Distribution of global earthquakes used in this study. Credit: Geophysical Research Letters (2025). doi: 10.1029/2025gl116411
When a slab slides beneath a dominant slab in a subduction zone, the slab acquires a property called anisotropy, which means its strength is not the same in all directions. Anisotropy is what makes a wood board break more easily along the grain than in other directions. In rocks, the alignment of minerals such as clay, serpentine, and olivine can lead to anisotropy. Pockets of water in rocks can also cause and enhance anisotropy, as repeated dehydration and hydration commonly occur at depth in a subducted slab.
It is known that an earthquake generates a compression wave and a shear wave. If a shear wave passes through an anisotropic rock, it can split into a faster shear wave and a slower wave with different polarizations.
Although seismologists routinely measure shear wave splitting in subduction zones by analyzing recorded seismic waveform data, it is difficult to determine where splitting occurs along the wave propagation path.
In the past, researchers have studied the Earth's interior rotation for answers, especially in the mantle wedge region above and below the plate. However, Sharmila Abene and her colleagues propose a different explanation: Contrary to popular wisdom, it is the descending plate that causes most of the shear wave splitting. Their findings were published in the journal Geophysical Research Letters.
The researchers tested their theory using recordings of 2,567 shear waves from the Alaska-Aleutian subduction zone. They found that the way waves split as they propagate across the slab varied depending on the location of the earthquake, and that these differences were consistent with the anisotropy observed in the dipping slab. They also used a forward model to predict that the splitting pattern would differ depending on the direction the shear wave was coming from, which they verified by observing the data. Previously, scientists thought the difference in splitting patterns was due to complex mantle flows.
Moreover, the anisotropic slab also explains why unusual seismic wave radiation patterns are found in earthquakes deep within the slab. Other recent findings also indicate that the formation of subducting plates causes anisotropy, the authors write.
The researchers suggest that if the slab carries most of the variability, rather than the mantle wedge or subplate zone, then this discovery has far-reaching consequences that could radically change long-held ideas about how mantle dynamics work and how rocks deform.
The authors say these results underscore the possibility that plate anisotropy is a poorly studied element of seismology and geodynamics.
More information: Sharmila Abney et al., Analysis of shear wave splitting patterns in Alaska: Evidence for strong intra-slab heterogeneity, Geophysical Research Letters (2025). doi: 10.1029/2025gl116411
This story is republished with permission from Eos, hosted by the American Geophysical Union. Read the original story here.
Citation: New seismic model goes against the grain (2025, October 27) Retrieved October 28, 2025 from https://phys.org/news/2025-10-earthquake-grain.html
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