The network of GPS (global positioning system) stations set up around a Hawaiian volcano in 1996 was designed to track a very noisy phenomenon. The geophysicists working with the system wanted to see how the land moved when the Kīlauea volcano rumbled. They wanted to use such data to help them understand volcanic processes and to predict future eruptions.

As far as these noisy events are concerned, their work was a success. But, as the paper on page 71 of this issue reveals, that wasn't all that the team encountered. Their data also provided an unexpected insight into a much quieter phenomenon: 'silent slips'. These are earthquakes that usually occur when one tectonic plate slides very slowly under another.

Silent slips were first recorded in 1998, but weren't really labelled as such until 2000, says Paul Segall, a geophysicist at Stanford University who helped set up and run the Kīlauea GPS network. “When we set up this network, no one knew about these,” says Segall. “Slow earthquakes hadn't yet been discovered.” And even after they were discovered, they weren't well understood and were not associated with volcanoes. So Segall didn't expect to see or hear any signs of them in Hawaii.

But Segall's graduate student, Peter Cervelli, saw 'steps' in the data, indicating that slight, sudden movements had taken place. “He first thought it was a mistake,” says Segall. “But after working on it and working on it, he realized it was real and looked like an earthquake.” To add to the mystery, no earthquake had been recorded or categorized for the time period that matched the steps.

The team was receiving batches of data every 24 hours. When these were broken down into minutes and seconds, the researchers saw evidence that the land around the volcano's crater was slowly slipping and sliding, causing small, low-frequency events that set off little earthquakes. This was unusual, as previous silent earthquakes had been associated with the movement of tectonic plates. The Kīlauea events, they concluded, were caused by the slippage of land around the volcano's cone.

“These observations occurred very serendipitously,” says Segall. “We had no particular hypothesis to test, although we did have particular goals in mind.”

There had been some earlier evidence that Kīlauea's activity was triggering other events. “We knew from previous measurements that there is a steady sliding of the volcano towards the ocean,” explains Segall. This slippage had been linked to earlier earthquakes, such as a major event in 1975, and there had been speculation that the slippage had helped to generate tsunamis. The latest data should offer a fresh perspective on the volcano's influence, Segall says.

But getting a clearer picture of Kīlauea's silent phenomena might prove difficult. It was costly putting up the 20 or so GPS receivers around Kīlauea, not to mention the technology to transmit and process the data. The team will almost certainly need more GPS instruments, and would like to have data transmitted more frequently than once a day. Other instruments, such as seismometers and tilt meters, should also help the researchers capture aspects of the silent earthquakes. But much of Kīlauea is a national park, so it might be difficult to secure permission to install these devices. Nevertheless, Segall is optimistic that new technology will eventually provide a clearer view of the silent slips. “We keep getting new tools for measuring things,” he says. “When we put them out we are see things we never expected.”