By Some 230 million people in the United States face the potential of damaging earthquakes in 100 years, according to the latest U.S. National Seismic Hazard Model, or NSHM. That’s about 40 million more people than the NSHM previously suggested.
“This hazard model forecasts where we think the future earthquakes will occur,” and moreover, where there’s “the chance of having any damage from an earthquake,” says geophysicist Mark Petersen of the U.S. Geological Survey in Denver.
The NSHM draws from data on some 130,000 quakes recorded in seismic catalogs and the geologic record, as well as nearly 500 new active faults. It also incorporates new ground motion models that estimate the shaking at a site during a quake. These updates contributed to an increase in the mean earthquake hazard across the country, Petersen and colleagues report December 29 in Earthquake Spectra.
Damaging quakes
This map from the updated National Seismic Hazard Model shows the probability of a damaging earthquake occurring anywhere in the United States in 100 years. A damaging quake is any temblor with a Modified Mercalli Intensity, or MMI, rating of VI or above. MMI ratings are roman numerals that ascribe quakes a severity based on their observable effects. An earthquake rated as MMI VI is felt by all, frightens many, moves some heavy furniture and causes a few instances of fallen plaster, resulting in relatively slight damage overall.
Earthquake probabilities across the United States
One key difference in this update is the improved characterization of shaking in sedimentary basins, Petersen says. Basins with deep soil can amplify some of the waves generated by an earthquake. That boost can be significant for a quake’s longer and slower waves, causing more damage to tall buildings and long bridges. Accounting for the amplification elevated the ground shaking hazard for such structures in some major cities, like Seattle, Los Angeles, Salt Lake City and Portland, Ore.
The new NSHM also includes updated ground shaking models of the subduction zones in the Pacific Northwest and southern Alaska. These are areas where one tectonic plate pushes under another, and they’re notorious for generating large quakes (SN: 1/13/21). In general, the changes in the new NSHM have increased the hazard very close to the subduction zone and decreased the hazard away from it.
That’s not to say quakes are restricted to the plate boundaries, though. In and around southeastern Missouri, quakes sometimes rumble along ancient rifts in the crust. And in 1886, an unidentified fault near Charleston, South Carolina birthed a devasting temblor that led to 60 deaths and damaged thousands of structures.
