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Uncertainty in Ground-Motion-to-Intensity Conversions Significantly Affects Earthquake Early Warning Alert Regions

Jessie K. Saunders, Annemarie S. Baltay, Maren Böse, & Sarah E. Minson

Submitted September 11, 2022, SCEC Contribution #12161, 2022 SCEC Annual Meeting Poster #221

We examine how the choice of ground-motion-to-intensity conversion equations (GMICEs) in earthquake early warning (EEW) systems affects resulting alert regions. We find existing GMICEs developed for the United States can underestimate observed shaking at short rupture distances or overestimate the extent of low-intensity shaking. Updated GMICEs that remove these biases would improve the accuracy of the alert regions for the source-characterization-based algorithms of the ShakeAlert EEW System for the West Coast of the United States. ShakeAlert employs two types of ground-motion models: ground-motion prediction equations (GMPEs), which calculate spatial distributions of peak ground acceleration and velocity from estimates of the earthquake source (magnitude as well as rupture distance); and GMICEs, which translate GMPE output into modified Mercalli intensity (MMI). We find significant epistemic uncertainty in the alert distances; near-source MMI estimates from different GMICEs can differ by over 1 MMI unit, and for larger magnitude earthquakes (M~6.5+), the MMI 3.5 and MMI 2.5 alert distances used for public EEW can differ by hundreds of kilometers. We use a catalog of “Did You Feel It?” shaking reports to evaluate how well GMICE predict observed shaking, and we find that the observed shaking is not well-modeled by either the GMICE recommended by Worden et al. (2012) (which underestimates near-source MMIs) or the GMICE currently used by ShakeMap for West Coast earthquakes (which overestimates the MMI 2.5 extent). These results motivate updating the GMICE relationships more generally, including in ShakeMap applications.

Saunders, J. K., Baltay, A. S., Böse, M., & Minson, S. E. (2022, 09). Uncertainty in Ground-Motion-to-Intensity Conversions Significantly Affects Earthquake Early Warning Alert Regions. Poster Presentation at 2022 SCEC Annual Meeting.

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