I thought that I remembered that the following has been reported in the past. And with a search, the following reference appeared. It discusses earthquakes related to past mining activity etc. in Colorado. When the timbers etc. get old the mines collapse and there is a shockwave, or earthquake. I don't know if that applies to any recent earthquakes in that area.

Just glancing at it I am surprised that the earthquake magnitudes would be that high.

• Colorado Mining Report