One of North Korea’s most recent nuclear tests may have set off aftershocks so powerful that they moved the Earth’s crust, the U.S. Geological Survey said Saturday.

The test from early September was the likely cause of two tremors detected Saturday near the North Korean nuclear testing site Punggye-ri, the USGS said. Though the tremors were minor, registering at magnitude 2.9 and 2.4, they were still powerful enough to literally shift the planet.

“They’re probably relaxation events from the sixth nuclear test,” a USGS official told Reuters about the tremors.

“When you have a large nuclear test, it moves the Earth’s crust around the area, and it takes a while for it to fully subside. We’ve had a few of them since the [September] nuclear test.”

12-10 north korea tremors
12-10 north korea tremors

Waveform data on the North Korean nuclear test in September from the Japan Meteorological Agency.Reuters

The tremors appeared to come from an H-bomb test conducted September 3 in the isolated nation. Experts at the time said the weapon was 10 times more powerful than the atomic bomb the United States dropped on Hiroshima in 1945. The U.S. bombs dropped on Hiroshima and Nagasaki that year, at the end of World War II, remain the only nuclear weapons ever used on civilians.

North Korea’s nuclear tests, which Kim Jong Un has ramped up this year, have been taking a tremendous toll on the local environment — and on Kim’s citizens. The same September test studied by USGS triggered a 6.3 magnitude earthquake that reportedly collapsed nearby buildings, including a school with more than 100 students in it, many of whom were feared dead in the aftermath. Defectors have also alleged that radiation from the tests at Punggye-ri has spread a “ghost disease” that has sickened adults and deformed babies.

Even so, Kim has shown no signs of stopping the missile tests as he continues to lob threats at President Donald Trump about a possibe strike. After the September test, North Korea took a two-month hiatus from weapons tests that sparked speculation that Kim might be backing off his ambitions for destruction. But North Korea last month ran yet another test on a missile that defense experts said flew higher and longer than any other produced by the country, and was capable of hitting anywhere in the mainland U.S. However, North Korea has not proven that it can launch such a weapon so far with a nuclear warhead attached.