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Sunday, April 11, 1999
Copyright Las Vegas Review-Journal

If The Big One Hits Here, Will We Be Ready?

Valley building officials say structures are safe, but geologists say too little is known about the threat quakes pose.

By Keith Rogers

      When the Aladdin came crashing down last April from the force of 233 pounds of explosives, the demolition company's president observed, "It is not a building I'd want to be in, in an earthquake."
      The 32-year-old landmark was so easy to drop that Controlled Demolition Inc. President Mark Loizeaux reduced the amount of explosives by 137 pounds from what he initially thought it would take.
      If the strategically placed charges had been detonated below ground, they would have delivered about the same amount of energy as a magnitude-1.1 earthquake, a small blip on a seismograph probably not strong enough to be felt by people.
      But with the charges positioned above ground instead of within the crust -- where the release of strain results in powerful earthquakes -- the Aladdin implosion didn't even register on the nearby seismograph at the University of Nevada, Las Vegas, according to geology professor Dave Weide.
      Building supervisors for Clark County and Las Vegas say they are comfortable with the current seismic safety requirements for construction in Southern Nevada, which follow the Uniform Building Code and the new International Building Code that was developed for 2000 by engineers and building officials from several professional organizations.
      Older buildings such as the Aladdin, they say, were built according to codes of the time.
      The Aladdin "was in no way ready to fall down," says county Building Assistant Director Ron Lynn, who is also chairman of the state Seismic Safety Council.
      "Mr. Loizeaux is the foremost at exploding structures. However, I'm not sure he is an expert on erecting structures," Lynn said, noting that the Aladdin, because of stress caused by settling soil, "had been reinforced, all the way to the top floor."
      Many older buildings and parking structures were bulked up with reinforced steel, retrofitted to withstand stronger ground motion or equipped with braces and tie-downs to reduce the risk of falling objects through the years.
      Examples of such upgrades in the downtown area include modifications to the Las Vegas Club parking structure and some structures in the Fremont Street Experience. Several transportation structures, such as the Oakey Boulevard underpass, have been reinforced with steel bars surrounding piers that support the weight of the roadway.
      In most cases, according to Las Vegas Building Director Paul Wilkins, buildings that haven't been retrofitted have been torn down.
      In addition, five aging casino buildings -- the Aladdin, the Sands, the Landmark and two from the Dunes -- have been imploded since 1993.
      From Weide's perspective, the chance of an earthquake collapsing buildings in Las Vegas "is the risk you live with."
      "Las Vegas exists because of probability. Gaming is probability," he said.
     -- -- --
      Builders gamble on the very low probability of a really big earthquake ever hitting along the seven faults that cross the Las Vegas Valley.
      Local building experts say the structures are safe enough to endure a potentially damaging earthquake based on the types of construction used, construction upgrades, soil conditions and on the likelihood of a large earthquake occurring here. Nevada ranks third behind Alaska and California in the frequency of strong earthquakes, according to the U.S. Geological Survey.
      Transportation structures such as freeway overpasses and bridges are built to withstand 15 percent of the acceleration of gravity, according to Nevada's chief bridge engineer, Bill Crawford.
      While he said that is a "fairly low seismic design," it is based on the probability of an earthquake strong enough to damage such structures occurring once every 500 years.
      Acceleration of gravity, sometimes referred to as g, equals 32 feet per second, which is the speed of an object falling into the Earth's gravitational field. Large earthquakes, at their point of origin, generally deliver forces of up to 1g.
      Earthquakes are measured by magnitude. With every increase in magnitude, the size of the seismic waves that travel through the crust increase by a factor of 10. In terms of energy released, that translates to a 30-fold increase, which means a magnitude 6.7 earthquake sends out 900 times more energy than a 4.7 earthquake.
      Despite the tremendous difference in energy, seismic designs are not solely related to magnitude. The bottom line, according to Lynn, is how a structure performs while the ground is shaking. Unless a building or a structure is sitting directly on the place where a fault ruptures to the surface, the real test is its ability to ride out the motion.
      Freeways, roads and bridges are designed to resist seismic activity based on criteria developed by the American Association of State Highway and Transportation officials.
      "We don't use the Richter scale," Crawford explained. "It's a measure of energy, and that doesn't reflect how it affects a structure."
      He said construction of overpasses and ramps as part of the overhaul of the Spaghetti Bowl in Las Vegas relies on single columns to support the weight of the roadway. The pillars are anchored into clay and rocklike caliche.
      "It's designed to resist a significant seismic load," Crawford said.
      He described the difference between seismic design being used for the Spaghetti Bowl and the one that was used for the Cypress overpass, a double-deck structure built atop mud soil in Oakland, Calif. The overpass collapsed during the magnitude-7.2 Loma Prieta earthquake on Oct. 18, 1989.
      Centered in the Loma Prieta Mountains between Santa Cruz and the San Francisco Bay area, it was the largest earthquake to occur on the San Andreas Fault since the San Francisco earthquake in April 1906. Sixty-three people were killed, and damage -- much of it to freeways along the bay and structures in San Francisco's Marina District -- totaled $6 billion.
      During the Loma Prieta quake, seismic waves had a dramatic effect on the soft bay soils, up to 65 miles from the epicenter, causing liquefaction -- where, for a matter of seconds, solid ground reacts as if it is liquid.
      "(The Cypress overpass) was founded on soft soils that tended to amplify the earthquake forces. The (clay) soils in the Las Vegas Valley tend to be much better and don't amplify earthquake forces from the bedrock," Crawford said. "The softer soils, like mud or lake bottom, tend to amplify it like a plate of Jello. You shake the plate and the Jello moves more than the plate."
     -- -- --
      On Dec. 14, a magnitude-2.7 earthquake rattled homes and buildings in northwest Las Vegas. It shook the pro shop at Angel Park Golf Club but wasn't powerful enough to stir gamblers at Calico Jack's Saloon, about a mile or two from where geologists believe the earthquake struck without rupturing the surface. That jolt packed 450 times the amount of energy it took to drop the Aladdin with explosives.
      During the quake, Las Vegas resident Burt Slemmons, professor emeritus of geology at the University of Nevada, Reno, happened to be at Hualapai Way and Sahara Avenue, just a few miles from the epicenter.
      "We heard it more than felt it," he said at the time. "It was more or less a window rattler. It was really loud, almost like an explosion."
      Just across from Angel Park Golf Club, Quinton Boshoff, senior vice president of The Resort at Summerlin, said he didn't feel the earthquake. The tremor didn't budge the seismic safety features of the resort, which was under construction.
      Mustapha Assi, Summerlin resort project manager for Martin & Peltyn Inc. Structural Engineers, said the tremor was "very soft, very light, but I definitely felt it." His company also designed the structures for Bellagio, MGM Grand, Treasure Island, The Mirage and the Paris hotel.
      Assi said the shaker assured him that designs used in The Resort at Summerlin are safe for the type of earthquakes and the soil conditions in the Las Vegas Valley.
      In January, a year after construction began on the resort, visitors to the project could see seismic design features in the framework. Huge tube-steel K braces, shaped like inverted V's, support the main casino. The braces are designed to hold up during the side-to-side movement generated by an earthquake. Frames that resist similar lateral motions also have been put in the resort's two six-story hotels.
      A two-story, reinforced concrete slab with reinforced concrete shear walls built to withstand seismic loading was constructed to keep the resort's parking garage from wobbling during severe shaking. The shear wall supports the garage in the same way a large piece of plywood positioned between the legs of a table would keep the table from wobbling because it would move as one unit instead of four separate legs, Assi explained.
      The elevator shaft leading to the garage is "not connected to anything else." That means the free-standing structure would react independently of the buildings it connects.
     -- -- --
      Craig dePolo, a research geologist with the Nevada Bureau of Mines and Geology who is studying faults within the Las Vegas basin, said knowledge about the faults is scarce. Given the concentration of people who live here -- 1.3 million -- a more in-depth understanding is warranted, he said.
      "There are questions that have to be answered," dePolo said during a break in February's Nevada Earthquake Safety Council meeting. "Maybe we're lucky if we have 10 percent of that knowledge."
      Just hours after dePolo's comment on Feb. 19, a magnitude-2 tremor shook North Las Vegas. Only one monitoring station in the Nevada Seismological Laboratory's network picked it up, an instrument in the Sheep Range on the northern outskirts of the valley. With only that one piece of information, geologists were unable to pinpoint the epicenter. A month later the scientists were speculating whether the instrument was triggered by an earthquake or noise, such as blasting or a sonic boom.
      As seismologist David von Seggern put it, "Again, we're severely handicapped by having almost no instruments in Southern Nevada."
      Some North Las Vegas residents reported ground shaking in the early afternoon, but they might have been confused by bombing demonstrations at Indian Springs by aircraft from Nellis Air Force Base, von Seggern said. The bombing exercise registered as noise on seismic equipment around 2:20 p.m., but residents had reported feeling an earthquake about two hours earlier, at 12:15 p.m., the same time the Sheep Range instrument was triggered.
      DePolo agrees the Las Vegas basin needs a more extensive network of seismic instruments. Ideally, he said, two sets of instruments would be anchored into outcroppings at various locations. Two types of seismographs are preferred because if only the most sensitive instruments are installed, then strong motion waves from larger, less frequent earthquakes could distort the recordings.
     -- -- --
      Wilkins said the Las Vegas city building code dates to 1927, when information about earthquakes in the valley was virtually nonexistent. For the most part, blueprints for those years and the next six decades have been discarded. Since the mid-1980s there has been a requirement to keep building plans for commercial structures indefinitely.
      In Clark County, Lynn said building design records are stored on microfilm and, more recently, on optical discs. Records date back to 1981 for most commercial buildings. A few have been kept from as early as 1960. The same records for subdivisions date back to 1978.
      After 1980, more emphasis was put on seismic resistant designs as building codes were tightened based on lessons learned from destructive quakes.
      "Back in 1982 there were six pages on earthquake design," Wilkins said. "By 1997 there were 14 pages and this (the 1 1/2-inch-thick International Building Code) is not going to be the end of it."
      More than just the likelihood of a strong earthquake and soil conditions go into the formula for safety designs. The distance from faults where large earthquakes occur is part of the equation of today's safety codes, as are the types of structure involved.
      Seismic waves from the shifting of rock masses ripple through the crust, losing their punch at greater distances. There are two types, the fast moving "P" waves, or primary waves and the slower, side-to-side moving "S" waves, or secondary waves. Those are the motions that newer buildings in Las Vegas are designed to ride out.
      Critical structures, such as Hoover Dam and hospitals, need to be the most resistant to ground motion. The dam, built in the early 1930s, blocks the Colorado River and creates a drinking water supply for more than 18 million in the Southwest. Disaster lurks if a breach of the 3.25 million cubic yards of concrete occurred during an earthquake. Bureau of Reclamation officials, who oversee the dam, say the structure is designed to sustain seismic waves sent out from the nearest threats -- the Black Hills Fault, the Mead Slope Fault and background earthquakes.
      They point to the dam's track record of having survived several moderate earthquakes since it was built.
      During construction of the dam, it was revealed that two significant faults -- the Mead Slope Fault and the Black Hills Fault -- were upstream and downstream of the dam's foundation, respectively. But it was reported by Charles Berkey, a consulting geologist during the dam's construction, that "the dam site could not have been located more accurately to take advantage of the best local physical condition."
      While predicting the potential effects of earthquakes is not an exact science, Bureau of Reclamation engineers say the massive dam is designed to withstand a significant earthquake without failure. In fact, a magnitude-6.75 earthquake occurring on either of the two faults near the dam likely would cause only cracking in its upper portion.
      Although there weren't many structures in 1932, Las Vegas survived one test from strong ground motion generated from outside the valley during the magnitude-7.2 Cedar Mountain earthquake, which struck Dec. 21 that year near Gabbs, 240 miles northwest of the valley.
      The valley was tested again during the 6.2-magnitude Clover Mountain earthquake near Caliente, 120 miles north of Las Vegas, on Sept. 22, 1966; and again during the magnitudes-7.4 and -6.5 Landers, Calif., earthquakes on June 28, 1992, 150 miles southwest of Las Vegas. In California, the twin quakes killed a boy, injured 350 people, destroyed 20 homes and damaged 1,100 more. Ten businesses also were destroyed.
      Still, buildings such as the Golden Gate Hotel have endured those and smaller earthquakes. The hotel, the first in Southern Nevada constructed of concrete, opened in 1906 as the Hotel Nevada. The original structure -- the first two floors -- is part of today's establishment on Fremont Street, which has endured several expansions in its 93-year history including the addition of a third floor in the 1920s.
     -- -- --
      Engineers from professional organizations that developed the Uniform Building Code and the International Building Code based requirements on lessons learned from earthquakes such as the 1994 Northridge, Calif., earthquake -- a magnitude-6.7 temblor -- and the 1995 magnitude-7 Kobe, Japan, quake.
      Las Vegas is ranked as a "2b" area for seismic safety under the Uniform Building Code. That means structures must incorporate certain earthquake-safe features such as reinforced walls and foundations designed to withstand the type of ground motion expected from earthquakes in the area.
      Construction in the Las Vegas Valley and most of Southern Nevada must meet the mid-level requirements on a scale of zero through 4, where 4 is the most stringent for seismic safety design. A 4 ranking would be required in urban, coastal areas of California, parts of Alaska and parts of central Nevada where strong earthquakes strike more frequently.
      High-rise buildings in Clark County, according to building chief Lynn, have been designed and constructed to a standard much higher than the minimum requirements for the 2b zone. Some parking structures, he said, are on par with what is required for seismic safety of parking structures in Los Angeles, including such features as earthquake clips -- metal rails that prevent heavy objects from crashing down.
      Geologists believe several faults around Las Vegas deliver strong earthquakes about every 10,000 to 50,000 years. But Slemmons and others say the intervals might be more frequent, or about every 10,000 to 20,000 years, or less in the case of the Eglington Fault.
      Lynn's perspective is that civilizations, such as the Roman Empire, which existed for about 1,000 years, will rise and fall before an earthquake strong enough to drop buildings occurs in the Las Vegas Valley. In fact, he puts earthquakes sixth on hazard rankings for buildings in Clark County, preceded by soil problems, first; fires, second; wind, third; water, fourth; and tornadoes, fifth.

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A tube-steel K brace, shaped like an inverted V, supports the main casino of The Resort at Summerlin during construction in January. K braces are designed to hold up during side-to-side movement generated by an earthquake.
Photo by John Gurzinski.

A tube-steel K brace, shaped like an inverted V, supports the main casino of The Resort at Summerlin during construction in January. K braces are designed to hold up during side-to-side movement generated by an earthquake.
Photo by John Gurzinski.

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