Incorporating ground-motion uncertainties into earthquake early warning alert distance strategies using the July 2019 M6.4 and M7.1 Ridgecrest, California, earthquakes

Jessie K. Saunders, Brad T. Aagaard, Annemarie S. Baltay, & Sarah E. Minson

Published August 14, 2020, SCEC Contribution #10596, 2020 SCEC Annual Meeting Poster #214

The ShakeAlert earthquake early warning system aims to alert people who experience Modified Mercalli Intensity (MMI) IV+ shaking during an earthquake by using source estimates (magnitude and location) to estimate the median-expected peak ground motions with distance, then using these ground motions to determine the median-expected MMI and thus the extent of MMI IV shaking. Because median ground motions are used, even if the magnitude and location are correct, there will be people outside the alert region who experience MMI IV shaking but do not receive an EEW alert (“missed” alerts). We use 91,000 Did You Feel It? survey responses to the July 2019 M6.4 and M7.1 Ridgecrest, California, earthquakes to determine which ground-motion to intensity conversion equation (GMICE) best fits the median MMI with distance calculated from the ground-motion prediction equation (GMPE) used in ShakeAlert. We then explore how incorporating uncertainty from the GMPE and the GMICE in the alert distance calculation can produce more accurate MMI IV alert regions for a desired alerting strategy (for example, aiming to alert 95% of the people who experience MMI IV+ shaking) assuming accurate source characterization. Without incorporating ground-motion uncertainties, we find that the MMI IV alert regions using the median-expected ground motions alert less than 20% of the population that experiences MMI IV+ shaking. In contrast, we find that over 94% of the people who experience MMI IV+ shaking can be included in the MMI IV alert region when two standard deviations of ground-motion uncertainty are included in the alert distance computation. The optimal alerting strategy depends on the false alert tolerance of the community, particularly for situations such as the M6.4 earthquake where alerting 95% of the 5 million people who experience MMI IV+ requires over-alerting 14 million people who experience shaking below this level and do not need to take protective action.

Key Words
earthquake early warning, ShakeAlert, Ridgecrest

Citation
Saunders, J. K., Aagaard, B. T., Baltay, A. S., & Minson, S. E. (2020, 08). Incorporating ground-motion uncertainties into earthquake early warning alert distance strategies using the July 2019 M6.4 and M7.1 Ridgecrest, California, earthquakes. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions