In a study led by geophysicists at UCLA, it was found that powerful supershear earthquakes happen more often than was previously thought.
The researchers examined 87 strike-slip earthquakes with a magnitude of 6.7 or higher that occurred around the world since 2000 and found 12 of the supershear type, or about 14% of them. Four of those earthquakes had never been reported before.
Less than 6% of strike-slip earthquakes had previously been classified as supershear, which means that this percentage is more than twice what scientists had anticipated.
Supershear Strike-slip Earthquakes
Strike-slip earthquakes occur when the edges of two tectonic plates push sideways against each other. Supershear quakes are a variant of that category that occur when faults under the surface break faster than shear waves — the seismic waves that shake the ground back and forth — can flow through rock. The effect constricts energy, which is then violently released, similar to a sonic boom.
As a result, compared to other earthquakes of the same magnitude, supershear earthquakes tend to cause more shaking and may be more destructive.
Because they are more efficient at producing seismic waves with greater shaking, which may result in greater damage, supershear earthquakes have the potential to be more destructive than other types of earthquakes, according to the paper’s corresponding author Lingsen Meng, UCLA’s Leon and Joanne V.C. Knopoff Professor of Physics and Geophysics.
Disaster Planning Implications
The research also showed that supershear earthquakes are just as common under the oceans as they are on land, and that they are more likely to happen along strike-slip faults like the San Andreas Fault in California.
The results imply that efforts to plan for disasters should consider whether nearby faults are capable of producing supershear earthquakes and, if so, take precautions to prepare for a higher level of shaking and potential damage than could be caused by non-supershear earthquakes.
Researchers primarily study earthquakes that occur on land, according to Meng, which explains why relatively few supershear earthquakes have been discovered.
Seismic Wave Backprojection
To estimate how quickly an earthquake moves along the fault, the researchers used a technique called backprojection to determine the direction in which seismic waves arrived. The method uses an algorithm to look at the short gaps of time between seismic waves picked up by several sensors. The process is analogous to finding someone by following the signals that person’s smartphone sends to cell towers.
Supershear earthquakes, according to the data, tend to occur on mature strike-slip faults where the borders of two continental plates scrape laterally against each other. In a mature fault, this event has occurred long enough to form a zone of damaged rocks that acts as a dam surrounding the fault, slowing or obstructing seismic wave propagation and concentrating their energy.
The results might improve our understanding of what causes a fault to rupture in the ways that cause supershear earthquakes, said co-author Jean-Paul Ampuero, a senior researcher at Université Côte d’Azur.
Are All Supershear Earthquakes Destructive?
On the area map on the left, the colored background is the ground displacement induced by the Palu earthquake and the thin black line is the fault, both derived from satellite radar images. The black dot is the city of Palu. The circles are spots that radiated waves during the earthquake; their color indicates time (blue at the beginning, red at the end). The right figure shows the timing and position of these earthquake radiators. Their alignment indicates a steady earthquake speed of about 4.1 km/s
Credit: © Han Bao et al., Nature Geoscience
At least one big supershear earthquake has occurred in California in the last century: A 6.5-magnitude earthquake in Southern California’s Imperial Valley region in 1979 harmed people as far away as Mexico and severely damaged irrigation infrastructure. Even though it happened before modern observation methods were in use, the 1906 earthquake that destroyed San Francisco was definitely a supershear event.
Not all supershear earthquakes are this destructive.
The geometry of the fault, the rocks surrounding it, and other factors can all affect seismic wave propagation and limit energy accumulation. Curved faults tend to impede, deflect, or absorb seismic waves, whereas straight faults allow them to flow freely.
The devastating 7.5 magnitude earthquake that struck the Indonesian island of Sulawesi in 2018 was classified as a supershear event in a previous study by Meng’s research team. At least 4,000 people were killed by the tsunami and subsequent earthquake.
Despite the curve in the Indonesian earthquake fault, the terrible damage happened because the fault moved faster than any previously recorded and because energy from earlier temblors was probably trapped in the rocks, waiting for a chance to explode, according to Meng.
Challenging Classical Earthquake Models
S-waves, which shear rocks and travel at a speed of about 3.5 km/s, and P-waves, which compress rocks and travel at a speed of about 5 km/s, are the two main seismic waves that cause shaking along a breaking fault.
The study raised questions about prevalent earthquake models. Such studies on other faults around the world could help us predict earthquake effects better because the impact of an earthquake depends greatly on its speed.
Real faults are encased in a layer of rocks that have been fractured and softened by previous earthquakes as opposed to the faults that live in classical earthquake models’ idealized intact rocks. Damaged rocks can actually rupture steadily at speeds that wouldn’t be expected for rocks that are still whole. This is because the speed of seismic waves is slower in damaged rocks.
Fortunately, supershear earthquakes in the ocean are less likely to produce tsunamis than earthquakes that cause the sea floor to shift vertically. In contrast, the San Andreas Fault is largely straight and has the potential for a rupture that is even more explosive than the Sulawesi earthquake.
References:
Bao, H., Xu, L., Meng, L. et al. Global frequency of oceanic and continental supershear earthquakes. Nat. Geosci. (2022).
Bao, H., Ampuero, JP., Meng, L. et al. Early and persistent supershear rupture of the 2018 magnitude 7.5 Palu earthquake. Nat. Geosci. 12, 200–205 (2019).