New map reveals highest hazard risks for earthquakes in the United States

The latest U.S. National Seismic Hazard Model (NSHM) suggests that approximately 230 million individuals in the US are at risk of experiencing hazardous earthquakes within the next century. This forecast is around 40 million more than what was previously estimated by the NSHM.

Geophysicist Mark Petersen from the U.S. Geological Survey in Denver explains that this model estimates future earthquake locations and the possibility of damage resulting from these earthquakes.

The NSHM is based on data from about 130,000 earthquakes recorded in seismic catalogues and the geologic record, incorporating nearly 500 newly discovered active faults. It also uses new ground motion models to predict the extent of shaking in a particular location during an earthquake. These updates have led to an overall increase in the average earthquake hazard throughout the country, as mentioned in the December 29 report of Earthquake Spectra by Petersen and his team.

An updated map from the NSHM depicts the likelihood of a damaging earthquake anywhere in the US within a century. A destructive earthquake is defined as any tremor with a Modified Mercalli Intensity (MMI) rating of VI or above. The MMI ratings use roman numerals to assign a severity level to earthquakes based on their visible effects. An earthquake rated as MMI VI causes minor damage, moves some heavy furniture, frightens many, and is felt by all.

The revised model highlights the improved understanding of shaking in sedimentary basins. According to Petersen, basins featuring deep soil can enhance some of the waves created by an earthquake. This can be substantial for slower, longer waves of an earthquake, leading to more damage to tall buildings and lengthy bridges. This realization caused an escalation in the risk of ground shaking in major cities like Seattle, Los Angeles, Salt Lake City, and Portland, Oregon.

The NSHM update also features improved models of ground shaking in the subduction zones of the Pacific Northwest and southern Alaska. These are areas where tectonic plates converge, known for generating large earthquakes. Generally, the revisions in the NSHM have increased the hazard near the subduction zone and reduced it further away.

However, earthquakes are not limited to these plate boundaries. For example, in southwestern Missouri, earthquakes occasionally occur along old crustal rifts. Additionally, in 1886, an unidentified fault near Charleston, South Carolina caused a significant earthquake that resulted in 60 fatalities and damaged thousands of buildings.