Earthquake effects apparently not unique to Blue Lake

Two weeks ago, a Caldera poet witnessed sudden waves and turbulence at Blue Lake. (The Nugget, November 6, 2002)

As it turns out, the phenomenon observed by Gwen Morgan was seen in other areas around the country.

Forest Service geologist Larry Chitwood said that the major earthquake in Alaska triggered waves in lakes as far away as Louisiana, where water disturbances were generated in Lake Pontchartrain.

"There's a pretty good case that the wave actions observed at Blue Lake were probably induced by the big Denali earthquake," Chitwood said. "There were reports coming in from all over the contiguous 48 states that were triggered by that quake."

He said that waves were recorded in the Great Lakes and "at Lake Union in Seattle, boats were banged together and damaged."

Chitwood doubted that the phenomena observed were the result of anything other than direct transmission of seismic waves.

He noted that seismographs throughout the Northwest registered the event, but the top of the wave's signature was "clipped" at most sites because the temblor exceeded the sensitivity settings of most units.

One exception, however, was the seismograph at the Mt. St. Helens dome.

"The one at the dome was turned down because there are so many rock falls there that they saturate the seismograph," Chitwood said.

"Since it was turned down, there was not any clipping. It showed beautiful waves."

Chitwood explained that the first wave through is the P-wave, or primary wave.

"It's very noisy," he said, "and the one you actually hear. Seven minutes later, the S-wave (sheer wave) was really noticeable and came through in big pulses. That's the wave that causes damage, because it causes the ground to move sideways.

"The waves came at six cycles per minute," he said. "Almost all of its energy produced back and forth motions that required 10 seconds to complete each swing.

"That has quite an effect on lakes. It's like taking a bowl full of water and moving it back and forth."

Chitwood suspects that all lakes within reach of the tremor responded in some way to the release of energy. He could not say for certain why the effects at Blue Lake were so noticeable.

One important factor, he said, is that someone was in such a good position to directly observe at the time of the incident.

Chitwood consulted other geologists about the Blue Lake observations and all agreed that the most interesting aspect of the incident was the fact that the lake level was suddenly raised and remained at the higher level for some period of time.

"That's pretty remarkable," he said, explaining that the shaking during an earthquake causes underlying layers of sand and gravel to "densify."

Deposits at the bottom of a lake have interstitial spaces filled with air and water.

"When that (densification) happens," Chitwood said, "the water has to go somewhere. It's just water trying to get out of the ground when sand and gravel settle."

As a result, he said, water levels can be temporarily raised.

In this case, however, the water level remained a couple of feet above what it had been.

Blue Lake resident Gary Lovegren said that after three days, he went to the dam to lower the level because he was worried about the possibility of damage from the high water.

"It's never been that high before," Lovegren said.

Scientists from the Cascade volcano project expressed interest in the Blue Lake observations and may seek to develop additional information. At this point, however, Chitwood says that there is very little chance that the incident is related to any local volcanic or seismic activity.