You don't want to be woken up by a magnitude 5.1 earthquake under any circumstances. But you really don't want to be woken up by one that indicates North Korea's just tested a nuclear weapon. Especially this one, since it means North Korea's nukes are getting larger.

North Korea's third nuclear test, conducted overnight while much of the United States slept, follows a pattern set by its first atomic detonations in 2006 and 2009. It doesn't occur outside, where satellite imagery might spot it and where radioactive fallout could block back on the testers. That means understanding the detonation requires understanding the geology of it. (Just a bit.)

For one thing, the earthquake in North Korea is unlikely to be a routine geological event. Check out this chart on seismic patterns in northeast Asia since 2005, offered by the Vienna-based organization that oversees the international ban on nuclear testing, known as CTBTO: North Korea is typically spared. (Though seven years may be an insufficient period of time to for geological data.) CTBTO terms the North Korean earthquake a "seismic event with explosion-like characteristics."

More saliently, according to Robert Avagyan, a research analyst with the Institute for Science and International Security, international seismic monitoring stations around North Korea pick up seismic waves emanating from the center of the blast, believed to be at Punggye-ri. Seismic waves don't travel uniformly through the earth, but the size and speed of their travel – as well as their wave patterns – enable an extrapolation that indicates the yield of a nuclear device. If you want to get really technical, here's the raw data from the seismic event, compiled by the United States Geological Survey.

That seismic data may be difficult for laymen to interpret, but there's a clear conclusion from it. The North Koreans "have a bigger yield than previous [nuclear] devices," Avagyan says. "They're getting better at doing this."

In 2006, the North's first nuclear test led to a seismic blast of magnitude 4.3. That allowed nuclear experts to estimate that its device yielded a blast of less than 1 kiloton. The 2009 nuclear test was around magnitude 4.7, leading scientists to estimate the North had reached a much higher yield, of between 4 and 7 kilotons. (The estimated relationship between earthquake magnitude and blast yield isn't linear.)

The overnight seismic event, according to the U.S. Geological Survey, was magnitude 5.1. Avagyan estimates the device the North detonated was 10 kilotons. The Director of National Intelligence hedges and estimates only a "several kiloton" yield.

There are caveats to all of this. The world now has more and better monitoring stations closer to North Korea than in previous tests, Avagyan says, which might mean that the 2006 or 2009 detections of Pyongyang's nuclear tests might have been inexact. And North Korea might have used "boosters, specialized materials," he adds, for bigger explosive yields unrelated to the nuclear device itself to make a bigger boom.

There's another big unknown in the North Korean nuclear test: what nuclear material Pyongyang used. North Korean bombs often rely on plutonium, but in 2010, a former Los Alamos National Laboratories director revealed that North Korean officials had showed him an advanced and heretofore unknown plant for enriching uranium. The plant appeared configured for a crash program. Since bombs reliant on high-enriched uranium are considered more powerful than plutonium ones, a switch in nuclear material might indicate – and, perhaps, account for – a decision to go for more powerful bombs.

Avagyan says it'll take another day or two for atmospheric readings to indicate if the third North Korean nuclear test used highly enriched uranium or plutonium. Until then, the geologic record from the detonation says plenty, and none of it welcome in Washington.