Scientists for the first time have mapped out the deep underground plumbing of Hawaii’s active volcanoes by locating and plotting hundreds of thousands of earthquakes over a 3-1/2-year period.

The study, published Thursday in the journal Science, also offers strong evidence that Mauna Loa and Kilauea are connected deep underground — a question that was on many minds earlier this month when the two volcanoes went silent almost at the same time.

Geoscientists at California Institute of Technology used a machine-learning algorithm to help model a structure of pancakelike chambers, called sills, that channel magma laterally and upward to recharge the chambers of both volcanoes.

“Before this study all we knew was that magma was generated at a great depth,” said John D. Wilding, a Caltech graduate student and co-lead author of the paper. “We know pretty well what the magma is doing in the shallow part of the system. But from 10 to 100 kilometers, it was a mystery. Now we have a high-definition map of an important part of the plumbing system.”

With data on more than 192,000 small temblors of less than magnitude 3.0 that occurred from 2018 to mid-2022, the team was able to map out a plumbing structure in never- before-seen detail.

Wilding still remembers the day the 3D plotting software finally spit out the final map showing the scores of dots on a three- dimensional “point cloud” map.

“My jaw hit the floor,” he said. “I stood up, and I was pacing around the office. I knew this was something that was never seen before, and I was showing it to anyone I could find.”

What appeared before him were more than a dozen sills stacked on top of one another, the largest among them about 3.7 by 4.3 miles.

The sills, described as the Pahala sill complex, were generally more than 300 yards thick, separated by a distance of about 550 yards. They appear to sit at depths ranging from around 22 to 27 miles. By comparison, the deepest humans have ever drilled into the earth is about 7.5 miles.

All of the data used for the study was gathered by U.S. Geological Survey seismometers on Hawaii island.

The key, however, was the machine-learning algorithm developed in the lab of Zachary Ross, Caltech assistant professor of geophysics. The advanced algorithm was taught to identify earthquake signals using a dataset of millions of previously identified earthquakes, allowing it to find patterns that distinguish quakes from background noise.

This technique gave the Caltech team the ability to separate signal from noise and plot out a 3D map showing the shape and location of the Big Island sills.

“It’s analogous to taking a CT scan, the way a doctor can visualize the inside of a patient’s body,” Ross told the Caltech communications office. “But instead of using controlled sources with X-rays, we use passive sources, which are earthquakes.”

The team was able to pinpoint about 10 times as many earthquakes as was previously possible, the researchers said, and they were able to pinpoint their locations with unprecedented accuracy.

During the study, the researchers were able to identify the magma’s progress as it pushed up through the sills and then connect that to Kilauea’s volcanic activity.

For Kilauea’s eruptions during the study period, the scientists saw immediate upwelling that was “almost like sucking through a straw,” Wilding said.

In the sill structure, the team identified bands of seismicity that appeared to connect Kilauea and Mauna Loa to a common source in the mantle.

“The Pahala sill complex may serve as a common magma source at (25 miles) depth for Kilauea and Mauna Loa,” according to the study. “This degree of volcanic interconnectivity is remarkable in light of geochemical and past seismological results that imply Hawaiian volcanoes have distinct plumbing systems sourced from distinct regions of the underlying plume.”

Because the study ended in May, the scientists cannot say whether they can spot the magma flow that led to the Nov. 27 eruption of Mauna Loa.

But Wilding said the team intends to look at that topic in the next year or so.

The scientists could not say whether the plumbing beneath the Hawaii volcanoes are unique to the islands.

“Hawaii is the best-monitored island in the world, with dozens of seismic stations giving us a window into what’s going on beneath the surface. We have to wonder: At how many other locations is this happening?” Wilding said.

Also unknown is exactly how the magma’s movement triggers the small earthquakes. It could be that the injection of a lot of magma into a confined space generates the stress needed to rattle the earth, according to the researchers.

The paper, titled “The magmatic web beneath Hawai‘i,” was funded by the National Science Foundation and Jet Propulsion Laboratory, which Caltech manages for NASA. Computations for this research were conducted in the Resnick High Performance Computing Center, a facility supported by the Resnick Sustainability Institute at Caltech.