Deep under the islands of Japan, the crust and mantle of the Earth move and crack. Four lithospheric plates—the Eurasian, Okhotsk, Pacific, and Philippine Sea plates—crunch together there. The strong interactions at the interface of those plates make the Japan subduction zone a prime location for active volcanoes, tsunamis, and earthquakes.
As earthquakes and other seismic events occur, they send waves of energy rumbling through the Earth. In studying how those waves move in three dimensions, Liu and Zhao developed an increased understanding of how the lithospheric plates interact with the area around them and react to the energy pulsing through them.
Seismic events like earthquakes release energy through the Earth. As the waves propagate through varying materials, they can reveal the seismic anisotropy, or how the velocity of energy traveling through the Earth is affected by the direction or angle of propagation. When there are vertical cracks in the Earth—as is particularly common in subduction zones where the crust and mantle are most stressed—it causes azimuthal anisotropy, which means that the horizontal direction of wave propagation has a greater effect on variations in velocity.
The researchers used the Kiban network of 1852 seismic stations to record the travel times of seismic waves from 2528 earthquakes in and around the Japan Islands. They also recorded seismic wave travel times from 747 other teleseismic events, or earthquakes that originated more than 3000 kilometers from the station sites. From these two data sets, the authors were able to recreate the motion of the waves, building a high-resolution three-dimensional map of the azimuthal anisotropy structure of the Japan subduction zone down to an unprecedented depth of 700 kilometers.
The study found that energy waves traveled faster parallel to trenches along the subducting Pacific and Philippine Sea plates, perhaps because of the orientation of certain minerals or faults on the ocean floor. However, things get more complicated in Earth’s mantle: Plate subduction and dehydration joined with convective circulation to cause energy to flow perpendicularly to the trenches and even in a toroidal pattern around a hole in the Philippine Sea plate. Unexplained anomalies remain, particularly under the Pacific slab beneath northeast Japan, but this study has provided a more detailed description of the Japan subduction zone than any previous research. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2016JB013116, 2016)
—Leah Crane, Freelance Writer