From Earthquake Source Parameters to Ground-Motion Warnings near You: The ShakeAlert Earthquake Information to Ground-Motion (eqInfo2GM) Method

We present a new near-real-time method for converting earthquake source parameters into ground motion (GM) at locations across the west coast of the United States. This method, called earthquake information to ground motion (eqInfo2GM), has been implemented as part of the ShakeAlert earthquake early warning (EEW) system and makes estimated GM accessible to users on the EEW timeframe of seconds, as the U. S. Geological Survey ShakeMap does at higher resolution and accuracy in the minutes following a seismic event. Whereas the higher fidelity ShakeMap comes at the cost of longer processing times, eqInfo2GM effectively provides a predicted ShakeMap before shaking arrives at many locations. We describe key design details, including ground-motion prediction equations (GMPEs) implemented, modifications made to optimize for speed, and formats created for conveying GMseverity. The GMoutput format determines added latency and reflects a trade-off between speed and accuracy; for our test earthquake data set, added latency is in the 0.01-1.5 s range after earthquake source parameters have been generated. GMPE implementations are validated against predicted ShakeMaps (without observations), with almost all events showing minimal mean shaking intensity level differences, reflecting variations only in treatment of source distances and V-S30 data. Comparison against ShakeMaps computed with observations (a proxy for true GM) show larger differences, demonstrating the challenges of working in the EEW timeframe, when full source characterization and peak ground motion observations are both unavailable. Although specific configurations and features of the method will evolve as the needs of the EEW user community become evident, eqInfo2GM is expected to improve the overall utility of EEWalerts by providing end users with estimates of predicted local GM hazard. Such near-real-time estimates will enable users to decide more accurately what action to take to reduce the impact of imminent, potentially damaging shaking. Supplemental Content: Comparison of behavior of ground motion prediction equations (GMPEs) implemented in earthquake information to ground motion (eqInfo2GM) across west coast regions; comparison of the output of eqInfo2GM and ShakeMap for a shallow Cascadia scenario event; comparison of the map and contour outputs of eqInfo2GM for two finite-fault scenarios; information on the historic earthquake events used for offline testing; information on synthetic finite-fault events used for offline testing; information for scenario events used for ground-motion accuracy validation against ShakeMap and internal consistency assessment between contour and map products; example ground-motion message formats for ground-motion map and ground-motion contour outputs.