The deep-seated bedrock of Bermuda, long assumed to have simple volcanic origins, attracted unexpected world attention last year when a US scientist discovered something a little stranger deep below the bulge in the Earth’s crust that the island sits upon.

Exploring more of what lies beneath is the next step for William Frazer, a seismologist with Carnegie Science in Washington, who has come back with an array of instruments to get a better picture not just of the structure but the subtle ways that it shifts geologically.

The Earth often rumbles beneath or near Bermuda — the vast majority of the time in ways nobody on the surface would notice.

There have been exceptions. The island was notably rattled back in 2011 by a tremor measuring a relatively minor 4.6 on the Richter Scale. It was still enough to crack the tiled floor at a Hamilton establishment.

In fact, Dr Frazer told The Royal Gazette, tremors beneath and around Bermuda are plentiful, and undetectable, apart from by those with the instruments to listen in.

As the rumbles move through rock, they deliver a picture of the geologic structures they pass through. Profiling deep-earth structures would be impossible by other means.

Dr Frazer, who visited the island this year as part of a team to set up the listening instruments, explained: “Bermuda sits on, depending on where you are, about 20 to 50 or so metres of carbonates and then the volcanic rocks that formed Bermuda are down below those. But the rocks that I’m interested in are about ten to 15km deeper than that. We can’t drill a core that deep.

“That’s why I use these seismic waves to make inferences about the structures beneath Bermuda without having to come up with some sci-fi apparatus.”

Bermuda and its environs sit upon a huge body of rock that Dr Frazer said is, geologically speaking, “buoyant” in comparison with its surroundings.

Instead of magma forcing its way through the Earth’s crust to the surface from the hot mantle beneath, a portion of the mass that Bermuda sits upon pushed up and then, in geological terms, froze in place.

Those deep-buried and less dense rocks created the support for an area far larger than the island itself.

Interviewed last week, Dr Frazer explained: “The most likely formation mechanism for this layer is the process of magmatic underplating, which is where magma coming from deep below Bermuda in the mantle goes up just below the oceanic crust.

“Some of it erupted and formed Bermuda but not all of it was able to erupt and these rocks solidified into the part of the mantle below the oceanic crust. They are then less dense than the mantle rocks they replace and they were able to support the swell.”

The “swell“ is a massive bulge compared with what can be seen above sea level.

Dr Frazer became interested in Bermuda as a seismologist exploring structures 600km deep in the Earth’s mantle, in a region called the mantle transition zone. Deep-drill research here suggested there was something more to be investigated.

“I learned there was a permanent seismic station in Bermuda and I started looking at some of the data from it to do some seismic imaging,” he said.

Bermuda already held special scientific interest, Dr Frazer explained: “It’s a very small area on the surface but it is a very large geologic feature because it’s sitting on top of this giant swell on the ocean floor.

“My work that I published last year showed that beneath Bermuda there’s this very thick, unusual layer of rock that is formed probably through a process called underplating, where these rocks were placed when Bermuda was formed volcanically about 30 million years ago.

“This base supports Bermuda today, that Bermuda sits atop.”

This spring, he and a team backed by Bermuda scientists installed ten seismometers across the island to get a better view.

Dr Frazer said: “On the surface, they’re not particularly exciting to look at. It’s a black plastic box a couple of feet wide with a solar panel sitting on top to power the system. The actual magic is the sensor, which is buried about arm’s length deep beneath the ground.”

His visit last month was to do what scientists do with gadgets: make sure the devices were working as well as collecting “the first set of data”.

“What we’re looking for with this seismic network is faint earthquakes that cannot be felt at all.

“There are historical records of seismicity felt on Bermuda and in places that are not very seismically active. Occasionally there are earthquakes that are felt and Bermuda is no different. But Bermuda also has these very small earthquakes that cannot be felt, that are not hazardous.”

As the faint rumbles give a profile of what lies beneath, Dr Frazer said the plan was “to keep these stations in place for about two years”.

“I’m hoping that at the end of the project I will have a couple of new results to share,” he said.

“And it’s totally possible that we are able to find something really interesting sooner but you never know. It’s very difficult to predict the pace of discovery.”

Dr Frazer is likely to return in November to see how the instruments progress.

“Roughly every six months is going to be how often I visit now to collect data and check on the stations.”

The global press coverage last year of a mere geological study “definitely surprised” him, Dr Frazer admitted.

Some appeared to be generated by the Bermuda Triangle mystique.

He added: “I didn’t expect it; I had no experience with that before. But it’s really nice to see that people got excited about science and hopefully learned something about the world around them.”