A global survey of large strike-slip earthquakes (greater than magnitude 6.7) reveals that over a 20-year period, 14 percent of such earthquakes sustained supershear rupture speeds.
By Laura Fattaruso, Simpson Strong Tie Fellow (@labtalk_laura)
Citation: Fattaruso, L., 2022, Speed kills: fast, potentially damaging earthquakes more common, Temblor, http://doi.org/10.32858/temblor.289
The phenomena known as supershear earthquakes — those that travel super fast and can cause more shaking than slower quakes — may be much more common than previously estimated, according to a study of global earthquake data recently published in Nature Geoscience.
Supershear earthquakes occur when the rupture front of an earthquake moves faster than the seismic waves it produces. The rupture front is the leading edge of the breaking fault plane that generates an earthquake. The rapid advancement of the rupture front causes the waves to pile up together and form a much larger wave, known as a Mach cone wave — the earthquake equivalent of a sonic boom. The Mach cone wave generates larger-amplitude waves that produce stronger shaking than a typical earthquake. The authors of the new study suggest that estimates of seismic hazard for faults should take into account whether they can produce the more damaging supershear ruptures.
Long, straight and overstressed
According to Lingsen Meng, a seismologist at UCLA and co-author of the study, the most likely candidates to host such earthquakes are strike-slip faults that are long, straight, overstressed and mature (meaning they have generated many earthquakes before). The San Andreas Fault in California, the North Anatolian Fault in Turkey, and the Alpine Fault in New Zealand are all examples of faults that meet the criteria, according to Meng.
The first direct measurements of supershear earthquakes were in 1999 when two supershear temblors struck the North Anatolian Fault in Turkey, first in İzmit and then in Düzce (also site of a quake in late November 2022). The magnitude-7.6 İzmit earthquake that struck on Aug. 17, 1999, was one of the deadliest in modern Turkish history, and was followed a few months later by the magnitude-7.2 Düzce quake on Nov. 12, with both quakes generated by different sections of the North Anatolian Fault.
In 2018, a magnitude-7.5 supershear earthquake struck the Indonesian island of Sulawesi, including the capital city, Palu. The quake caused the ground to behave like a liquid (called liquefaction), and triggered mudflows and landslides that resulted in widespread and severe damage from shaking. That earthquake also instigated a tsunami much larger than had been predicted for the strike-slip rupture; strike-slip quakes don’t typically produce the necessary vertical motion that displaces water and causes tsunamis. While some researchers have concluded that liquefaction-induced landslides caused the unexpectedly large tsunami (Sassa and Takagawa, 2019), others concluded that seafloor uplift along the fault combined with enhanced shaking from supershear rupture were the cause (Ulrish et al., 2019).
How unusual are supershear events?
To estimate the global prevalence of these extreme events, the researchers focused on strike-slip earthquakes greater than magnitude 6.7. While smaller earthquakes could also reach supershear speeds, the current methods of observation can only identify supershear rupture that is sustained for 50 kilometers (31 miles) or more, meaning that for now, only these larger earthquakes will have detectable supershear ruptures.
Out of the 87 large strike-slip earthquakes that occurred between 2000 and 2020, 12 exhibited supershear speeds. From these data, researchers estimate that about 14 percent of large strike-slip earthquakes reach supershear speeds, more than double the previous estimate of about 6 percent.
Using new methods that refine estimates of rupture travel speeds from distant earthquakes, the researchers found four previously unidentified supershear quakes. All of the newly identified supershear earthquakes occurred on oceanic faults. Previous estimates of how common supershear earthquakes are did not include underwater faults, but the ocean floor is home to many long strike-slip faults — the exact type of fault that can generate this type of rupture. By considering earthquakes from both continental and oceanic sources, the researchers have provided a clearer picture of how common these events are.
“There has never been a global survey like this because we didn’t have adequate tools available before,” explains Meng.
The study used multiple methods to identify supershear earthquakes. Back-projection — a method first developed for use with sonar and radar data — has been advanced and honed as a method within seismology, allowing for better resolving rupture speeds. Using arrival times of P-waves at different seismic stations around the planet, the travel speed of the earthquake rupture front can be estimated. When back-projection showed supershear speeds, the researchers also looked for the Mach cone wave in the seismic data, which produces a distinct pattern of shaking compared to slower earthquakes.
From theory to observation
“In general, monitoring the rupture speed is a challenge,” says Elif Oral, a geotechnical earthquake engineer at Caltech who was not involved with the study. The existence of supershear earthquakes was predicted by numerical models and observed in lab experiments producing fractures in plastic before the tools for measuring them on the Earth’s surface existed. Determination of whether they truly happen has been controversial in the past, but over the last two decades, the methods for measuring them have improved. “This emergent technique of back-projection that was applied to large earthquakes, like the Indonesian earthquake in Palu, reduces the doubt and highlights that this phenomenon really happens,” Oral explains. “Now we can monitor it better.”
While the study has answered some questions about the global prevalence of these events, many outstanding questions remain. Several supershear events were identified in the Caribbean on faults that have oceanic crust on one side and continental crust on the other side, suggesting that juxtaposition of different materials across a fault may also promote the super-fast ruptures. The theory behind supershear earthquakes also implies that they must move above a certain speed to sustain their high-speed propagation. However, many of the quakes observed in the latest study moved slower than that threshold, opening up questions about what may be missing from the theory. Meng suggests that damage along preexisting fault zones could be one explanation for the discrepancy. The global survey has set the stage for future investigations of supershear earthquakes and the conditions that promote them.
Laura Fattaruso is Temblor’s Simpson Strong Tie Fellow. They are a Ph.D. candidate at U Mass Amherst, where they study how rocks break to better understand earthquake processes (laurafattaruso.com). Simpson Strong Tie is sponsoring a science writing fellow to cover important earthquake news across the globe.
References
Bao, H., Xu, L., Meng, L. et al. Global frequency of oceanic and continental supershear earthquakes. Nat. Geosci. 15, 942–949 (2022). https://doi.org/10.1038/s41561-022-01055-5
Sassa, S., & Takagawa, T. (2019). Liquefied gravity flow-induced tsunami: first evidence and comparison from the 2018 Indonesia Sulawesi earthquake and tsunami disasters. Landslides, 16(1), 195-200.
Song, S. G., Beroza, G. C., & Segall, P. (2008). A unified source model for the 1906 San Francisco earthquake. Bulletin of the Seismological Society of America, 98(2), 823-831.
Ulrich, T., Vater, S., Madden, E. H., Behrens, J., van Dinther, Y., Van Zelst, I., … & Gabriel, A. A. (2019). Coupled, physics-based modeling reveals earthquake displacements are critical to the 2018 Palu, Sulawesi tsunami. Pure and Applied Geophysics, 176(1), 4069-4109.