Analyze This: Where are U.S. earthquakes most likely?
Some 230 million people in the United States face the risk of damaging earthquakes in the next 100 years. That’s according to the latest U.S. National Seismic Hazard Model, or NSHM. The NSHM estimates the risk of earthquakes based on historical data and seismic studies. The number of people expected to be at risk by the new NSHM is about 40 million more than NSHM had suggested in models from 2018 and earlier.
Let’s learn about earthquakes
“This hazard model forecasts where we think the future earthquakes will occur,” says Mark Petersen. A geophysicist, he studies earthquakes at the U.S. Geological Survey in Golden, Colo. The new work also reveals where there’s a chance of damage from a quake.
The NSHM draws from data on some 130,000 quakes. That includes recent ones and some that happened long ago. It also considers data from nearly 500 active faults. A fault is a split in Earth’s crust where rocks rub past each other. The NSHM also uses new methods that estimate ground shaking at specific places during a quake.
All this new data revealed that, on average, earthquake hazards have increased across the United States. Petersen and his colleagues shared their findings in February in Earthquake Spectra. Their new map will help people prepare for possible temblors.
Explainer: Seismic waves come in different ‘flavors’
A quake’s energy ripples out in ground-shaking vibrations called seismic waves. The updated model is better at estimating the shaking of sedimentary basins, Petersen says. Those places have deep soil that can amplify seismic waves. That can really boost certain waves from a quake, causing more damage to tall buildings and long bridges. Accounting for amplified waves increased the hazard forecast for cities such as Seattle, Wash., Los Angeles, Calif., and Portland, Ore.
But quake hazards aren’t limited to the U.S. west coast. Or even to the places where tectonic plates meet. In the middle of the country, in southeastern Missouri, quakes sometimes rumble along ancient rifts in Earth’s crust. And in 1886, a then-unknown fault near Charleston, S.C., caused a devastating temblor. It led to 60 deaths and damaged thousands of structures. The new map draws attention to the earthquake risks those more eastern states may face in the future as well.
This map shows the probability of a damaging earthquake occurring anywhere in the United States in the next 100 years. It’s based on the updated National Seismic Hazard Model.
U.S. Geological Survey National Seismic Hazard Program
An earthquake counts as “damaging” if it is a level VI or higher on the Modified Mercalli Intensity, or MMI, scale. MMI ratings describe earthquakes’ severity based on the effects observed during a quake. An earthquake rated as VI is felt by all in the area and can be frightening. It moves some heavy furniture and may knock a bit of plaster off of walls. Overall, it results in slight damage. Click image to enlarge.
Data Dive:
Look at the figure’s legend. What does it mean when an area is dark red? What does it mean when an area is blue?
What places in the United States have the highest risk of a damaging earthquake in the next 100 years?
Which of these high-risk places have high population densities? Which have low population densities?
Which areas of the United States have the lowest chance of a damaging earthquake in the next 100 years?
What other information would be useful to those in places at high risk of damaging earthquakes?
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