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February 20, 2009
By Craig Depolo
The 2008 Wells Earthquake was the largest earthquake to occur in Nevada in the last 30 years. The following is an abridged description of the earthquake and its effects, some of the results from scientific studies of the event, and some of the lessons that Nevadans should learn in light of this event.
The Wells Earthquake struck northeastern Nevada on Thursday morning at 6:16 a.m., February 21, 2008. It was a damaging magnitude 6 earthquake that originated about 5 miles north-northeast of Wells. In Wells, it began with a sharp bang that most thought was an explosion of some sort, followed by shaking that built in strength and lasted fully 20 to 40 seconds. People were in many places about town at the time: at home, at work, and getting ready for school. When the shaking began, they braced themselves and watched in amazement as the contents of their homes, offices, and stores began to react in unison. Bricks were thrown from old buildings and one abandoned, two-story building collapsed. The shaking caused a temporary propane tank to roll over, breaking its connection and spraying liquid propane that formed a ground-hugging cloud. A water main broke, the large chimney on the high school auditorium was cocked backwards, and a 51-ton transformer began to walk off its pad, making its grounding cable taunt and pulling on the electric wires. Over 30 buildings in town were damaged. Debris was thrown down from over 20 buildings, in some cases covering the sidewalk with bricks, glass, and rubble. Two buildings partially collapsed. Three homes were lost. Most buildings and homes in town had cracks in them. Over 60 chimneys (10% to 15% of all in town) were damaged; some toppled. This added to the challenge in the February days where temperature highs were in the 20s and 30s. The effects from an earthquake are dramatic, instantaneous, and anticipatable. Being prepared and mitigating potentially dangerous situations is the key.
Wells experienced a major "background" earthquake, one that didn't break the ground surface and the type that could occur anywhere, anytime in this seismically active piece of the Earth's crust known as Nevada. It is a reminder that all of Nevada needs to be prepared for earthquakes.
Emergency preparedness, planning, and training were all used in Wells that morning. Local emergency responders and residents took care of all critical incidents within the 40 minutes it took for the first outside help and equipment to reach Wells. Outside assistance bolstered the response and the teams that canvassed the town looking for injuries and dangerous situations. By all accounts, Wells and surrounding Elko County personnel responded expertly and effectively. This success and momentum was carried into the recovery efforts of the town, where volunteers worked in cold and snowy conditions to help clean up and rebuild. Nevadans were at their finest!
The damage to several of the older, unreinforced brick buildings was severe. Parapets, the upper parts of walls, and concrete tops of walls fell on sidewalks and roofs. A problem with these buildings is that the lack of reinforcement means they are not tied together. Therefore they can shake as different pieces rather than as a unit that supports its members. Several of these failures exposed poor building practices, such as rubble in-filled walls or use of adobe and unfired bricks in walls, all of which make walls weaker when shaken.
The Wells Earthquake was important to document. It was the most damaging earthquake in Nevada since the 1954 Fallon - Dixie Valley Earthquakes and was a major earthquake recorded by modern instruments, allowing a lot more to be learned than just a location and magnitude. Understanding the Wells Earthquake helps us estimate likely shaking at other locations in Nevada, including other towns and Yucca Mountain. Teams of seismologists and technicians from the Nevada Seismological Laboratory, the University of Utah Seismograph Stations, and the U.S. Geological Survey raced to the area with seismometers and transmission equipment. As their earthquake-response duties and assistance to the community subsided, the Nevada Department of Transportation, Wells Rural Electric, and the Elko County Sheriff's Department helped the seismologists with snowcats to put stations into remote areas, and the Nevada Department of Information Technology assisted with data transmission. Twenty-one portable seismometers were installed within a week, allowing scientists to locate aftershocks precisely and outline the subsurface fault along which the earthquake occurred.
What scientists found was the earthquake began at about 5 miles deep and the fault break quickly raced in all directions from that point, rupturing an area of between 3 and 6 miles in length. It ruptured up to within about 1 mile below the surface and down to about 6 miles deep. The fault is oriented in a northeast-southwest direction and is inclined (or dips) to the southeast about 50° from the surface. The movement of the fault during the earthquake was normal dip-slip, meaning, in this case, that the southeast side slid down relative to the northwest side. Hundreds of aftershocks were recorded. Geodesists from the Nevada Bureau of Mines and Geology used GPS measurements from continuous stations in northeastern Nevada and found that, although weak, the offsets measured matched the earthquake model. A comparison of before and after radar images from satellites clearly showed the footprint of the earthquake on the surface of the earth. The radar comparison, known as interferometric synthetic aperture radar (InSAR), found that the ground surface had fallen 5 inches over the middle of the earthquake rupture.
The Wells earthquake occurred in a small step, or cross-fault, that connects two, larger fault zones, the Eastern East Humboldt Range fault zone and the Eastern Snake Mountains fault zone. Geologists from the Nevada Bureau of Mines and Geology and the Utah Geological Survey went to Wells immediately following the earthquake to look for ground cracks and other hazards; no ground breakage from the fault was ever found. They did find evidence that a paleo-earthquake offset the ground by about two-feet just north of town, indicating earthquakes larger than the 2008 event have occurred in the Wells region.
Careful measurements of small variations of gravity can give specific information about the subsurface geology. Geophysicists from the U.S. Geological Survey made precise measurements of the gravity around Wells and found that the bedrock was buried by over a mile of sediments. The measurements indicate this is a small basin, which likely trapped and amplified seismic waves from the Wells Earthquake. Trapped waves also may have caused the shaking in Wells to be longer than usual from a magnitude 6 earthquake. Many of Nevada's communities are located in basins which can have similar effects.
The 2008 Wells Earthquake struck a Nevada community hard, but that community and their surrounding neighbors have a pioneering spirit and wherewithal that can be credited with effectively responding to the emergency and recovering quickly to keep Wells going strong. Wells residents have been gracious in allowing the earthquake effects to be scrutinized so lessons can be learned. These lessons need to be heeded and applied in Nevada so that damage patterns, injuries, and deaths can be prevented from future earthquakes.
The lessons learned for the Nevada public are: Major earthquakes can occur anywhere in Nevada; Nevadans should secure, relocate, replace, or remove dangerous items that can fall on people and hurt them; If you are inside a building during an earthquake, stay inside; if you are outside, get away from buildings if it is safe to do so; Know, and make sure your family, friends, and employees know, to Drop, Cover, and Hold during an earthquake, and how to turn off the natural gas if they smell a gas leak; Please check on your neighbor's condition following a strong earthquake and help them if you can; Volunteers were essential to the success of the response and early recovery efforts at Wells. Nevadans need to continue to be willing to help their own communities, and neighboring communities, in earthquakes and other disasters.