Introduction. by Dr. Tao Lai, Project Manager & Dr. Polsak Tothong, Analysis Engineer

Transcription

1 AIR Post-Disaster Survey for the M6.0 Wells (Nevada) Earthquake Editor s note: Days after the rural town of Wells, Nevada was hit by a magnitude 6.0 earthquake, AIR sent a team of engineers to survey the damage. The event provided a valuable opportunity to gain further insight into the vulnerability of structures in the intermountain region of the U.S. a region of moderate seismicity, but one often overlooked when assessing seismic risk. While the Wells earthquake did not result in large insured losses, a similar event in the Salt Lake City area, for example, would clearly be significant. by Introduction On February 21, 2008, at 6:16 a.m. local time, a magnitude 6.0 earthquake ruptured a previously unmapped fault about 5.5 miles (9 km) northeast of Wells, Nevada in the northeast corner of the state. With a relatively shallow focal depth estimated at 4.2 miles (6.7 km), the shock was felt as far away as Twin Falls, Idaho nearly 100 miles from the epicenter. In Salt Lake City, one hundred fifty miles to the east, a seven-story office building belonging to the Salt Lake Tribune shook east to west on the morning of the quake. Light fixtures swayed up to a foot. Wells itself is located at the southwest tip of a smaller basin bounded by the East Humboldt Range and the Snake Mountains (Figure 1). Geologists Wesnousky and Willoughby (200) 2 found paleoseismic evidence of an earthquake of about magnitude 7 at the northern end of the Ruby Mountains, just south of the East Humboldt Range. They further estimate that this event occurred between about Ground motion records and the pattern of aftershocks indicate that the causative fault was oriented in a roughly north/south direction and that it ruptured, at least in part, toward Wells. 1 This exacerbated both the ground shaking and damage in the town. The town of Wells (pop. 1,600) lies in the Great Basin, an arid region between the Wasatch Mountains in Utah and the Sierra Nevada in California. Though there are numerous smaller ranges across Nevada that are the consequence of a complex plate tectonic region, the northeast corner of the state is at only moderate risk. Figure 1: Geographic map of northeastern Nevada (Source: AIR)

2 4,800 and 7,600 years ago. In the past one hundred years, however, four tremors registering about Mw 5.0 all on nearby but unrelated faults have gently rocked the region. The Physical Consequences of the Quake The February 21 earthquake hit the town hard, as indicated by the USGS ShakeMap (Figure 2), which shows strong shaking in Wells. The ShakeMap is a real time intensity estimate based on seismogram recordings. One of seismograms, which is about 5.5 miles (9km) southwest of the epicenter and close to Wells, recorded PGA (Peak Ground Acceleration) of 0.12g. Wells is a rural town whose economy is largely driven by the fact that it lies on a crossroads for truckers and other travelers on Interstate 80 and U.S. 9. Earthquake insurance penetration is very low; one State Farm insurance agent told a local newspaper that he had sold only one earthquake policy there in the past 10 years. The district, however, did hold insurance coverage enough to cover damage to the high school and other civic structures. The Historic Downtown Not surprisingly, hardest hit were unreinforced masonry (URM) buildings in Wells historic commercial downtown particularly those in the Front Street District (7th Street and Lake). URM is among the most vulnerable construction types to earthquake ground shaking. Erected in the nineteenth century, the buildings here had recently been designated for revitalization and only about half were in use when the earthquake struck. That fact, along with the early hour of the quake, kept injuries to a minimum. The fate of these buildings is now highly questionable, and many will likely have to be demolished. Collapsed roofs and walls as shown in the bottom of Figure were common. Had the shaking been only a little more severe or longer lasting, complete collapses would likely have occurred. Newer masonry buildings fared better. Figure 4 shows a large crack and displacement of the brick veneer on a laundromat. The underlying concrete block wall remained relatively unscathed. Most residential buildings, made either of woodframe or of newer masonry, also sustained limited building damage. Figure 2. USGS ShakeMap showing ground motion intensity of the Wells earthquake (Source: USGS) Figure. Front Street before (top, source: Google Earth) and after (bottom, source: AIR) 2

3 Figure 4. the brick veneer of a masonry structure was displaced by more than three inches. (Source: AIR) Manufactured Homes Manufactured homes dominate some areas of Wells. Based on the AIR team s communications with residents and another field investigator on the scene, 95% of residential buildings are not anchored to their foundations. As a result, some manufactured homes fell off their jacks or supporting cinder blocks. Others were shifted, but remained on their foundations. According to reports, these homes are being tied down as they are repaired. Wells High School A significant percentage of the insured loss was accounted for by Wells High School, the first sections of which reopened only on April 14, nearly two months after the event. The main building, which is of reinforced concrete, was constructed in 1954 and additions were added in 1961 and The school s gymnasium is a steel structure. The AIR surveyors received access to the facility to find contents still strewn about among them broken trophy cases and toppled bookshelves. Fortunately, most computers and monitors remained in place on desk and table tops. worker, the infill was not part of the original construction, but was added during the most recent renovation. The high school s chimney was in the process of being demolished when the AIR team visited the site. According to an onsite worker, the chimney had towered 10 feet above the roof before the quake. The shaking had caused spalling of bricks at a discontinuous transition zone of the steel rebars, causing the upper part of the chimney Figure 6. demolition of chimney at Wells High School after upper section became misaligned (Source AIR) to misalign. The chimney had been nonfunctional since a previous renovation and, as the worker pointed out, should have been demolished at that time. Damage to Nonstructural Components The most ubiquitous nonstructural damage was to chimneys. According to a survey conducted by the Nevada Bureau of Mines and Geology, 4 some 58 chimneys sustained severe damage or were toppled. Tall, unreinforced chimneys fared the worst. All three chimneys on the building in Figure 7 sustained damage. Rather remarkably, the AIR team saw no instance in which a chimney fell and punctured the roof. Outside, cracks were visible on the southwest wall of the gymnasium at the top corners of a metal infill, close to the end of a large steel roof beam. During the quake, the beam sustained large axial forces that were transferred perpendicularly to the southwest gym wall, resulting in cracks in the wall and inclination of the infill. Better anchoring between the infill and the surrounding wall would have reduced the damage. According to an on-site Figure 7. All three unreinforced chimneys of this Wells building were crippled after the quake. (Source: AIR) Figure 5. Contents in disarray at Wells High School. (Source: AIR)

4 Workers at one convenience store spent nine hours refilling shelves that took only seconds of ground shaking to empty (Figure ). At a nearby casino, ceilings fell and gambling machines toppled. Figure 8: A light and well-anchored steel chimney (left) and the hybrid chimney with a low masonry section (right) survived the earthquake without damage (Source: AIR) The kind of masonry chimney damage seen across Wells could have been mitigated with proper anchoring techniques. Modern chimneys, like the well-anchored metal chimney and the hybrid chimney shown in Figure 8, fared well. Damage to other nonstructural components, such as suspended ceilings (Figure 9), unreinforced parapets and roof tiles was also common. Conclusion AIR routinely performs post-disaster surveys of significant events as part of its model evaluation process. While the M6.0 Wells, Nevada earthquake did not result in large insured losses, it provides valuable insight into the seismic hazard, building performance, and damage patterns in the intermountain region of the U.S. and insight into the magnitude of the potential losses were a similar earthquake to occur in a more populous area of this region, such as Salt Lake City. It also informs us more generally. For example, if a town in California were similarly shaken, we know that building damage would be limited due to stricter codes and stricter code enforcement, but damage to contents and nonstructural building components would likely be considerable. If a town in New Madrid area were similarly hit, in addition to contents and nonstructural damage, severe damage to the older building stock there could be expected, and to masonry construction in particular. The event also served as a wake-up call for communities and insurers writing in regions of moderate seismic risk, and it highlights the need to take mitigative measures Figure 9: suspended ceilings collapsed in this Wells office building. (Photo courtesy of Wells resident Cindy Mauchley) Contents Damage After the earthquake struck on the morning of February 21, most residents in Wells reported some type of property damage, including cracked walls and fallen chimneys and parapets. The prevailing damage, however, was to contents; the quake shook items off walls and shelves and tipped furniture in nearly every household and business establishment in the town. Figure 10: surveillance camera shows contents damage inside a Wells convenience store (Source: KUTV)

5 such as seismic retrofit of older structures. For example, parapets and chimneys are not expensive to reinforce, and communities should coordinate renovation efforts to obtain maximum benefit from seismic retrofit. An understanding of where future earthquakes are likely to occur is greatly facilitated when the locations of faults are known and mapped. In some regions, faults can easily be seen on the surface of the earth the San Andreas Fault in California is a prime example. In many seismic zones, however, there is little or no expression of faults on the surface, so their locations must be inferred from the seismic activity of the area. Many faults remain undiscovered, as had the fault that produced the February 21 Wells, Nevada earthquake. Because of the uncertainty surrounding the exact locations of faults and the epicenters of pre-instrumental earthquakes, the AIR earthquake models allow simulated earthquakes to occur not only directly on known faults, but also, with some probability, anywhere within a seismic source zone. Modeling thus becomes a particularly critical tool in assessing earthquake risk in regions of low to moderate seismicity For a discussion of the socio-economic impacts of this event, see ch/reports/depolo-14report/initial_observations_ pdf 4 About AIR Worldwide Corporation AIR Worldwide Corporation (AIR) is the scientific leader and most respected provider of risk modeling software and consulting services. AIR founded the catastrophe modeling industry in 1987 and today models the risk from natural catastrophes and terrorism in more than 50 countries. More than 400 insurance, reinsurance, financial, corporate and government clients rely on AIR software and services for catastrophe risk management, insurance-linked securities, site-specific seismic engineering analysis, and property replacement cost valuation. AIR is a member of the ISO family of companies and is headquartered in Boston with additional offices in North America, Europe and Asia. For more information, please visit www. air-worldwide.com AIR Worldwide Corporation. All rights reserved. 4