Seismologists’ Blast Jolts Whittier Area

A blast set off early Sunday morning by researchers hoping to learn more about the Oct. 1 Whittier Narrows earthquake produced a stronger jolt than had been expected, setting off burglar alarms within a mile of the site.

Although geologists had said the explosion would not be felt by anyone more than 100 yards from the site in the Whittier Narrows Recreation Area, the 4:10 a.m. blast rattled windows and was felt as far as five miles away. Radio stations and police departments were flooded with calls from nearby residents who thought another earthquake had occurred.

At the site itself, the explosion was somewhat louder than the report from a shotgun and it sent a plume of mud and water more than 50 feet into the air. It left two craters in the ground, one about 10 feet wide and 15 feet deep, the other perhaps 12 feet wide and 10 feet deep.

There were no reports of damage resulting from the experiment.

Seismologist Kate Hutton of Caltech said the explosion registered on the Caltech monitoring network as the equivalent of a magnitude 2.0 earthquake, but experts were quick to note that no earthquake actually occurred.

More than 20 startled residents called the Los Angeles County sheriff’s substation in Temple City shortly after the blast. “They were concerned because they were awakened from a full sleep,” Deputy Doug Gatlin said. The explosion triggered about 10 burglar alarms in South El Monte and Rosemead, he said.

Geophysicist Douglas Given of the U.S. Geological Survey said the jolt was stronger than expected, apparently because the explosives were in very good contact with the ground. Given said it would be days or even weeks before the researchers will finish processing information from the blast.

Speed of Shock Waves

The most important reason for conducting the experiment, he said, was to get precise information of the speed of shock waves through the soil in the Whittier Narrows area to calibrate the seismometers that have been recording aftershocks of the Oct. 1 temblor. The quake, which measured 5.9 on the Richter scale, caused three deaths and produced at least $213 million in damages.

This calibration will enable researchers to “refine the locations of the aftershocks to see how the faults are moving and where they are,” Given said.

The researchers will also record shock waves that are reflected by underground rock structures to get a better picture of the geology of the area and of the nature of the underground faults. And, he said, the researchers hope to get a better understanding of why the brunt of the damage from the Oct. 1 temblor occurred in downtown Whittier, several miles from the earthquake’s epicenter.

Seismic Data

To gain that information, a U.S. Geological Survey team from Pasadena and Menlo Park spent Friday and Saturday setting out 60 portable seismometers in a rough circle within 10 miles of the Whittier Narrows area. Timers started the instruments rolling at 4 a.m., recording the seismic data on magnetic tape. They picked the early morning hour to minimize “cultural noise,” such as freeway traffic.

Blast master Ron Kaderabec spent Saturday afternoon preparing the explosion. A local drilling company had just completed two 100-foot-deep holes in an old riverbed just behind the rifle range at the Whittier Narrows Recreation Area. They lined them with six-inch PVC pipe.

He then loaded each hole with 600 pounds of ammonium nitrate, a blasting agent that has 83% of the explosive potential of dynamite. “But it’s a lot safer than dynamite, and we don’t have to worry about it going off while we transport it,” Kaderabec said.

Setting Off Explosions

The ammonium nitrate, which looks like ready-made spackling compound, was packed in four-foot-long, 60-pound sacks that fit comfortably inside the lining of the holes. As Kaderabec hoisted each bag over a hole, he would cut open the top before dropping it in. Then he filled the hole with dirt.

Kaderabec has been setting off such explosions for the U.S. Geological Survey for 10 years, averaging about 10 per year, he said. He and geophysicist Scott Schapper have traveled all over the United States, including Alaska and Hawaii, to help conduct such research in the aftermath of earthquakes.

“People set off explosions like this hundreds of times a week, and it has never triggered a quake before,” Schapper said. “There’s no reason why this one should.”

To set off a quake, a small nuclear bomb the equivalent of at least 50,000 tons of dynamite is required, said seismologist Lucille M. Jones. “And even then, you only get a magnitude 3,” she added.