Here, we do not simulate or model an individual extreme event, but rather,
we statistically extrapolate data to an extreme event based on long time series of historical events using the methods described earlier. For this analysis, we follow the same bootstrap with replacement (Efron & Tibshirani,
1994) procedure outlined in Love et al. (
2017).
The extreme values we present are the median values from a 100-sample bootstrap analysis. The bootstrap samples also provide a 95% confidence interval for each magnetotelluric survey site centered around the median, m, with an
average confidence interval over all survey sites of [0.74 m, m, 1.37 m]. The confidence interval for the magnetotelluric sites is again less than a factor of 2, which is much smaller than the site-to-site variability in extreme values.
Figure
8 shows the
estimated once-per-century values of the geoelectric field at the 1,079 magnetotelluric sites spread across the United States. Regional investigations close to geomagnetic observatories have previously shown the large influence that geology plays in determining geoelectric hazards in small regions (e.g., Love et al.,
2017; Love, Lucas, Bedrosian, et al.,
2018; Love, Lucas, Kelbert, et al.,
2018).
We now expand that view to the entire United States and can see how variable the geoelectric hazards are across the nation.
The largest
estimated once-per-century geoelectric field is 27.2 V/km at a site located in Maine (MEE62), while the lowest
estimated once-per-century geoelectric field is 0.02 V/km at a site located in Idaho (IDK15). That is more than 3 orders of magnitude variation in geoelectric field that spans the entire country. Across the upper two thirds of the United States that has been surveyed, estimated geoelectric fields for a once-per-century geomagnetic storm are
predicted to exceed 1 V/km at 322 of the 1079 magnetotelluric sites, nearly 30% of the surveyed land area in the United States.
