Instant Earthquake

Since April 1962, no less than 700 mild earthquakes have been recorded in the area around Denver. The tremors have done practically no damage, but in a part of the country that knew no quakes at all for 80 years before the current flurry, Denver's citizens were understandably concerned. What was causing the trouble? Would the quakes get worse?

Not until last November, though, did anyone offer reasonable answers. Then Consulting Geologist David Evans suggested that the quakes under the suddenly shaky Colorado terrain could be traced to a deep well at the nearby Rocky Mountain Arsenal. Military and civilian experts scoffed, but Evans backed up his theory with impressive evidence.

Probable Relation. To dispose safely of contaminated water containing the waste products of a deadly nerve gas and other products manufactured at the arsenal, the Army had sunk a 12,045-ft. shaft and pumped down the first 4,000,000 gallons of waste water in March 1962. The quakes began the next month; they have been rattling the area ever since at a rate that has varied with the amount of waste water disposed of in the well. Between April and September of 1965, for example, when the Army pumped 5,800,000 gallons per month into the earth, an average of 44 quakes per month was recorded. From October 1963 to September 1964, when no contaminated water was put down the well, the quakes fell off to only five per month. Even more convincing, the rough data that Evans had collected placed the epicenters (surface points above the earthquake centers) of all of the recorded quakes within five miles of the arsenal's deep shaft.

To Evans, at least, the answer was obvious. When water was pumped deep into the Pre-Cambrian rock around the bottom of the well, he said, it lubricated the surfaces of vertical fractures, allowing the rock faces to slide against each other, causing recurring tremors. The theory sounded good enough for Colorado Congressman Roy McVicker, who called for a full-scale scientific investigation. Beginning in December, the U.S. Geological Survey and four Colorado colleges and universities set up seismographs on the arsenal grounds; they recorded quakes while Army technicians systematically reduced both the volume and pressure of waste water entering the well, finally shutting it off completely on Feb. 20.

The results of the study seemed to strengthen Evans' argument, though other geologists feel that the cause must be something more than mere lubrication of the fracture surfaces. Both the frequency and intensity of quakes diminished dramatically as less water was pumped down the well. Furthermore, the study established that the epicenters of the quakes were located within only a mile of the well and the quake centers themselves were at 12,000 ft.—close to the bottom of the suspect well, where an earth fault was also found. The Colorado earthquakes and the Army's disposal of waste water, said the Geological Survey, "probably are related."

Large Dreams. To establish that conclusion beyond a doubt, the Army is allocating $150,000 for a further geological study. The Colorado School of Mines last week received a $98,000 federal grant, and will shortly get another $122,000 from the Colorado legislature for its own investigation of the phenomenon.