Geophysicists at the Geological Survey had initially concluded the blast was the source of the Nov. 4 tremors because they were centered near a McCook quarry that was conducting blasting around the same time. Survey scientists also said the wave patterns did not appear consistent with an earthquake.

But after assembling a team of "seismological experts, quarry operators and technical consultants," and examining records from the quarry, the Geological Survey said it has now concluded that "the quarry blast itself was too small to account for the signals recorded by the regional seismographic network.