"University of Utah seismograph stations recorded seismic waves of 3.9 magnitude early today in the area of the mine, causing speculation that a minor earthquake had caused the cave-in. Scientists later realized the collapse at the Genwal mine had caused the disturbance.

“There is no evidence that the earthquake triggered the mine collapse,” said Walter Arabasz, director of the seismography stations.

Since the mid-1990s, at least a half-dozen other mine collapses have caused similar seismic waves, including a 1995 cave-in in southwestern Wyoming that caused readings as high as 5.4 on the Richter scale."

Regarding your reasoning, you have given one of the two explanations. Another reason for stronger motions at the surface is the low impedance of the material there. A shear wave will wiggle the surface much more than the rock deep in a mine because it has a lower modulus. Just as for springs, if the spring constant is smaller, the spring wiggles more violently per unit of energy in the waves.

• Detroit Free Press source