A groundbreaking study published in The Seismic Record reveals that significant deformation occurs deep within Earth's mantle, particularly in areas where ancient tectonic slabs have gradually sunk over millions of years.
Global Mapping of Mantle Deformation
Researchers have long theorized a link between deep mantle deformation and these submerged slabs, but this study offers the first comprehensive global perspective. The team analyzed nearly 75% of the lowermost mantle, situated just above the core-mantle boundary, approximately 2,900 kilometers (1,800 miles) beneath the Earth's surface.
Led by Jonathan Wolf from the University of California, Berkeley, the research team utilized an extensive dataset, compiling and analyzing over 16 million seismograms from 24 data centers worldwide. This effort has resulted in one of the most detailed seismic datasets ever created.
Decoding Seismic Waves
Earthquakes generate shear waves that traverse through the Earth's interior, with their speeds varying based on direction and material properties. This phenomenon, known as seismic anisotropy, enables scientists to pinpoint areas of mantle deformation.
By examining these seismic patterns, researchers can better understand the dynamics of mantle flow and circulation over time. "While we know that upper mantle deformation is primarily influenced by the movement of tectonic plates, large-scale insights into the lowermost mantle remain limited," Wolf stated. "Our goal is to enhance this understanding."
Unlocking Patterns with Seismic Data
Employing what Wolf describes as "the largest collection of earthquake seismic data to date," the researchers studied various seismic wave phases that travel through the mantle, penetrate the core, and return to the mantle. These waves effectively map seismic anisotropy over extensive distances, revealing how deformation is distributed in the deepest mantle.
The findings indicate anisotropy across approximately two-thirds of the analyzed regions. While the patterns are intricate, most deformation seems concentrated in areas where deeply subducted slabs are believed to exist.
Understanding Subducted Slabs
Scientists continue to investigate the causes of seismic anisotropy in these slabs. One hypothesis suggests that the slabs may retain some "fossil" anisotropy from their earlier positions. However, a more plausible explanation is that intense deformation occurs as the slabs descend and interact with the core-mantle boundary, reshaping surrounding materials and altering minerals under extreme heat and pressure, thus creating a new anisotropic "fabric."
Future Research Directions
Wolf cautioned that regions without detectable anisotropic signals should not be assumed to be devoid of deformation, as some signals may be too weak for current detection methods. The extensive dataset from this study continues to serve as a valuable resource for ongoing research into Earth's deep interior.
"I envision a future where we have sufficient data to comprehensively understand global flow directions in the lowermost mantle, illuminating its complexities from multiple perspectives," Wolf expressed.