Military experts expect North Korea to test a nuclear bomb this weekend -- underground -- inside its borders. No doubt, you'll hear about this on the news shortly after it happens. But, it will take a few minutes or longer for official confirmations to be completed before it can be broadcast. However, from home using the publicly-available USGS earthquake detection network, you can determine if it's happened -- before it's news.
When a nuclear weapon is detonated near the ground, (in this case -- again -- it is expected to be tested underground, isolating the radiation) detectable, but harmless, compression shock-waves are sent through Earth's crust and circle the planet in seconds. The nature of these seismic waves can instantly tell seismologists the exact location, depth and strength using a network of seismic detectors.
CLICK FOR LINK FOR REAL-TIME USGS NORTH KOREA QUAKE MAP
A tell-tail sign it's a nuclear explosion and not a natural earthquake is by observing the depth of the, "shake". Natural quakes originate from far under the ground. The epicenter of a nuclear bomb's localized quake is right at the surface.
For comparison, a natural quake today near Fuji registered 5.8 magnitude at 285 miles deep. A September 9, 2016 nuclear bomb test in North Korea registered a 5.3 on the Richter scale at 0.0 km (0.0 miles) deep.
Data from @USGS says North Korea earthquake depth at 0 km - consistent with nuclear test.— James Pearson (@pearswick) September 9, 2016
Of interest: today's MOAB bomb ("mother of all bombs") dropped in Afghanistan, was the largest non-nuclear bomb ever used in conflict, with 11 tons of explosive. Even then, it did not produce a shockwave large enough to register on the Richter Scale. Nukes are in a league all their own.