REGION — To help make the best decisions to protect communities from earthquakes, new USGS maps display how intense ground shaking could be across the nation.
The USGS recently updated their U.S. National Seismic Hazard Maps, which reflect the best and most current understanding of where future earthquakes will occur, how often they will occur, and how hard the ground will likely shake as a result.
While all states have some potential for earthquakes, 42 of the 50 states have a reasonable chance of experiencing damaging ground shaking from an earthquake in 50 years (the typical lifetime of a building). Scientists also conclude that 16 states have a relatively high likelihood of experiencing damaging ground shaking. These states have historically experienced earthquakes with a magnitude 6 or greater.
The hazard is especially high along the West Coast, intermountain west, and in several active regions of the central and eastern U.S., such as near New Madrid, Mo., and near Charleston, S.C. The 16 states at highest risk are Alaska, Arkansas, California, Hawaii, Idaho, Illinois, Kentucky, Missouri, Montana, Nevada, Oregon, South Carolina, Tennessee, Utah, Washington and Wyoming.
In California, earthquake hazard extends over a wider area than previously thought. Most notably, faults were recently discovered, raising earthquake hazard estimates for San Jose, Vallejo and San Diego. On the other hand, new insights on faults and rupture processes reduced earthquake hazard estimates for Irvine, Santa Barbara and Oakland. Hazard increased in some parts of the San Francisco Bay Area and Los Angeles region and decreased in other parts. These updates were from the new Uniform California Earthquake Rupture Forecast Model, which incorporates many more potential fault ruptures than did previous assessments. Recent earthquakes in Alaska, Mexico and New Zealand taught scientists more about complex ruptures and how faults can link together. This insight was applied to California for which approximately 250,000 potential complex ruptures were modeled.
New research on the Cascadia Subduction Zone resulted in increased estimates of earthquake magnitude up to magnitude 9.3. Deep-sea cores were collected that show evidence within the sea-floor sediments of large earthquake-generated mudflows. Earthquake shaking estimates were also increased following abundant data gathered from the magnitude 9.0 earthquake in Tohoku, Japan in 2011 and the magnitude 8.2 earthquake offshore of Chile in 2014, as those events ruptured along subduction zones similar to the Pacific Northwest zone.
Some states have experienced increased seismicity in the past few years that may be associated with human activities such as the disposal of wastewater in deep wells.
One specific focus for the future is including an additional layer to these earthquake hazard maps to account for recent potentially triggered earthquakes that occur near some wastewater disposal wells. Injection-induced earthquakes are challenging to incorporate into hazard models because they may not behave like natural earthquakes and their rates change based on man-made activities.