Scientists Testing Sound Waves to Study Ground Motion

“You might want to plug your ears with your fingers,” geophysicist Robert A. Williams cautioned a resident who came out of his house Saturday to watch the U.S. Geological Survey conducting tests on the ground under the road.

“Whack!” A mechanical sledgehammer dropped a 100-pound weight onto Kinzie Street, sending sound waves down more than 100 feet and back to sensors along the curb.

Resident Hal Joberg was disappointed.

“You get more [noise] than that at a skeet range,” he said.

Nevertheless, the team members from the Geological Survey’s Denver headquarters, on their third trip to the San Fernando Valley since the 6.7-magnitude earthquake on Jan. 17, 1994, took care to explain what they were doing. The scientists are visiting about two sites a day over the long Labor Day weekend in an effort to better forecasts how much ground shaking could occur in future temblors.

“We’re measuring the strength and stiffness of the soil, which will help explain the differences in earthquake ground motion,” said Williams, who with two colleagues passed out a flyer and booklets to the approaching curious.

“We’re not looking for faults,” said geologist Jack Odum.

“Aw, shucks,” Joberg said. “I was hoping you were going to tell me if we had one right under my house.”

“No,” Odum replied. “We find enough of our own faults; we don’t go looking for other people’s.”

Williams said the team was compiling data on the velocity of sound waves at its chosen locations. Each tested site was previously monitored by the Geological Survey during the first few weeks of aftershocks after the Northridge quake.

Results will be used to draw a “ground-shaking hazard map” of the area, and should be ready by April, Williams said.

The team began with tests at two locations in Sherman Oaks Friday, then worked Saturday in Northridge and Granada Hills. Today and Monday, the geologists hopes to visit bedrock sites in Sherman Oaks and near the Encino and Chatsworth reservoirs, he said.

On Northridge’s Kinzie Street Saturday, at a cul-de-sac just east of Balboa Boulevard, the scientists placed small seismometers, or geophones, along the curbside to catch echoes from the soil and rock layers and transmit them to a seismograph in a Geological Survey van. The geophones pick up the noise signals reverberating through the soil.

Inside the van, electronics technician David M. Worley gave the order to drop the weight.

“Fire!”

Once, when he didn’t tell his co-workers that he got a good reading on his seismograph, he was asked, “Are you happy?”

“I’m just tickled to death,” Worley replied, explaining that the banter comes from having worked with the same crew for five to 10 years.

They have been up and down the West Coast, but also to the Missouri-Arkansas-West Tennessee region where a large earthquake occurred in 1811, and to South Carolina where a big one hit in 1886, he said.

To record the velocity of vertical sound waves relatively close to the surface, the team swings an ordinary sledge hammer wired up to the apparatus in the van. To record the horizontal, or sheer, waves on the strata of the earth, the sledgehammer was swung against the end of a 10-foot-long wooden pole that was placed on the street and held firm under the front tires of a truck.

“I let Rob [Williams] do the hitting; he’s a bigger guy,” said Odum, who moved into a spot of shade and wondered how hot the rest of the Labor Day weekend was going to be.