Blast to Snap ‘Photo’ of Underground

U.S. Geological Survey seismologists will set off 1,200 pounds of dynamite at 5 a.m. Sunday in a 110-yard-deep hole at the Whittier Narrows Recreation Area, near the epicenter of the Oct. 1 earthquake, but assured Southern Californians Friday that there would be no repeat quake as a result.

Seismologist Lucille M. Jones of the geological survey agency said Friday that the agency’s phones were “ringing off the hook” with anxious inquiries about the blast and protests against it.

Jones emphasized that the blast will not be felt by anyone who is more than 100 yards from the hole, and that there is no danger of it triggering another earthquake.

“Geologists exploring for oil set off hundreds of such explosions around the United States every week, and none has ever set off an earthquake,” Jones said.

She also noted that Caltech seismometers record several larger blasts every day from quarries in the Los Angeles area.

The researchers hope that the explosion will give them a better idea of how the Oct. 1 quake occurred.

Shock waves from the explosion will bounce off underground rock layers and return to an array of 40 to 50 seismometers that the geologists will place around the area before the explosion. By measuring the time required for the shock waves to reach the seismometers, the researchers will compile a “picture” of geological structures underground.

The explosion will also help them calibrate seismometers that are permanently located in the Los Angeles Basin.

A Caltech spokeswoman also said Friday that two small earthquakes struck Friday morning. A magnitude 3.0 aftershock of the Whittier Narrows earthquake occurred at 1:27 a.m. Friday, about nine miles southeast of Pasadena--near the epicenter of the original temblor. It was the 31st aftershock measuring magnitude 3 or larger.

Also, a magnitude 3.1 quake occurred at 4:59 a.m. at Devore, northwest of San Bernardino.