Nukes and earthquakes look very different on a seismometer.

Firstly, the waveforms look very different. While an earthquake generates strong S-Waves, the seismograms of underground nuclear test lack most of these waves. Instead, the P- (or primary or pressure) waves dominate the seismogram from the detonation of an atomic bomb below ground (see figure 1).

A second way to distinguish between the origins of the elastic waves is to analyze the data collected by many stations in what is known as a “Moment Tensor Solution.” By performing this computation, seismologists trace the elastic waves back to their origin. That not only pinpoints the precise location of the focus, it also shows the mechanism of the forces initially shaking the rocks. During an earthquake, rock breaks in a shear fracture, which results in the rapid sideways movement of two flanks of a fault. In an explosion, however, the origin is indeed a point, from which elastic pressure waves travel concentrically outward.

If they used a nuke, they would not be able to keep it a secret.

Why were they not already avoiding it? This isn’t the first time that a passenger plane was shot down flying in or near a war zone. Hell, it’s not even the first time Russia shot one down over Ukraine. Commercial flights really should stay away from war zones.