“It’s like a doctor who has been examining blood circulation with ultrasound for decades and finds arteries exactly where he expects them,” ETH Zurich Professor Andreas Fichtner stated. “Then if you give him a new, better examination tool, he suddenly sees an artery in the buttock that doesn’t really belong there. That’s exactly how we feel about the new findings.”
Under the colossal Pacific Ocean, almost 600 miles deep, scientists have found seismic anomalies defying traditional notions of the Earth’s interior. The finding, thanks to improvements in full-waveform inversion (FWI) seismic imaging, has created waves in the world of geoscience. These unanticipated forms in the lower mantle areas where seismic waves conduct themselves in manners incomprehensible given known geological processes raises fundamental questions regarding the dynamic past of the planet and the forces that continue to mold it.
The path to this realization began by investigating seismic waves, the vibration waves generated by earthquakes that pass through the planet’s layers. Seismologists use the unique characteristics of P waves and S waves to make conclusions about Earth’s interior condition and structure. P waves, which compress and expand the material in the direction of travel, can travel in solids, liquids, and gases, while S waves, which shear material at right angles to their direction of travel, can travel in solids only. The contrasts in wave speed and travel enable researchers to map the planet’s internal structure more precisely than ever.
Conventional seismic tomography has long speculated that positive velocity anomalies of seismic waves patches where seismic waves travel more quickly than usual are linked with oceanic lithosphere slabs being pushed beneath the Earth’s surface. When plates descend into the mantle at subduction zones, they form cold, dense bodies that persist for millions of years. But Pacific anomalies are well out of range of any of the existing subduction zones and pose a challenge to existing models. According to a new paper in Scientific Reports, “these findings suggest more diverse origins for these anomalies,” evoking processes more complex than simple plate recycling.
The strength of full-waveform inversion is that it uses the entire seismic waveform, rather than arrival times, to build high-resolution, three-dimensional images of the mantle. This computationally demanding technique, with the help of supercomputers and large data sets, has shown a far more complex mantle structure than was conceived. UT Dallas’ Dr. Hejun Zhu explained how FWI allowed his research group to directly image 3D mantle flow fields beneath subduction conditions, verifying the hypothesis that plates that are sinking have broken up, not in one large chunk. Some researchers have hypothesized that this fragmentation occurs, and our imaging and modeling provides evidence that supports that view.
It is what makes the Pacific anomalies so intriguing, their solitude. Whereas most of the positive wave speed anomalies are caused by earlier subduction over the past 130 million years, the western Pacific anomaly is under the track of the oldest part of the Pacific Plate, far removed from recent subduction. The east Pacific anomalies are under the East Pacific Rise, an ancient spreading center. This geometric configuration negates the previous assumption of one-to-one correlation between positive anomalies and slabs being subducted. To the contrary, recent studies hold that as few as 60-70% of reconstructed subduction geometries agree with positive anomalies and statistical tests abandon a correlation also at the 1% significance level.
The causes of these enigmatic features are still contentious. One theory suggests they are a remnant of ancient, silica-rich material from initial phases of mantle formation almost four billion years ago. Another theory is that they are regions where iron-bearing rock has accumulated through convection of the mantle. These alternative theories are proof of the deep Earth’s complexity, including plate tectonics, mountain building, and volcanism fueled by convection in the mantle the slow, heat-fueled circulation of rock. The flow of the mantle is controlled by a mix of thermal gradients, gravity, and physical rock properties, so it is an active system partially comprehensible.
The significance of these discoveries extends beyond. The distribution and character of deep-mantle anomalies are important to make models of plate tectonics and convection of the mantle more refined. These models then control our comprehension of the manner in which continents migrate, in which ocean basins open and close, and in which the surface of the world evolves on geological time scales. Having unseen structures lurking beneath the Pacific implies that plate tectonic and mantle history is maybe even more elaborate than one had in mind previously, involving phases of slab detachment, fragmentation, and possibly even the preservation of ancient domains within the mantle.
Seismic wave passage, the basis upon which this finding relies, continues to be a pillar of geophysical inquiry. As seismic waves travel through the Earth, speed and course are controlled by the constitution, density, and state of the materials through which they are traveling. The intricate reciprocal dependency of P and S waves and surface waves such as Love and Rayleigh waves opens an epistemological window during which areas that cannot be seen directly become readable. The ongoing progress in seismic imaging technologies, combined with computing advancements, is the key to unlocking further secrets buried deep beneath our feet.
As Professor Fichtner and his team continue probing mantle secrets, the recognition of such anomalies says much about the solidity of modern geophysics. The Earth, it appears, still has many surprises to keep in store in its interiors reminders that the history of the Earth remains far from complete.
