged and people injured, reaching as far away as Mexico. The 1906 earthquake that severely damaged San Francisco, though it occurred before scientific monitoring, was also probably a supershear event.

Not every supershear earthquake is extremely destructive. The seismic wave propagation and the amount of energy that can accumulate can be influenced by the fault's shape, the rocks nearby, and other elements. Seismic waves are more likely to be slowed, refracted, or absorbed by curved faults than by straight ones.

The devastating 7.5 magnitude earthquake that struck Sulawesi in 2018 was classified as a supershear event in a previous study by Meng's research team. According to Meng, tsunamis are less likely to be produced by supershear earthquakes in the ocean than they are by earthquakes that trigger vertical movement on the seafloor.

Contrarily, the San Andreas Fault is largely straight and has the potential for a rupture that is even more explosive than the Sulawesi earthquake, Phys Org reports.

The UCLA study was just released in the Nature Geoscience journal.