SAN FRANCISCO, CALIFORNIA—Ask seismologists when they’ll be able to predict earthquakes, and the answer is generally: sometime between the distant future and never. Although there have been some promising leads over the years, the history of earthquake forecasting is littered with false starts and pseudoscience. However, some scientists think that Earth’s crust may give hints before it ruptures, in the form of electromagnetic anomalies in the ground and atmosphere that occur minutes to days before an earthquake. Last week, here at the fall meeting of the American Geophysical Union, researchers shared their evolving understanding of these phenomena—and how they might be used to predict deadly quakes.

Kosuke Heki, a geophysicist at Hokkaido University in Sapporo, Japan, first got interested in the subject when he spotted an increase in the total electron content of the ionosphere—the charged outermost layer of the atmosphere—above Tohoku about 40 minutes before the magnitude 9.0 earthquake struck in 2011. Heki had long used GPS data to study ionospheric responses to earthquakes, which occur when the sudden movement of Earth’s crust reverberates through the atmosphere. Ionospheric disturbances interfere with the communication between GPS satellites and receivers, leaving a fingerprint at specific radio frequencies that researchers can tease out.

In 2011, Heki was skeptical of electromagnetic precursors. But since then, he has used the world’s growing array of GPS stations to identify similar signals before nine other major earthquakes, he explained at the meeting. In addition, Heki has found that earlier anomalies precede stronger earthquakes, potentially reflecting the longer time needed to initiate rupture along larger segments of a fault. Now, he says he’s convinced there’s really something going on: “Seeing is believing.”

However, scientists have yet to agree on a mechanism by which the crust could create electromagnetic signals. One idea is that rocks can generate positive charges when heated or stressed in the build-up to an earthquake, says Friedemann Freund, an adjunct professor of physics at San Jose State University in California and a senior scientist at NASA’s Ames Research Center in Mountain View, California. “When you stress a rock, it turns into a battery,” Freund says. “Not an electrochemical battery that you find in your car, but a new type of semiconductor battery that produces electrons and holes.”

These “holes” are positive charges that come from molecular defects known as peroxy bonds, which occur in most crystalline rocks and involve two oxygen atoms bonded together instead of to silicon or another element. At high temperatures and pressures, peroxy bonds break, causing them to pull in an electron from a neighboring atom, and leave behind a positively charged “hole.” This creates a chain reaction of electrons flowing toward the peroxy defect, effectively creating a cloud of positive charge flowing away, potentially to the surface and beyond.

With rocks, Freund says, “the faster you stress them, the more electricity becomes available.” He says this mechanism could explain numerous observations of strange occurrences before earthquakes, like mysterious lights emanating from the ground and the reported tendency for compass needles to dance around. Such positive charges may also propagate into the atmosphere to cause the ionospheric disturbances seen by Heki and others, although exactly how remains unclear.

Freund and his collaborators, including John Scoville, a physicist at San Jose State University and the SETI Institute, have demonstrated the semiconducting behavior of rocks in laboratory experiments where they drop a 90-kilogram mass onto a slab of igneous rock. The electromagnetic pulses they create in their experiments are shorter and weaker than those observed before earthquakes, but Scoville attributes that to the vast differences in scales. “We just can’t possibly recreate the volumes of rock that are active in the Earth during earthquakes,” he says.

The pulses they see in the lab do have the same shape—rising faster than they fall—as those seen in the field. At the meeting, Jorge Heraud of the Pontifical Catholic University of Peru in Lima reported that his team has detected magnetic pulses more than 2 weeks before recent earthquakes near Lima, using a pair of ground-based magnetometers designed specifically to look for earthquake precursors. In addition, Heraud said that his team has successfully identified the locations and depths of scores of future earthquakes each time, sending a letter to colleagues at his university in advance.

Although magnetometers may be capable of picking up precursor signals in surface rocks, they have one major drawback: They must be located within 100 kilometers or so of the epicenter. That’s why Angelo de Santis, director of research at the National Institute of Geophysics and Volcanology in Rome, and others have turned to satellites to get a global view of what happens in the atmosphere before earthquakes. Through a new research initiative called SAFE (Swarm for earthquake study), de Santis and his colleagues will pair data from the European Space Agency’s Swarm satellites, launched in 2013, with those from traditional earthquake monitoring devices like seismometers and GPS stations.

Their goal, de Santis says, is first to understand the link between Earth’s crust and atmosphere. “If you understand the physics, you double the chances to make predictions,” he says. They will also study past earthquakes to identify any patterns that precede known ruptures, and then see whether those same patterns precede future earthquakes. Already, at the meeting, they reported finding ionospheric anomalies before the 2014 Iquique earthquake in Chile and the recent disaster in Nepal.

However, many seismologists remain skeptical, including Tom Jordan, the director of the Southern California Earthquake Center in Los Angeles, who chaired a report on earthquake prediction and forecasting commissioned by the Italian government after the deadly L’Aquila earthquake in 2009. “We concluded that there was no evidence that ionospheric or electromagnetic precursors provide any diagnostic information about earthquakes in advance,” Jordan says.

After all, true earthquake prediction requires determining the time, location, and magnitude of a quake in advance. So far, researchers have mainly looked for anomalies after the fact. That’s OK, as long as researchers don’t consider finding something a confirmation of their hypotheses, says Jeffrey Love, a geophysicist at the U.S. Geological Survey (USGS) in Denver. “The acid test is always predicting something that hasn’t happened yet,” Love says. One possibility, frequently cited by critics, is that these precursors are simply coincidences arising from the fact that Earth and its ionosphere are fairly noisy places, geomagnetically speaking.

Another more problematic issue is that many seismologists are convinced, after decades of scrutiny, that the crust doesn’t undergo major changes before it ruptures that would produce precursor signals. And if anything does happen, it’s not clear that it would portend the size of an impending earthquake—the most critical parameter. “If it is the case that all earthquakes start alike, and that a big earthquake is simply an earthquake that went farther and longer, then that’s going to put some real cold water on the idea of earthquake prediction from precursors,” says Michael Blanpied with the USGS Earthquake Hazards Program.

Blanpied thinks it’s unlikely that electromagnetic signals—or any other precursors, for that matte—will turn out to be “a smoking gun,” but he does think they merit attention, especially as researchers collect more and more data from an expanding network of sensors. “It may be that earthquakes are predictable only at a very, very marginal level,” he says, which may only become apparent by studying large datasets.

These kinds of possibilities are worth pursuing, says Craig Dobson, a program scientist and manager at NASA headquarters in Washington, D.C., whose division funds research like Freund’s. As an investment strategy aimed at reducing the devastating toll of earthquakes, “it’s good to be supporting some things which may have a potentially very high pay off but seem somewhat risky and are not necessarily part of the mainstream,” Dobson says. “Ultimately, that’s often how large advancements are made.”