The 5.1 earthquake for this bomb would suggest a smaller explosion than the bombs at the tail end of WW2, however that depends on a lot of things. Looking at this list it would suggest about a ~600 ton explosion http://self.gutenberg.org/articles/richter_magnitude_scale
Counterpoint: wikipedia shows a formula that puts it at 44kT[1]. However there is a note that it can vary greatly depending on the fraction of energy that is converted to seismic waves (this formula assumes 0.5%);
That being said, 44kT is still way below what one would expect for a fusion bomb, and low even for a boosted fission bomb.
Not to downplay the seriousness, but that site is quite surprising. I grew up about an hour outside of DC and according to that site, greatly overestimated the danger of a nuclear blast in the nation's capital.
Seismic, acoustic, and, especially, radiation signatures tend to strongly differentiate natural and nuclear device events.
Earthquakes virtually always develop over both time and space -- the "epicenter" of an earthquake is the center or origin of the movement. Some quakes occur over very long rupture zones -- the 2004 Boxing Day quake off the coast of Indonesia involved several hundred km of fault line, and moved along that fault at several hundred km/hr (this is among the reasons why major quakes can be felt over several minutes -- the rupture is literally happening over that duration, and, depending on the location of the observer, the seismic waves have to travel back to them).
An atomic blast begins and ends within a few thousands of a second -- at the seismic scale it's both instantaneous and a point event.
A bomb will vent radioactive fission/fusion products and their decay products, many of which are radioactive, and have signatures specific to the type of weapon and design used. Even underground blasts will typically vent detectable quantities to the surface and atmosphere.
The wave forms are completely different.[1] That's how you can tell it's artificial. Radiation let's you know that that it's a nuke, and not just a pile of TNT.
Wikipedia has it listed at about 10ish kT, with higher end estimates up around 40ish. By comparison, the Trinity test during the Second World War was roughly 20 KT.
Were they actually able to produce a 9MT weapon, it would be more than half as strong as the largest weapons test the US ever conducted.
