The latter question has a reasonably simple answer. It is easy to differentiate between an earthquake and an artifical explosion. The Italian National Institute of Geophysics and Volcanology, explains the primary characteristics of an artificial seismic event:
"Since the explosion is typically an impulsive source concentrated in space, it produces elastic energy that is characterized by waves with a short duration and with frequencies that are greater than the earthquakes ones."
The difference is easier observed when comparing the seismograph output of artificial and natural seismic events (via NIGV):
Underground nuclear explosion (A) performed on 14th July 1984 in Kazakistan and earthquake (B) occurred on 5th October ’85 in Western Canada, both the recordings are from in the Castel Tesino seismographic station of the I.N.G.V. National Seismic Network.
It is easy to see the difference between ithe high frequency and short wavelengths of the nuclear explosion and the relatively longer ones of the natural event. That does not explain the difference between a chemical based exploson and a nuclear one. In a recent LA Times article, they explain the current quandary the experts are in:
"But officials said the relatively small size of Monday's blast might make it difficult to use seismic data to determine whether the explosion was caused by a nuclear device or conventional explosives. Analysts haven't ruled out that the seismic shock could have been caused by conventional explosives, but a consensus seemed to be forming Monday that a nuclear device had been detonated."
However, in order of pull off a hoax of this magnitude, two million pounds of TNT would be required. The shear amount of conventional explosives required makes this extremely unlikely. Furthermore, there the experts allude to nuclear weapons having a distinct seismic signature.
The former question, about the size of the detonation is more difficult to answer. The geological characteristics of the region must be taken into account as well as the facility it was detonated in. Little analysis has been forthcoming about the current event. However, Armscontrol.org provides insight into India’s 1998 nuclear test does provide some information about the present situation.
"Geological explanations for the discrepancy between announced yield and the recorded seismic signals could include the possibilities that the explosion was poorly coupled to the surrounding rock, or the region surrounding the test site does not transmit seismic waves efficiently. Recordings of earthquakes and other seismic evidence indicates that the region of the Indian test site transmits seismic waves quite well, ruling out the latter explanation. Poor coupling to surrounding rock could occur if the seismic signals were muffled by detonating the explosion in a large underground cavity."
The actual calculation of the size is quite naturally left to the experts. However, it can likely be assumed that the test was either a fizzle or was significantly damped in order disguise the true size.
For additional reading, Wikipeida has an interesting article about the size of earthquakes, their yearly frequency, the associated magnitude in tons of TNT, and their equivalent in weapons. Also, there is my coverage of the location of the event.
