The other Gutenberg

When German seismologist Beno Gutenberg accepted a position as professor of geophysics at the California Institute of Technology in 1930, he was delighted by the chance to work in a “wonderful seismological laboratory” in sunny Pasadena. He was also “very much afraid that we would have a second World War,” according to a 1980 interview with his widow, Hertha.

So, to further the scientific career that he had been struggling to sustain at home, Gutenberg moved his family away from troubled Germany.

Among his important scientific contributions, Gutenberg identified the depth of Earth’s core–mantle boundary (see figure 1), a discontinuity where seismic secondary waves are absorbed by the liquid core. At the top of the mantle, some 40 to 100 km beneath the ocean basins, another sharp seismic discontinuity exists. In that region a rheological change indicates the transition from cold, rigid lithosphere to the low-seismic-velocity, hot, convecting asthenosphere. Both the core–mantle discontinuity and the boundary above the low-velocity zone have been referred to as the Gutenberg discontinuity, or simply the G.

One theory of plate tectonics says that a layer of molten rock lubricates the boundary between lithosphere and asthenosphere and allows crustal plates to slide about. If that’s the case, the discontinuity should extend across an entire plate.

But it doesn’t—which means some mechanism besides a layer of melt is allowing the plates to slide.

“The G discontinuity appears to be intermittent in depth and brightness. Thus it only shows up well in some regions, so it is difficult to tell whether it is a continuous feature,” says seismologist Peter Shearer of the University of California, San Diego.

In past studies, scientists could not determine whether the G was present everywhere beneath the plate or only in isolated patches. In a new study, Nick Schmerr of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, (http://www.sciencemag.org/content/335/6075/1480.abstract ) imaged the area below the Pacific plate and verified that the discontinuity is not a Pacific-wide phenomenon. Rather, it is only detectable beneath regions with recent surface volcanism.

In his study, Schmerr used a type of seismic wave that travels from earthquake to seismic station and reflects at the surface near the center of the path (see figure 2). If there is any underlying discontinuity near this reflection point, a small amount of initial wave energy will also reflect from the discontinuity. Because its path length is slightly shorter, that wave arrives at the station earlier than the surface reflection and so is called a “precursor.” Scientists use those precursors to image Earth structures halfway between the earthquake and seismic station, rather than just those directly below a seismic station.

Schmerr proposes that the melt associated with the G is formed by partially molten rock produced in the asthenosphere which collects and ponds at the base of the lithosphere. Decompression of hot rock at upwellings or mantle plumes may be responsible for generating the melt and thermally reheating the lithosphere, making it shallower than expected underneath older crust.

“Ultimately, the lack of a G in many regions of the Pacific essentially says the [lithosphere–asthenosphere boundary] in these areas arises from other properties of the mantle, and does not require melt,” explains Schmerr. Other mechanisms that enable plate tectonics could include mantle composition or grain size.

A hundred years after Gutenberg’s first seismic forays into Earth’s mantle, his namesake discontinuity is still generating questions. Understanding the variability in the discontinuity could also provide clues about convection and plate tectonics on other planets with different inner structures.

Gutenberg’s legacy of investigating boundaries extends beyond the enigmatic properties of the inner Earth. In 1932, he and his wife issued affidavits, provided financial support, and wrote letters of recommendation that enabled 32 German Jews and Catholics to emigrate to the US, where many of them pursued scientific careers