[InvertPrete]

Just read up on the Seismic effect of nukes:

Cannikin was detonated on November 6, 1971, as the thirteenth test of the Operation Grommet (1971–1972) underground nuclear test series. The announced yield was 5 megatons (21*PJ) – the largest underground nuclear test in US history.[23] (Estimates for the precise yield range from 4.4[34] to 5.2[35] megatons or 18 to 22*PJ). The ground lifted 20 feet (6*m), caused by an explosive force almost 400 times the power of the Hiroshima bomb.[36] Subsidence and faulting at the site created a new lake, over a mile wide.[3] The explosion caused a seismic shock of 7.0 on the Richter scale, causing rockfalls and turf slides of a total of 35,000 square feet (3,300*m2).[23] Though earthquakes and tsunamis predicted by environmentalists did not occur,[32] a number of small tectonic events did occur in the following weeks, (some registering as high as 4.0 on the richter scale) thought to be due to the interaction of the explosion with local tectonic stresses.[37] http://en.wikipedia.org/wiki/Amchitka
I guess a 3-400 kt TNT equivalent size of a nuke could create an earthquake of 5.8 magnitude, and it could create smaller aftershocks as was seen after the Amchitka nuke.

As a sidenote a 150 kton nuke at wtc should have created an earthquake that was at least 5.3 or so on the richterscale. If I recall it correct the seismic force of the collapsing towers were less than 3 which is at least 2 magnitudes lower...