Seismic waves reveal Earth has 'innermost' core

Two Harvard University seismologists, Miaki Ishii and Adam Dziewonski, first proposed the idea of the innermost inner core in 2002 based on peculiarities in the speed of seismic waves passing through the inner core

The inner core of Earth appears to hold an innermost secret. Geology textbooks almost inevitably include a cutaway diagram of Earth showing four neatly delineated layers: a thin outer shell of rock that we live on known as the crust; the mantle, where rocks flow like an extremely viscous liquid, driving the movement of continents and the lifting of mountains; a liquid outer core of iron and nickel that generates the planet’s magnetic field; and a solid inner core.

Analysing the crisscrossing of seismic waves from large earthquakes, two Australian scientists say there is a distinctly different layer at the very centre of Earth. “We have now confirmed the existence of the innermost inner core,” said one of the scientists, Hrvoje Tkalcic, a professor of geophysics at the Australian National University in Canberra.

Tkalcic and Thanh-Son Pham, a postdoctoral researcher, estimate that the innermost inner core is about 800 miles wide; the entire inner core is about 1,500 miles wide. Their findings were published last week in the journal Nature Communications.

Although the cutaway diagram appears to depict clear-cut divisions, knowledge about the deep interior of Earth is unavoidably fuzzy. It is nearly 4,000 miles to the centre of Earth, and it is impossible to drill more than a few miles into the crust. Most of what is known about what lies beneath comes from seismic waves — the vibrations of earthquakes travelling through and around the planet. Think of them as a giant sonogram of Earth.

Two Harvard University seismologists, Miaki Ishii and Adam Dziewonski, first proposed the idea of the innermost inner core in 2002 based on peculiarities in the speed of seismic waves passing through the inner core. Scientists already knew that the speed of seismic waves travelling through this part of Earth was not always the same. The waves travelled fastest when going from pole to pole along Earth’s axis and slowest when travelling perpendicular to the axis. The difference in speeds — 3 per cent faster along polar paths — arises from the alignment of iron crystals in the inner core, geologists believe.

But in a small region at the centre, the slowest waves were those traveling at a 45-degree angle to the axis instead of 90 degrees, the Harvard seismologists said.

The data were too sparse to convince everyone.

The best measurements would be seismic waves travelling from an earthquake’s origin straight down into Earth and through the innermost inner core. However, detecting those generally requires a seismometer located almost exactly on the other side of Earth, and that point is in the middle of the ocean.

The new paper takes advantage of the fact that seismic waves also bounce back. Thus a seismometer close to the epicentre could detect the reflection of the wave that travelled through Earth and bounced back, passing through the innermost inner core twice. They could also be reflected back-and-forth a second time, travelling through the innermost core four times.

This article originally appeared in The New York Times.