Lake Mead shaken by quakes

Thursday, Oct. 7, 2004 | 11:19 a.m.

Here's one more thing for which the drought can be blamed: lake quakes.

A series of low-magnitude earthquakes at Lake Mead in the last two weeks was caused by the drought, geology experts say.

The dropping lake level has caused the quakes, scientists say, because of a little-known phenomenon.

The phenomenon came to light in 1935, when the first detailed evidence of reservoir-induced earthquakes came from filling Lake Mead for Hoover Dam.

Earthquakes at reservoirs are caused by fluid injections or withdrawals that create pressure in pores inside rocks along faults. Such increased pressure causes rocks to slip, causing earthquakes, scientists say.

Geologists and seismologists also say that the Lake Mead quakes mirror larger tremors that occurred in late September and earlier this month in California, Northern Nevada and at the Nevada Test Site.

Since January 2002, there have been 78 quakes in the area of Lake Mead, with 20 of those this year, said Charles Watson, president and chief geologist of Seismo-Watch, Inc., a private Reno-based company that produces earthquake reports and histories. All of those Lake Mead quakes have been minor, less than a magnitude of 3.0, Watson said.

He says that while there were no tremors around the lake in August, there were six quakes there in September, all occurring between the 20th and the 29th. Watson said the September lake tremors were caused by the decline in water levels, a result of the five years of drought in the region.

The lake is at the 1,126-foot level, about 50 to 70 feet below what some experts have termed as normal, said Roxanne Dey, spokeswoman for the Lake Mead National Recreation Area,

Scientists caution against drawing too many conclusions from the earthquakes at Lake Mead and other earth-moving action, including volcanic activity at Mount St. Helens in Vancouver, Wash., because the incidents likely are unrelated.

"It is coincidental but probably not connected," said UNLV geophysicist Catherine Snelson, who has done extensive earthquake research. "The activity is not all that unusual because this is an active Earth."

Snelson said the quakes that have occurred are on different plates and that the Lake Mead tremors are caused by the lake levels dropping. The others, she said, are "tectonic" -- caused by the force of plates rubbing together.

Watson, who writes an earthquake column for 37 U.S. newspapers including the San Jose Mercury News and the Orange County Register, also is cautious about theorizing whether the quakes are related to one another.

"When you add Mount St. Helens to the mix, the temporal relationship is striking, but not necessarily a direct line," he said.

"What we can say is that it is happening on the West Coast of the North American continent clustered in time and space. It is a coincidence where further research is needed. Still it may be part of a bigger picture. It is perhaps an indication of a more regional situation."

Of the earthquakes at Lake Mead this year, a magnitude 2.8 tremor at Virgin Canyon on the eastern side of the lake on May 14 was the largest.

On Sept. 20, the other two above a magnitude 2 occurred at the southern end of the Spring Mountains near the Bird Spring Range by Goodsprings -- one a 2.2, the other a 2.1 five hours after the first, according to Seismo-Watch.

Two small earthquakes occurred at the Nevada Test Site between Goldfield and Beatty, both last Friday -- a magnitude 2.8 tremor 135 miles northwest of Las Vegas and a 3.9 followed by a 2.8, north of Scotty's Junction 130 miles northwest of Las Vegas, Watson said.

On Sept. 28, an earthquake with a magnitude of 6.0 was recorded in Parkfield, Calif., and a day later a 5.0 earthquake was registered 40 miles east of Bakersfield. Both quakes were at about the same latitude as Las Vegas, Watson said.

Between Sept. 18 and Sept. 20 at Mono Lake near Mammoth Lake on the Nevada border south of Hawthorne earthquakes with magnitudes of 5.5 and 5.4 were recorded followed by four aftershocks of greater than 4.0, Seismo-Watch said.

Experts say the quakes around Lake Mead and in Las Vegas in general have been and are expected to remain in the low-magnitude range, below a 3.0.

Before the mid-1930s, earthquakes were rare in Southern Nevada. But, between 1936 and 1946, seismographs registered at least 600 earthquakes. Snelson said some were a magnitude 5 or greater.

Most of the earthquake activity around Las Vegas in the last 65 years has been recorded near Lake Mead, Watson said. However, he said, investigations by the Nevada Bureau of Mines and Geology and other agencies have found several significant active fault traces around the Las Vegas Valley.

That, he said, is an indication that large earthquakes have occurred in Las Vegas in the not so distant geologic past. Watson said that revelation, however, was not surprising given that the tectonic process built the region's rugged mountains.

For agencies that protect the lake, the small recent earthquakes have caused no disruption in business.

"We have sensors that measure the shocks and when we get one that registers we make a quick visual inspection of the area," said Hoover Dam spokesman Bob Walsh. "No facilities at Hoover Dam were affected by recent quakes. But we do feel them in Boulder City."

While movies have depicted earthquake-induced catastrophic disasters at dams, Walsh said Hoover Dam was built to withstand a 9.0 quake, noting that 6.5 million tons of concrete was poured to create the 726-foot tall structure that is 660 feet thick at the base.

"It is a good, solid piece of concrete construction," he said.

Lake Mead spokeswoman Dey said no earthquake-related problems have resulted at the lake or on the roads leading to and from it this year.

Seismo-Watch's president, Watson, said,"What people think most about earthquakes in Las Vegas is not what happened as a result of the building of the dam and Lake Mead, but rather from the years of nuclear testing at the Nevada Test Site."

Since below-ground nuclear bomb testing ended in September 1992 as a result of the Nuclear Test Ban treaty, earthquakes from the test site have been far less frequent. Watson said there have been some petroleum-based blasts at Indian Springs that have rocked Southern Nevada slightly.

Other significant earthquakes in the region in recent years according to Seismo-Watch include: