Waiting for the Big One
Geologists create a hypothetical quake and attempt to find ‘faults’ in the system
It’s a cool autumn morning in the Truckee Meadows. Parents wake their children and get them ready for school. Televisions are tuned to network news programs. Drivers speed down Interstate 80 and U. S. Highway 395 on their way to work.
Nothing seems out of the ordinary. Then the earth starts moving, slowly at first, and then violently. Everything becomes a blur. The shaking lasts for only 30 seconds, but it seems like an eternity.
Families scramble under tables as framed pictures fall off walls, bookshelves topple and dishes shatter on the kitchen floor. Older brick buildings on Fourth Street crack. Some collapse completely, sinking into heaps of rubble. Mobile homes shift off their foundations. A section of a runway at the Reno-Tahoe International Airport sinks into the ground. Car tires screech as drivers slam on their brakes in an attempt to move away from falling tree branches and phone lines. Window glass shatters. Large rocks tumble onto Geiger Grade and onto Interstate 80 near the Nevada-California border.
By the time it’s over, some 7,000 people have been injured, with 1,400 of them needing hospitalization. Power and telephone service are out in the Truckee Meadows and in Carson City. About 50 earthquake-related fires have broken out around the area. Hazardous materials have spilled in science labs at the University of Nevada, Reno. Raw sewage flows into the Truckee River from broken sewer pipes. Water treatment areas are shut down. Most dams and reservoirs have survived intact, but some have leaks.
Survivors are frightened or in shock. Several people have died of heart attacks. A large number of distressed pets have run away. Emergency management teams, fire departments, transportation crews, law enforcement agencies and utilities crews have a lot of work ahead of them.
Later, when the dust has settled, experts will tell the frightened populace what they know about the quake—that it registered magnitude 7.1 and that it occurred on the northern section of the Carson Range fault system, a 63-mile series of faults that extends from Woodfords, Calif., into southwest Reno. It’s one of the worst natural disasters ever to befall the area, they will say.
Sound implausible? Not to seismologists.
The description of a Reno-area earthquake given above is based on a scenario developed in 1996 by the Nevada Bureau of Mines and Geology. With some funding from the Federal Emergency Management Agency, the NBMG published the report “Planning Scenario for a Major Earthquake in Western Nevada.” In it, seismologists and geologists posed a plausible scenario of what could happen to the Reno-Carson City urban centers if a 7.1 earthquake struck them.
What they found was disturbing. A quake of that magnitude probably would produce a natural disaster the likes of which the area has not known since European settlers arrived. Some of the scary eventualities that could result from such an earthquake include damaged and collapsed buildings, liquefaction (when sandy, water-saturated underground deposits transform from a solid to a liquid state due to earthquake shaking, collapsing roads and buildings), power and telephone service going out and landslides that could block both ends of Interstate 80 for several hours or perhaps days.
The NBMG, the UNR Seismological Laboratory and various emergency management agencies and companies in Reno, Washoe County and Carson City collaborated on this project, using information contributed by local utilities, gas and other critical services that would be affected by such a large seismic event. The researchers also used data gathered during the 1989 Loma Prieta and 1994 Northridge earthquakes in California to predict what might happen here.
“The main driving force was to create a virtual earthquake, within a computer essentially, and try to create a visualization of the different effects, the different kinds of things that people would have to deal with,” says research geologist Craig dePolo, the main author of the report. “We try to [keep] it in perspective. It is one of the largest earthquakes that we could have here.”
Seismologists and geologists say that western Nevada has one of the highest seismic hazards in the state, based on its historic earthquake record and the amounts of active faults in the area. Evidence preserved in the geologic record indicate that a seismic event occurred in the Carson Valley about 500 to 600 years ago that may have been as strong as 7.5, according to the NBMG report. Shorter fault lengths and smaller surface offsets along the north Carson Range fault system suggest the largest quakes that occurred there were about 7.1.
In fact, in a 1997 report authored by dePolo and his colleagues, geologists estimate that the probability of the area being hit by a magnitude 6 or greater earthquake in the next 50 years is as high as 98 percent. They also estimate that the likelihood of one of 7 or greater in that time frame is as much as 50 percent.
Since the Carson Range fault system inclines, or dips, east under the Truckee Meadows, the scenario tremor’s epicenter—the point on the earth’s surface directly overlying an earthquake—is situated just east of Reno on the Virginia Range.
“This is a very close earthquake,” dePolo explains. “It probably would start with a good initial bang. The P wave [the primary wave, which people usually feel as a bang or thump] would be pretty distinct. … There would probably be a few seconds of somewhat random shaking before it really got going. … It’s pretty common to have a little bit of time between when you first feel things going and the very severe shaking. That’s a real critical period because, in this case, this is an earthquake where there’s no question you should take cover.”
dePolo says the most serious shaking could last for as long as 20 seconds in this particular earthquake scenario, followed by intermittent reverberations across the valley floor. Aftershocks would also occur.
Different sections of the Truckee Meadows and outlying areas probably would experience varying intensities of shaking. Seismologists measure how people perceive ground shaking and the extent of damage it can do by referring to the Modified Mercalli Intensity Scale. The scale ranges from Intensity I, or movement perceived by very few people under special conditions, to Intensity XII, or extreme shaking where the seismic waves can be seen on the earth’s surface, the ground is greatly disturbed and there is total damage across a wide area.
Walls come tumblin’ down
The highest level of shaking to occur in the Reno-Carson City region would probably be Intensity IX, or shaking that can cause general panic, considerable damage to—and some collapse of—unreinforced masonry buildings (brick, stone, etc.), buildings to shift off foundations, well-designed frame structures to be thrown out of plumb and damage to reservoirs and underground pipelines.
A scenario map shows that most of Reno, Washoe Valley, Carson City, Verdi, Mogul and much of Sparks and some areas of the north valleys could experience Intensity IX shaking, with other sections experiencing Intensity VII (general alarm, some rockfalls, weak chimneys breaking, heavy furniture being overturned, etc.) or Intensity VIII (alarm turns to panic, liquefaction occurs, trees shake strongly, slight damage in well-built structures with earthquake resistance, etc.).
Intensity IX shaking could cause a lot of non-structural damage (broken or fractured appliances, furniture, water heaters, etc.) as well as considerable structural damage (broken parapets, collapsed walls, poorly built structures crumbling, etc.).
The university and East Fourth Street, South Virginia Street and Victorian Avenue in Sparks are within the Intensity IX zone and could expect to see moderate to severe damage to the unreinforced masonry buildings in those areas. Without seismic retrofitting, most of the pre-World War II wood-frame dwellings in the IX zone and many in Intensity VIII areas would be uninhabitable after this hypothetical earthquake.
Most modern wood-frame homes should be spared major damage, says UNR Seismological Laboratory Director John Anderson, one of the authors of the 1996 report.
“What we believe is that in most cases, the new wood-frame houses are an extremely safe kind of construction,” Anderson says. “So in the vast majority of cases, the house itself being destroyed by the earthquake would be relatively unlikely.”
Most schools should survive structurally intact, although there could be a large amount of non-structural damage, according to the report. Several school buildings would likely be used as emergency shelters. Most downtown casinos have been built according to the earthquake safety codes in the area, Anderson says.
Gary Cavakis, director of facilities at Silver Legacy, says the hotel-casino is designed to withstand large earthquakes, but he cautions that you can never tell what will happen during an earthquake.
“To say how much damage you’re going to have, that’s the unknown, because you don’t know if that, [at] 7.1, if it was right underground, I don’t think anything could stand it,” Cavakis says. “I’m not a geologist or an earthquake expert. I just know that from the engineers that I’ve talked to, we have designed everything into this building that we can to minimize any damage.”
Press Clewe, program manager of the Washoe County District of Emergency Management, says some downtown casinos pose a safety hazard from falling window glass and other debris.
“Any building with lots of windows—dangerous, very dangerous,” he says. “It doesn’t matter how well they were built, that glass is going to come out.”
Jonathan Price, state geologist and director of the NBMG, says that in this worst-case earthquake scenario, the dollar amount of structural and non-structural damage could run from hundreds of millions of dollars into the low billons. But just because the scenario report lists certain regions as locations of potential high-intensity shaking, it doesn’t necessarily mean that all buildings will suffer mass destruction when the real thing hits.
“What’s clear from [past] earthquakes of similar size is that the damage is going to be spotty,” he says. “There will be some places that have a lot of damage … a building will collapse, [but] the building right next door is OK.”
The report shows potential moderate liquefaction occurring northeast of Sparks and in parts of the north valleys, south Reno and southeast Reno. The Reno-Tahoe Airport is located within this hazard zone. Washoe Valley, much of central Carson City, Lockwood and the canyon east along the Truckee River show a high susceptibility to liquefaction.
Price says the potential liquefaction hazards are because of the sandy and silty deposits in the area. Groundwater is also close to the surface in these areas.
He says that buildings with solid foundations could sink if the soil around them liquefies, causing them to float and partially submerge into the ground. A classic example of liquefaction occurred during the Loma Prieta earthquake, which caused homes in San Francisco’s landfilled Marina District to sink partially into the ground.
Since the Carson Range fault system crosses through Franktown, Galena Forest Estates and parts of south Reno, these areas could see varying degrees of surface offset. The ground surface could be displaced about six feet or more near Galena. Parts of Reno could experience smaller levels of surface faulting. Gas lines in these areas would likely break, Price says.
“Sometimes, the pipelines can be [built] to withstand the breaks, but the biggest earthquakes that we know of have as much as five, five and a half meters of vertical displacement, and it’s awful hard to engineer a pipeline to withstand 18 feet of vertical ruptures,” he says.
Rockslides and landslides would occur in mountain passes. The east and west directions of Interstate 80 could be closed because of rockslides and landslides in the canyons near Lockwood and Truckee. More slides could be triggered from aftershocks.
Living in the aftermath
Power, water and telephone services would be disrupted for anywhere from a few hours to several weeks after the quake, according to the report. All of this would affect residents, as well as emergency crews responding to various scenes of destruction.
“There will be broken water mains, broken telephone lines, broken sewer lines, broken gas lines, downed power lines,” Clewe says. “There will be fire. There will be broken buildings, there will be people trapped inside broken buildings, and we’re going to be needing to do a wide variety of first-response activities.”
People making non-emergency calls to dispatch or 911 centers can overwhelm the system. It is possible that some people’s calls for help may not be attended to during this time, Clewe says, because of the high amount of crises that state and local emergency response teams and law enforcement agencies would be responding to.
Water service and distribution centers would be affected after the quake. There is generally a one- to two-day water supply stored in the region, according to the report. But since the quakes could cause hundreds of breaks in water mains and service, power loss at groundwater pumping stations and storage tank failures, water availability would be limited.
It could be up to several days before water service was restored in some regions and up to several weeks in those that experienced heavy shaking. Water could be contaminated, so those who have service would have to follow boil-water orders and water-conservation measures. Wastewater systems could also experience numerous breaks, and repairs and restorations could take several days to weeks to finish.
Gas service would also be disrupted because of breaks in pipelines or from people turning off their gas. In the scenario earthquake, up to 20 percent of areas supplied by lines that cross fault zones would be without gas for up to a week. The petroleum distribution system could also be shut down for inspection for an even longer period, if repairs are necessary. Supplies are limited to a few days, and emergency response agencies and companies would have to address their needs for refueling vehicles and equipment during an emergency.
Preparing for the worst
Clewe says his department has exercised emergency operations using the NBMG report as their scenario. He says the report has been very helpful because of the wide variety of input from different aspects of the community, from medical services to utilities.
“We are constantly doing planning for this variety of event because … it is the kind of emergency event that is instantaneous, that if we can plan for this, we can plan for several other varieties of emergency events also,” he says.
He says earthquake awareness has increased since the report came out, citing earthquake drills that are done at schools in the Washoe County School District and the Nevada Department of Transportation’s seismic retrofitting of several bridges along Interstate 80. Scott Magruder, NDOT’s public information officer, says nine bridges in the Spaghetti Bowl and west of that intersection are being retrofitted as part of an $8 million project.
Seismologist Anderson also praises NDOT’s retrofitting of the freeway bridges. As the city’s lifelines, he says, the freeways and highways must be kept open so that food, medicine and other critical supplies can come through.
“The chance of our highways being shut down by an earthquake have been reduced enormously as a result of what [NDOT’s] done,” he says.
Seismologists can’t predict when earthquakes will strike or how large they will be. And no structure is fully earthquake-proof. Still, experts say it’s best to be prepared.
“It’s not the most probable earthquake," Anderson says about the hypothetical 7.1 quake. "The most probable earthquake is something smaller and not located within the city. But this earthquake, if it occurs, would be the earthquake that would stress the urban system more than any other. … If we are ready for this one, then we’ll be ready for anything."