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They compared activity at the sites year over year, first by examining imagery from January and March of 2015 and then comparing that with imagery collected this February, just after the latest nuclear test. The images showed a “substantive reduction of economic activity on the Sino-North Korean border” as evidenced by a huge drop in the number of railcars at the stations, trucks in customs areas, trucks on the bridge and undocked boats in the Yalu River.

At the Sinuiju rail station, most of the train cars appeared to be in storage early this year, with no engines attached to the freight cars. In the Sinuiju customs area, there were 111 trucks shown in the satellite image from January 2015, but in the February 2016 image, there were only five. On the Chinese side, there were 32 trucks spotted in the Dandong customs area in March 2015, but by this Februarythere were only six.

Official trade data regarding North Korea is notoriously unreliable, and Cha said comprehensive data on economic activity over the China-North Korea border does not really exist. But his team has been briefing U.S. and South Korean government agencies on what they found, and he said both governments have shown interest in pursuing the research.

In March, China signed on to a new United Nations Security Council resolution imposing fresh sanctions on North Korea in response to the January nuclear test, showing that Beijing was in fact upset with Kim’s actions. But the new data may show that Xi was much more upset than he let on and more than he wanted the rest of the world to know.

“The Chinese don’t feel like they need to get credit for punishing North Korea and they don’t want to be seen as [if] they are being pressured by the U.S. to do it,” said Cha.

The question going forward is whether Chinese economic pressure on North Korea, which will surely hit at Kim’s coffers, will compel the young ruler to think twice before his next dangerous provocation.