Seismologist Explodes Old Theories With New Shock Recording Machine

When many people find a window broken in their homes, or new cracks in walls or ceilings, the chances are that any blasting going on within a fifty mile radius will get the blame. Every year, thousands of irate householders file complaints against blasting companies claiming that the vibration and concussion from their operations has damaged their property.

To the rescue of these campanies has come L. Don Leet, associate professor of Geology and director of the University seismograph station. Leet has spent 15 years in research on the intensity of vibrations and the damage that they can do.

According to Leet, the vibrations set up around a house in Cincinnati, for example, due to a hurricane off Cape Hatteras, N. C., would be much greater than those produced by a passing truck or by dynamiting in a quarry only a few miles away.

Leet has found that most of the vibrations in the ground and in buildings come from sources that people do not expect, and the majority of industrial detonations produce tremors too small to do any damage to nearby dwellings. By use of a relatively new instrument, the Leet Portable Seismograph, actual measurements of the intensity of industrial vibrations at varying distances from their source have been made. The net result has been of great value to the blasting companies and quarry-owners who, up to the present, had not known just how much damage their blasts have done.

The detonation of the atomic bomb in New Mexico on July 16, 1945, gave Leet and his seismographs an excellent opportunity for measuring ground motion. Although the bomb was exploded in the air, its energy was delivered to the ground in a single vertical impact producing both airborne and earthborne waves.

Some startling facts have been brought to light through Leet's seismograph tests. In order to break window glass by concussion, charges must be fired in the open air, a most impractical thing in quarrying operations. Window glass must be subjected to pressures of 1 1/2 lbs. per square inch to break it, whereas air pressure at a few hundred feet from a quarry blast are 1 to two-thousandths of a pound per square inch, the same as those produced by a light summer breeze.

One of the largest blasts which has been recorded, that of 211,700 lbs. of dynamite at the Volunteer Mine, Mich., produced a maximum movement in a building a quarter of a mile away of only 52 thousandths of an inch.

Although Leet's instruments have proved scientifically that most industrial blasts cannot be of structural damage to nearby dwellings, the blasting companies still have to overcome most people's psychological reaction that where there's noise, there's damage.