The United State Geological Survey has mapped the locations where Sunday's blast was heard and felt. Their results confirm anecdotal reports that the sound did not travel uniformly from the site of the explosion, and provide an estimate of the intensity of the blast itself.
The 259 responses given to the USGS over the internet show that to the north, in Cromwell, residents did not even feel the blast, while to other directions, residents as far as 20 miles away felt the blast. The terrain and weather conditions may have strongly attenuated the blast shock wave in some directions.
Community surveys are used by the USGS to measure the effects, or intensity, of an earthquake. Intensity is a qualitative measure distinct from the magnitude of an earthquake, which is expressed in the quantitative Richter scale. Two earthquakes of the same magnitude, but happening at different depths or in different types of soil, can have similar magnitudes (Richter scale) but very different intensities.
The Kleen explosion registered as a IV (moderate) on the Modified Mercalli Intensity scale, which goes from I to XII. Surprisingly, the blast did not register at all on the earthquake monitoring seismograph at Yale, giving a 0.0 on the Richter scale, as reported in The Day, by Judy Benson. By contrast, an earthquake felt in Illinois on Wednesday, with the same Mercalli Intensity of IV, had a magnitude of 3.8.
Thus, the shock wave seems to have traveled great distances in the air, but not at all in the ground, perhaps because of the nature of the explosion, and perhaps due to the nature of the rock on which the Kleen Plant is located.
No damage to Aquifers Expected.
I asked Peter Patton, a hydrologist at Wesleyan, whether the blast might have caused any impact on either the wells in the area or the large aquifer which the city taps for much of our drinking water. He said that even if the explosion had shaken the ground enough to register on seismographs, it still would not have damaged the aquifers, which are recharged from below the Connecticut River. In fact, he said that surface explosions are sometimes used to increase the recharge rate for aquifers, by opening up cracks for water to seep through bedrock.