U of U Seismograph Stations Research: The Earthquake Clock in Southwestern Utah

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At 4:26 AM on September 2, 1992, a magnitude 5.8 earthquake occurred in southwestern Utah at a location about 10 miles east of St. George. The earthquake caused surprisingly little damage to buildings in the area (although damage estimates exceed $1.25 million), but it is well known for triggering the largest landslide ever for an earthquake of this magnitude. The University of Utah Seismograph Stations immediately sent a crew out with portable earthquake seismometers to monitor aftershocks and help refine the location of the fault that the earthquake occurred on. Although there were an unusually small number of aftershocks, it was identified as probably having occurred on the Hurricane Fault or a closely associated fault. The Hurricane Fault is the major fault of southwestern Utah and as is usually the case for many Utah earthquakes, there were no foreshocks (small earthquakes which precede the major shock) or other signs of imminent earthquake activity. Although this earthquake was highly unusual in terms of the minimal damage it caused for its size, there were structures which received notable damage in St. George and three homes were destroyed and many others threatened because of an earthquake-induced landslide. Shaking from the earthquake was felt over an area of over 55,212 square miles. People felt the earthquake in St. George, south to Lake Mohave (Nevada-Arizona border) and north to Delta, Utah. The shaking caused structural damage in older, unreinforced masonry buildings and included problems such as walls separating from roofs, cracking in walls and foundations, and chimney damage. Nonstructural damage included overturning of bookshelves, and objects knocked from shelves. Shaking also caused rock falls along the Hurricane Cliffs and major changes in water flow at the Pah Tempe Resort hot springs. The Springdale landslide was the most dramatic result of ground shaking. This slide destroyed two water tanks, several storage buildings, three homes in a subdivision, blocked State Route 9, and ruptured utility lines. Condominiums and businesses nearby were evacuated for a short time. Click Here For Photos of Landslide Eyewitness accounts from residents of two of the homes reported that they were awakened by the early morning earthquake, but returned to bed after the shaking stopped. Within 15-30 minutes they heard "snapping and popping" noises and began to feel the ground shifting beneath them. As they left their houses and tried to escape, they could feel the earth shifting and watched deep fissures form as they walked down the entrance road. One resident tried to escape by car, but found the road blocked by a series of scarps (see photo) several meters high. The landslide continued to move for about 10 hours after the earthquake and has not moved since. Click here for more information on the 1992 St. George Landslide, including newspaper accounts, personal accounts of what it felt like, and more photos. What about the "Earthquake Clock" in Southwestern Utah?? Although scientists have some idea how often large earthquakes occur in northern Utah, this information is less well known for southwestern Utah. The occurrence of moderate earthquakes, such as in 1992, are evidence that this area is "earthquake country". Studies of the major fault of this area, the Hurricane fault, reveal that much larger earthquakes have occurred on this fault and thus are likely to occur again in the future. This region has had magnitude 7 to 7.5 earthquakes in the past and will have them again in the future. However, there is not enough information for scientists to know when the last large earthquake occurred (a magnitude 5.8 earthquake is a moderate, not large earthquake), or to have some idea when the next large earthquake might occur. In general, the earthquake activity in southwestern Utah has a pattern of dense clustering of earthquakes in small areas. These dense clusters, or "swarms", of earthquakes are superimposed over a more diffuse pattern of earthquake activity.    Click image for a bigger view. The earthquake swarms consist of earthquakes with magnitudes less than 4 and are spatially associated with relatively recent volcanism (hundreds to thousands of years since the last eruption). It is possible that the earthquake swarms indicate magma movement. Historically, southwestern Utah has been subjected to several large earthquakes and in particular four have had estimated magnitudes between 6 and 6.5. These earthquakes are the 1901 Richfield earthquake, which was felt for over 50,000 square miles, 1902 Pine Valley, and two in 1921 in the Elsinore area, each magnitude of about 6.2. Click on each of these earthquakes for more information, including newspaper accounts and photographs. This lack of more detailed information should not deter local communities from preparing for future earthquakes and related hazards. The area of greatest risk is an area which trends from Richfield through Cedar City and on to St. George. Within this area, residents should be aware of hazards related to earthquakes such as ground shaking, dam failure and flooding, rock falls and landslides, and possible fault cliff formation (small cliff that forms when movement during large earthquakes reaches the Earth' surface ). For more information on the 1992 St. George earthquake. Consult "The September 2, 1992 Ml 5.8 St. George Earthquake, Washington County, Utah: G. Christenson, Ed.: Utah Geological Survey Circular 88. You can obtain a copy by calling the Utah Geological Survey bookstore at 801-537-3320.