Geologists try to date latest Stateline shaking

LAS VEGAS - The Stateline earthquake fault slumbers like a giant deep in the rocks west of Pahrump, and geologists wonder when it will wake up again and unleash a ground-shaking jolt toward the Las Vegas Valley 50 miles away.

They don't know the last time the fault moved, but a team of geologists led by University of Nevada, Las Vegas professor Wanda Taylor is trying to find out.

Taylor and student geologists including Jonathan Carter, who is writing a master's thesis on the subject, set out in January to dig three 10-foot-deep trenches in the fault zone's clay.

"The reason we're doing the research is we really don't know much about the fault," Taylor said. "Although we've known about it for decades, we're recognizing it has the potential for a larger earthquake than we thought before."

Whether the fault last ruptured a few centuries ago or tens of thousands of years ago, narrowing down the answer through clues left in the crust will help them estimate how much strain is building up along the fault to the point that it could snap.

Such an event would cause deep rock layers to slide against each other, similar to how the San Andreas Fault makes the Pacific tectonic plate slip northwestwardly against the North American Plate.

Taylor said the Stateline Fault, which was first mapped in the 1930s, could cause a powerful earthquake on the low end of a scale for what geologists consider to be large earthquakes.

"We don't think if the whole thing broke it would be a magnitude 9. Geologically that's unlikely," Taylor said last week at her office on the UNLV campus.

"We think it would rupture in segments with a most credible earthquake of about magnitude 7."

For comparison, an earthquake in the Indian Ocean in 2004, which had a recorded magnitude of between 9.1 and 9.3, sent out a series of tsunamis and was one of the deadliest natural events in history, killing more than 225,000 people.

Closer to the magnitude that the Stateline Fault could unleash was the 1989 Loma Prieta earthquake on the San Andreas Fault, which struck south of the San Francisco Bay Area.

The 7.2 magnitude Loma Prieta quake caused widespread damage and killed 57 people.

This year, the catastrophic earthquake that hit Haiti in January was magnitude 7, and the one that struck Chile in February was rated 8.8 magnitude.

Magnitude is a measure of the strength of an earthquake, or the energy from strain that's released from it.

An increase in one unit of magnitude, from 4.6 to 5.6, for example, equals a 10-fold increase in the amplitude, or size of waves.

That translates to a 30-fold increase in the amount of energy released for each magnitude.

Certainly if significant movement occurred on the Stateline Fault, seismic waves would impact the Las Vegas Valley.

"If it has a magnitude 7, Vegas is shaking," Taylor said.

Just how much damage would occur depends not only on the magnitude and distance involved but also the structural integrity of buildings and the type of soils beneath them.

Building codes for homes, businesses and resorts have been beefed over more than a decade to reduce the hazard from ground shaking and amplified ground motion caused by fine, soft sediments in some parts of the Las Vegas Valley.

However, Taylor said, "If your home is older than 1996, you need to check to be sure the water heater is strapped down."

At least 11 faults either cross the valley's floor or are located outside of it that are each capable of producing a substantial jolt.

The California Wash Fault, 50 miles northeast of Las Vegas, near Valley of Fire State Park, could deliver a magnitude 7 earthquake. It was one of the faults that Hoover Dam was designed to withstand.

Some faults in eastern California and in the Death Valley fault systems are also capable of producing ground motion that could shake Las Vegas.

Evidence of the Stateline Fault was found in two of three trenches from 70 meters to 100 meters long that Taylor's team dug a few miles west of Pahrump.

"The zone is several meters wide, and we know it has a little bit of a normal component, meaning that extension across it pulls it apart," Taylor said.

Soil from the trenches was collected in hopes that it contains enough pollen, charcoal or carbon remnants that can be age-dated based on the measured, radioactive decay of the isotope carbon-14 from the samples.

By comparing the dated soils on each side of the fault, the team will then have an idea when it last moved.

In addition, UNLV's Cathy Snelson conducted seismic reflection studies of the area, and researchers under Barbara Luke from the Civil and Environmental Engineering Department will collect ground-shaking data from a 100-meter-deep bore hole in the fault zone.

They also will evaluate the stiffness of sediments from core samples that were extracted.

"We'll be able to take all the data together and have a good package," Taylor said. "To have three sets of data is rare."

The project is funded by the Department of Energy as part of a $3 million, six-year effort with state and federal earthquake programs to better understand hazards in Southern Nevada.

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Information from: Las Vegas Review-Journal