28 January 2022–Stress transferred through the crust and upper mantle after the 1964 Great Alaska Earthquake may have been enough to trigger the 2018 Gulf of Alaska earthquake, according to a new study published in the Bulletin of the Seismological Society of America.
The model developed by Luyuan Huang at China’s National Institute of Natural Hazards and colleagues could help seismologists better understand the stress interactions between megathrust earthquakes that take place at tectonic plate boundaries, like the magnitude 9.2 event in 1964, and interplate earthquakes like the magnitude 7.9 quake in 2018.
The lower crust and upper mantle are viscoelastic materials, deformed by the application and relaxation of stress during earthquakes. (Toothpaste and play dough are common examples of materials that demonstrate viscoelastic properties.)
Huang and colleagues developed a viscoelastic spherical model to include the topographical relief and variations in viscosity in the Alaskan region, to calculate how stress might be transferred from the site of the 1964 earthquake to the 2018 rupture over time.
Based on this model, the researchers concluded that viscoelastic stress relaxation by the 1964 earthquake transferred through the crust could have created stresses that exceeded the amount needed to trigger the 2018 rupture fault. The closest distance between the two rupture zones is about 100 kilometers.
Huang and colleagues note, however, that this type of stress transfer isn’t the only factor that would influence the timing between megathrust events and triggered earthquakes. “The viscoelastic stress transfer will speed or delay the trigger events, but other parameters such as the elapsed time since the last event and the interseismic stress rate are also crucial in an earthquake cycle,” said Huang.
The 2018 Gulf of Alaska earthquake was a strike-slip event, which is an unusual occurrence in ocean crust near subduction zones. Another notable oceanic strike-slip event near a subduction zone is the 2012 magnitude 8.6 Indian Ocean strike-slip earthquake. Some researchers have proposed that viscoelastic relaxation of the 2004 magnitude 9.1 Sumatra-Andaman megathrust earthquake could have increased crustal stresses to trigger the Indian Ocean earthquake.
Huang and his colleagues think the occurrence of the two strike-slip earthquakes “may be hastened by the stress transfer of prior magnitude 9.0 thrust earthquakes,” he said. “However, no more similar earthquakes have been observed, and we do not think people should expect strike-slip intraplate events to occur after most interplate thrust events.”
To build their model, Huang and colleagues used GPS measurements taken from repeated surveys between 1992 and 2016 to look at deformation caused by the earthquake, as a way of learning more about how the continental mantle and oceanic mantle in the area deform and flow under stress. This information helped them determine the viscosities of the mantle for their model.
The researchers combined these viscosity estimates with fault slip models developed for the 1964 earthquake to calculate how viscoelastic stress might have been transferred under these conditions. Using a spherical Earth model allowed the researchers to account for the Earth’s curvature and lateral changes in viscosity to create a more realistic numerical model that they say could be used to explore viscoelastic stress transfer in other regions.