Geoelectric Hazard Maps for the Pacific Northwest

Abstract

Maps of extreme value, horizontal component geoelectric field amplitude are constructed for the Pacific Northwest United States (and parts of neighboring Canada). Multidecade long geoelectric field time series are calculated by convolving Earth surface impedance tensors from 71 discrete magnetotelluric survey sites across the region with historical 1-min (2-min Nyquist) geomagnetic variation time series obtained from two nearby observatories. After fitting statistical models to 1-min geoelectric amplitudes realized during magnetic storms, extrapolations are made to estimate threshold amplitudes that are only exceeded, on average, once per century. One hundred-year geoelectric exceedance amplitudes range from 0.06 V/km at a survey site in western Washington State to 9.47 V/km at a site in southeast British Columbia; 100-year geoelectric exceedance amplitudes equal 7.10 V/km at a site north of Seattle and 2.28 V/km at a site north of Portland. Systematic and random errors are estimated to be less than 20%, much less than site-to-site differences in geoelectric amplitude that arise from site-to-site differences in surface impedance. Maps of 100-year exceedance amplitudes are compared with the peak geoelectric amplitudes realized during the March 1989 magnetic superstorm; it is noted that some storms of relatively modest intensity can generate localized geoelectric fields of relatively high amplitude. The geography of geoelectric hazard across the Pacific Northwest is closely related to known geologic and tectonic structures.

The Role of Global/Regional Earth Conductivity Models in Natural Geomagnetic Hazard Mitigation

“…Similar conclusions have been reached by a Gannon et al (2017) study which compared simulated GICs using Fernberg (2012) 1D models vs Schultz et al (2006Schultz et al ( -2018 in the Pacific NW of the USA. This work is in some disagreement with other published and unpublished reports for reasons discussed in Love et al (2018b), but it does emphasize the role of geoelectric field orientation in GIC estimation.…”

A 100‐year Geoelectric Hazard Analysis for the U.S. High‐Voltage Power Grid

“…In the northwest United States, several geoelectric hazard analyses have been conducted with the nearby Victoria (VIC) and Newport (NEW) geomagnetic observatories (e.g., Gannon et al, ; Love, Lucas, Kelbert, et al, ), but separate extrapolation techniques have been applied to analyze a potential extreme event. Gannon et al () scaled the 1989 Ottawa magnetic observatory time series, whereas Love, Lucas, Kelbert, et al () generated a long time series and used a statistical extrapolation method equivalent to that employed in this paper. The northwest region does not produce the large amplitudes seen in the eastern United States, but the region reveals how polarization of the electric fields produce more pronounced geoelectric hazards along the east‐west transmission lines.…”

Extreme‐Value Geoelectric Amplitude and Polarization Across the Northeast United States

“…We have previously used similar empirically based methods in analyzing geoelectric hazards for other parts of the continental United States (e.g., Love et al, , ; Lucas et al, ). Our methods stand in distinct contrast to analyses of geoelectric hazards (e.g., Gannon et al, ; Marti et al, ; NERC, ; Ngwira et al, ; Pulkkinen et al, ; Nikitina et al, ; Trichtchenko et al, ; Wei et al, ) relying on idealized models for broad regions of the Earth with structure that is simply depth dependent, one‐dimensional (e.g., Blum et al, ; Ferguson & Odwar, ; Fernberg, ).…”

“…In estimating northeastern geoelectric hazards, we rely on geophysical measurements and simple derivatives and integrations of those measurements. Our methods build on those developed by Love et al (, ) and Lucas et al () in their analyses of the Mid‐Atlantic and Pacific Northwest United States. We estimate (retrospectively) geoelectric time series by convolving magnetotelluric impedance tensors obtained for various survey sites with geomagnetic time series acquired over decades of time at three magnetic observatories that straddle the geographic extent of the northeast.…”