Middletown explosion didn't register on Yale earthquake scale

The blast from Sunday's power plant explosion in Middletown, though heard by residents as far away as Mystic, "was not seismically detectable" on earthquake monitoring instruments, a geologist at Boston College's Weston Observatory said Monday.

The observatory, which tracks earthquake activity for the New England region, has one of its network of seismographs at Yale University, about 20 miles from the site of the explosion.

The monitor at Yale did not register any underground movement from the explosion, according to Michael Haggerty, a scientist at the observatory, meaning the shaking and blast noises were mainly the result of powerful sound waves traveling from the site.

Normally, Haggerty said, powerful explosions do register on the earthquake monitoring equipment when they reverberate into the ground beneath the site. It appears that in this case, however, the force of the explosion stayed mainly above ground.

Some of the properties in the immediate vicinity of the plant probably did experience some underground shock waves that caused cracked foundations and other damage reported by homeowners, but unlike the sound waves, these did not travel a great distance, Haggerty said.

At Wesleyan University, a few miles from the site, for example, students and staff heard the blast but there was no damage to buildings, said campus spokesman David Pesci. The college sent out an all-campus e-mail Sunday informing students what had happened, and five members of campus emergency response crews helped local responders at the site, he added.

The sound of the blast-wave energy was reported by residents in towns in a wide area around the blast site and even in southeastern Connecticut communities such as Ledyard and Mystic. The reports were made to the U.S. Geological Survey and posted on a graphic on its Web site. Acoustic energy can travel curved paths and sometimes skips over certain areas depending on wind, temperature and other atmospheric and weather conditions, Haggerty noted.

"There are shadow zones," he said.

The plant was being built in an industrial zone on a 137-acre site that had once been a feldspar and pegmatite mine. Mine operations have been closed there since the 1990s.

Ralph Lewis of Lyme, retired state geologist, said the property is a relatively remote site with few homes nearby that sits in proximity to existing gas transmission and power lines. The particular geology of the area was probably not a major factor in the choice of the site, he added.

Different types of soils and rock conduct blast energy differently, Lewis noted. Shock waves that travel through bedrock cause less shaking of nearby homes and other structures than those that travel through lake clay soils, for example, he said.

The property is relatively steep and is situated along an area of the Connecticut River known as The Narrows, where the river takes a sharp bend and there is bedrock on both sides along the shore, according to information on the state Siting Council Web site.

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USGS site

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