An image of the aftermath of the magnitude 9.2 Great Alaska earthquake in 1964
Subduction zones can produce destructive earthquakes, like the magnitude 9.2 Great Alaska earthquake in 1964, which occurred along the Alaska-Aleutian subduction margin. Credit: U.S. Army
Source: Geophysical Research Letters

Subduction zones produce some of Earth’s largest and most damaging earthquakes, including the 1964 Great Alaska earthquake and the 2011 Tōhoku-oki earthquake. The magnitude 9.2 Great Alaska earthquake produced tsunamis that caused deaths and damage as far away as California. The magnitude 9–9.1 Tōhoku-oki earthquake rocked the seafloor off the coast of Japan, producing a devastating tsunami that caused more than 18,000 deaths. Understanding how physical properties of fault rock affect slip behaviors at subduction plate boundaries will help scientists better assess where similar megathrust earthquakes might occur.

On the basis of measurements of slow seismic wave speeds, scientists previously hypothesized that rock along a subduction plate interface has a high pore pressure due to anomalously high fluid content. Miller et al. now provide an alternative explanation based on analyses of exhumed metamorphosed sediments from Kodiak Island, located off the southern coast of Alaska. These samples resemble rocks found at the plate interface of the Alaska-Aleutian subduction zone.

These metamorphic rocks are foliated and have repetitive sheetlike structures. The researchers made acoustic velocity measurements of these samples in the laboratory and found that measurements made perpendicular to the layers reproduced slow seismic wave speeds previously measured along the eastern Aleutian margin. Physics-based models confirmed that the mineral composition of the samples and cracks aligned with foliation and could explain the measured velocities.

The researchers propose that elevated pore pressure isn’t required for the slow seismic wave speeds that have been observed along megathrusts. Instead, they suggest that the velocities could simply be due to the presence of foliated metasediments. The new data underscore the importance of accounting for rock properties in studies of subduction zones. (Geophysical Research Letters,, 2021)

—Jack Lee, Science Writer

Citation: Lee, J. (2021), Rock structure explains slow seismic waves, Eos, 102, Published on 7 December 2021.
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