The energy industry may be increasing the risk of earthquakes  by pumping fluids underground, says a new federal scientific study. But the biggest danger is not due to extraction of fuel, but disposal of waste, said the report published Friday by the National Academies of Science.

Hydraulic fracturing of shale formations to unlock gas has been a hot-button issue, with concerns about the link to earthquakes. However, the fracking of the shale does not seem to be the main cause. There are about 35,000 fracked shale gas wells in the U.S., but, the report said, “hydraulic fracturing to date has been confirmed as the cause for felt, seismic events at one location in the world,” a weak but perceptible tremor in Blackpool, England.

Injecting the waste water from shale fracking operations, as well as waste water from other types of projects, may be the more significant cause for worry. There are about 30,000 waste water wells active in the U.S., the report said, with these wells a “suspected or likely cause” of earthquakes at eight sites, including in Ohio, where waste water from other states is being disposed.

(See: “Tracing Links Between Fracking and Earthquakes”)

A recent report by the International Energy Agency called for companies to engage better with communities where they are working and to increase transparency, to earn a “social license to operate.”

But the strongest concerns raised in the NAS report had to do with the fledgling, struggling technology  known as carbon capture and storage. The National Academies report noted that CCS poses seismic risk “because it involves the continuous injection of very large volumes of carbon dioxide [CO2] under high pressure.” The approach is advocated for capturing CO2 from power plants and storing it underground, so that coal and natural gas could continue to be burned while contributing much less to global warming than they do now.

Because the volumes of CO2 injected would be so large, the NAS said the technique could cause ruptures that would make the underground storage leak, allowing the CO2 back into the air—and undermining the reason for capturing the CO2 in the first place. Carbon capture and storage has only been used on a small scale so far, so the report called for more research into the risk of earthquakes and CO2 leakage.

(See: “Amid Economic Concerns, Carbon Capture Faces a Hazy Future”)

Other techniques appear to be associated with a  much higher risk of earthquakes, especially an approach called “enhanced geothermal.” Instead of normal geothermal heating, in which a well is drilled into a rock formation that holds hot water, dry rock is fractured in a way similar to fracturing shale, and water is circulated through the fractured rock, picking up heat as it passes through. Just eight pilot projects in California and Nevada are linked to a relatively high rate of earthquakes, from two to ten each year.

One final way that people are triggering earthquakes is through techniques to extract more oil and gas from old reservoirs, known as secondary and tertiary recovery. However, such problems are “very rare,” the report notes, with more than 100,000 secondary recovery wells linked to 18 felt earthquakes.

In all, the NAS report listed 13 states where seismic events related to all sorts of energy activity had been measured.