Desert quake could hit valley hard

Thursday, Aug. 9, 2001 | 10:49 a.m.

New research suggests that a system of faults west of Southern Nevada would create a more damaging jolt from an earthquake on one side than the other, a finding that could force a rewrite of building codes, including those in Clark County.

Scientists studying faults about 150 miles west of Las Vegas have found that the west side of the fault may be rigid and move hardly at all during a quake, while most of the action would occur on the east side, geoscience professor Kevin Furlong at Pennsylvania State University said.

That would likely send more shockwaves toward Las Vegas than into uninhabited desert, Furlong said.

If further study supports that view -- and that could take years of measurements during active earthquakes -- it could revolutionize the way builders and government officials create regional earthquake damage estimates, because until now scientists believed that both sides of a fault moved equally.

The study by Penn State and University of Miami scientists appears in the July 15 issue of Geophysical Research Letters.

Furlong and his team focused on a network of cracks called the Eastern California Shear Zone, which runs from about 150 miles east of San Francisco south into Southern California's Mojave Desert, paralleling the infamous San Andreas Fault.

They are using new techniques, combining temperature measurements from the rock with Global Positioning Satellite readings, which show stress building up in the crust. That stress appears stronger on the east side of the faults.

The fault zone moves about a half-inch a year. Earlier models assumed both sides of the fault move equally, but Furlong and the other scientists discovered that a quake could generate enough energy to send 90 percent of the motion east.

The scientists selected the area, because a magnitude 8.0 temblor shook Lone Pine, Calif., in 1872, a site right in the middle of the study area, Furlong said.

"It may be a wake-up call," said Clark County Acting Building Director Ron Lynn, who noted scientists did not pay attention to Southern Nevada's earthquake potential until major quakes struck close to the region in the 1990s.

Such temblors as the 7.6 magnitude Landers quake on June 28, 1992, followed by Little Skull Mountain 12 miles from the proposed nuclear waste repository site at Yucca Mountain at a 5.6 magnitude on June 29, 1992, and the 7.1 Hector Mine temblor in 1999, were felt in the Las Vegas Valley.

New geological information often changes building codes, Lynn said. "Even with new information near faults, building codes are designed to allow people to survive a quake, not to make the building earthquake-proof," he said.

There is no scientific, historical record for how often the Las Vegas Valley shook in the past. Written records of Las Vegas go back only 100 years, and the frenzied pace of construction has erased much evidence on the surface of seismic movement.

While Furlong's research may show stress building in the rock, that stress can be relieved either by a series of small quakes or a single big one, Lynn said. The trouble is that scientists cannot predict when a temblor will rattle the valley or how big it will be.

Southern Nevada building departments weigh the potential for damage with the cost of protecting homes from earthquake damage, Lynn said. "There is an economic reality when it comes to tightening the standards," he said.

However, he said, "It's refreshing more people are studying and discussing this data."

Furlong agreed that more study is needed to confirm the findings. The scientific team is returning to the California fault zone this fall to continue observations.

That research will try to duplicate the results, which were gathered by measuring the difference in temperature taken deep within the earth's crust, Furlong said. The Sierra Nevada mountains are rigid and have less heat flowing through them than the eastern side of the fault system, which is hotter and more easily deformed.

The temperature at 12 miles below the surface in the Sierra Nevadas was estimated at about 375 degrees Fahrenheit, and the Nevada side at 1,112 degrees, Furlong said.

The hotter the rock, the more stress builds before an earthquake and the ground movement creates more of a jolt. These temperature differences can be dramatic.

Before Global Positioning Satellite data became so good, it was impossible to do this kind of research, Furlong said. "We could not have seen the difference before," he said.