The iconic Arqam Babu Rahman Mosque on the coast of Palu City, which was damaged and partially submerged during the 2018 earthquake and tsunami (Image: Nur Rokhman on Unsplash)
The 1999 Izmit earthquake in Turkey (MW 7.6) and the 1905 San Francisco earthquake in the USA (MW 7.8) are the oldest known earthquakes thought to have exhibited supershear rupture propagation (by direct observation and inferred, respectively) – whereby the rupture velocity of the strike-slip earthquake exceeded the velocity of the shear wave it generated in the crust. Since then, a few more earthquakes have been shown or inferred to be supershear events, most recently in China and Indonesia.
In 2021, the MW 7.4 Maduo earthquake struck the Qinghai Province of China. Using seismic and geodetic data analyses, Qi Li at the China Earthquake Administration, Wuhan, and their team show that the earthquake was bilateral in nature, propagating both east and west from its epicentre. Based on the asymmetric rupture velocities, the team propose that the eastern part of the rupture entered supershear. This finding is supported by Xu Zhang at the China Earthquake Administration, Beijing, and their team, who use regional and global seismic data, strong motion and InSAR satellite data to reconstruct the event. The team identify a peculiar aftershock pattern, one that has previously been attributed to supershear ruptures, whereby the supershear portion of the rupture towards the east exhibits low aftershock activity compared to the western rupture. The team also identify a Mach wave – a cone-shaped, sharp wavefront created as the rupture propagates faster than the waves radiating from it, which is typical of supershear events. The rupture had a velocity of about 4.0 km/s, which exceeds the local shear-wave velocity of 3.5 km/s, a finding that is supported by a separate study from Mingzhe Lyu at the Southern University of Science and Technology, Shenzhen, and their team. This group use global navigation satellite system (GNSS) and InSAR data to confirm that, locally at least, the event was supershear, reaching velocities of 3.8 km/s.
Xu Zhang and colleagues go on to propose an interesting new method to validate supershear events. They compile estimates of the moment-scaled radiated energy (the relationship between the radiated seismic energy and the seismic moment or strain release) for large earthquakes globally between 1999 and 2021. They show that earthquakes that are thought to have been supershear have low moment-scaled radiated energy (at least in cases where the fault geometry is simple).
Another supershear earthquake is thought to have occurred in Palu, Indonesia, in 2018, an event that generated a devastating tsunami, despite its occurrence in a strike-slip setting, killing over 4,000 people. Kanghua Zhang at Tianjin University, China, and colleagues, show that this MW 7.5 earthquake reached supershear velocity on the northern part of its rupture, on a part of the fault that experienced rotation of the regional principal stress and peak ground acceleration. In contrast, Chuanyou Li at the China Earthquake Administration, Beijing, and their team argue that rupture accelerated in the middle and southern segments of the fault, reaching supershear speeds of up to 4.1 km/s. They suggest that while overall the fault acted as a strike-slip fault, a component of normal slip of up to ~2 m could have triggered the tsunami. Faisal Amlani at the University of Southern California, Los Angeles, USA, and their team further investigate the cause of the tsunami using ground motion data and a shallow water wave model. They are able to reproduce the evolution of the tsunami, as captured by CCTV cameras, and conclude that the Mach wave generated during supershear rupture could have interacted with the bathymetry, contributing to the devastating tsunami.
Together this flurry of fascinating studies published in 2022 provide new insights that are vital for disaster management, risk assessment and mitigation, as well as our understanding of supershear earthquake physics.
R. Arun Prasath
Details:
Seismological Research Letters 93, 1429-1439 (2022); doi.org/10.1785/0220210300
Geophysical Research Letters 49, p.e2022GL097984 (2022); doi.org/10.1029/2022GL097984
Tectonophysics 839, 229542 (2022); doi.org/10.1016/j.tecto.2022.229542
Geomatics, Natural Hazards & Risk 13, 1987-2005 (2022); doi.org/10.1080/19475705.2022.2104659
Geological Magazine 159, 893-903 (2022); doi.org/10.1017/S0016756822000012
Geophysical Journal International 230, 2089-2097 (2022); doi.org/10.1093/gji/ggac162
