Geoelectric hazard maps for the continental United States

Maps covering about half of the continental United States are presented of geoelectric amplitude that is exceeded, on average, once per century in response to extreme-intensity geomagnetic activity. These maps are constructed using an empirical parameterization of induction: local estimates of Earth-surface impedance, obtained at discrete geographic sites from magnetotelluric survey data, are convolved with latitude-dependent statistical maps of extreme-value geomagnetic activity, obtained from decades of 1-minute resolution magnetic observatory data. Geoelectric amplitudes are estimated for geomagnetic waveforms having 240-s sinusoidal period and amplitudes over 10 minutes that exceed a once-per-century threshold. As a result of the combination of geographic differences in geomagnetic activity and Earth-surface impedance, once-per-century geoelectric amplitudes span more than two orders of magnitude and are an intricate function of location. Specifically: for north-south induction, geoelectric amplitudes across large parts of the United States have a median value of 0.26 V/km; for east-west geomagnetic variation the median value is 0.23 V/km. However, at some sites, such as in northern Minnesota, once-per-century amplitudes exceed 14.00 V/km for north-south geomagnetic variation (23.34 V/km for east-west variation), while just over 100 km away, amplitudes are only 0.08 V/km (0.02 V/km). At some sites in the continental United States, once-per-century geoelectric amplitudes exceed the 1.7 V/km realized in Quebec during the March 1989 storm.