Answers About Seismic Shifts Deep in the Earth That Cause Most Devastating Earthquakes and Volcanic Eruptions

However, what governs the sinking motion of slabs within the mantle is not well understood.

A New Mexico State University researcher authored a paper published recently in “Nature Communications”that takes a look at how and why these giant slabs move. The article titled “Global Observations of Reflectors in the Mid-Mantle with Implications for Mantle Structure and Dynamics,” seeks to answer the question of why some slabs stop sinking at some depths, and become trapped there.

“In the mantle, there is hot material that rises up, and cold material that sinks back down, like in a pot of boiling stew. That is the plumes and the slabs. The downward-going material becomes trapped at a couple of depths,” said Lauren Waszek, NMSU assistant professor of physics who authored the paper. “We can see this in tomography models, which are analogous to CT scans of the Earth. Instead of X-rays, we use sound wave energy from earthquakes, or “seismic energy”. The tomography models map the speed that the waves travel. The slabs are cold from time spent at the surface, so waves travel faster, and they show up clearly in contrast to the warmer mantle material.”

Conceptual interpretation of observations of three distinct mantle domains. Potential sub-horizontal mid-mantle reflectors are denoted black (positive impedance) and white (negative impedance) and grouped by seismic domains: (1) Fast, cold downwelling regions (blue), with heterogeneities that are predominantly too small and variable to be resolved by SS precursors (major example region: Eastern Europe). (2) Slow, hot upwelling regions (red) (major example region: South-central Pacific). (3) Neutral regions (yellow), perhaps with compositionally or texturally distinct material (lighter yellow). Slabs may stagnate above these features, generating shallow reflections for the neutral domains (major example region: northern Pacific). Basaltic heterogeneity is denoted by black contours, with grey lines indicating heterogeneities that do not necessarily correspond to reflectors. Other known global reflectors in the mantle are indicated by black lines; e.g., at the 410 and 660 km depth.

Credit: NMSU photo

“We can see that some of the downgoing slabs stop sinking at about 1,000 kilometers depth and begin to move horizontally, but there is no good explanation for it. That was our question. Why do the slabs get stuck there? Hot plumes rising from the Earth’s deep interior are deflected sideways at this depth too: why?”

Waszek compiled and analyzed a dataset of more than 45,000 reflections of seismic energy in the mid-mantle. Reflectors are regional features that represent sharp changes in rock properties. The energy reflection is like a reflection from a mirror. It means there is a horizontal surface, where some material could potentially be trapped.

“We detect reflectors globally, with variable depths, geographic size, and reflection strength,” Waszek said. “The results identify three distinct domains in the mid-mantle, which are linked to the different up and downwelling features in the tomography models. The large variability of the reflectors indicates that the properties of these domains vary greatly.”

Waszek’s research determined that the three domains have highly distinctive observational signatures. Downgoing slabs, which do not stagnate, generate very few reflections since they are oriented close to vertically. Hot, upwelling regions produce many reflections, indicating where they are deflected in the mid-mantle. Domains with no hot material coming up or cold material going down contain laterally large and coherent reflectors. This last category may correspond to the trapped slabs.

Lauren Waszek, NMSU assistant professor of physics, authored a paper recently published in “Nature Communication” analyzing why slabs deep in the Earth become trapped at certain depths.

Credit: NMSU photo