September 24, 2009

Utah Geological Survey and U.S. Geological Survey geologists lead a field trip for geoscientists and engineers at the Rice Creek trench site on the Weber segment of the Wasatch fault zone.

A recently completed study has found that a major earthquake struck the Wasatch Front between North Salt Lake and North Ogden about 500 years ago.  The study, which focused on the Weber segment of the Wasatch fault zone, also found evidence of six earthquakes of magnitude 6.5 or greater recurring about every 1500 years. 
 
The Utah Geological Survey has published the results of a joint Utah Geological Survey and U.S. Geological Survey seismic-hazard evaluation of the Weber segment of the Wasatch fault zone.  The report, titled Paleoseismic Investigation of the Northern Weber Segment of the Wasatch Fault Zone at the Rice Creek Trench Site, provides important information on the timing, recurrence, and displacement of prehistorical large-magnitude earthquakes on the segment. 

These data are essential to characterizing the long-term earthquake hazard of the Wasatch fault and will ultimately help improve earthquake-hazard maps of Utah and seismic provisions in the building codes.  The report is the eighteenth volume in the Paleoseismology of Utah series.

The new report shows that the 35-mile-long Weber segment, which extends from North Salt Lake to North Ogden, has had no problem generating large earthquakes.  The UGS/USGS team excavated a large trench across the active trace of the Wasatch fault near North Ogden and found geologic evidence for six prehistorical earthquakes of magnitude 6.5 or greater. 

The study helps reduce uncertainties in the timing of these earthquakes, which have recurred on average every 1500 years during the Holocene (last 10,000 years).  The team also identified a previously unknown earthquake on the segment and determined that the youngest earthquake occurred about 500 years ago with 5–10 feet of ground-surface displacement. 

A similar earthquake could strike at any time along the Wasatch Front and cause substantial loss of life and damage to buildings and infrastructure.  Additional information on earthquakes in Utah is available in the Utah Seismic Safely Commission publication called Putting Down Roots in Earthquake Country – Your Handbook for Earthquakes in Utah, which can be found online at ussc.utah.gov.

Special Studies 130 is available for $19.95 at the Natural Resources Map & Bookstore, located at 1594 West North Temple, Salt Lake City (801-537-3320 or 1-888-UTAHMAP; geostore@utah.gov).  The report can also be viewed on the UGS website at: http://geology.utah.gov/online/ss/ss-130.pdf