Testing Two New Features in ShakeAlert: Magnitude Weighting with Data Duration: 100km/5s Pause Radius for Ground Motions

Deborah E. Smith, Jeff J. McGuire, & Angie Lux

Submitted September 11, 2022, SCEC Contribution #12580, 2022 SCEC Annual Meeting Poster #066

In the ShakeAlert earthquake early warning system, our goal is to provide estimates of earthquake source parameters and resultant ground motions that are both as timely and accurate as possible. The overarching objective is to provide information that can help save lives and prevent damage, while at the same time minimizing false alerts. Often, there are trade-offs between speed and accuracy. Two new features were introduced to ShakeAlert in an effort to maximize the benefits within the limits of these trade-offs. These two features were recently tested, approved, and deployed to the production system.

In the new system, the point source algorithm, EPIC, weights station contributions to the magnitude estimate by the amount of data available at the time. The ShakeAlert System Testing and Performance team found this change speeds magnitude estimates for large events by up to 2 seconds; however, earthquakes in the M4-6 range, which sometimes have initial overestimates due to uncertainty from sparse data (only a few seconds of data and very few stations), had an average increase in overestimating the initial magnitude by a little less than 0.1 magnitude unit. This is one demonstration of the trade-off of speed/accuracy of the ShakeAlert system, where improving our initial estimates of magnitude for large damaging events was prioritized over exact magnitude accuracy for smaller events.

The second new feature, called ‘alert-pause’, limits the area of an initial alert to a radius of 100 km for the first 5 seconds after event detection. During this time, alert levels may be modified inside 100 km, but the alert radius can only reach beyond 100 km after 5 seconds. Indeed, within 5 seconds, there is usually enough data (both in terms of data duration and number of stations used for the solution) to stabilize the magnitude solution and greatly reduce any overestimation that may have arisen; hence, the solution is more accurate after 5 seconds. This initial pause radius of 100 km limits the far-field overestimates of ground motions in the first 5 seconds while still allowing rapid estimation of the more intense ground motions for a 100 km radius surrounding the epicenter (where timely estimates are more critical).

Key Words
earthquake early warning, ShakeAlert, testing

Smith, D. E., McGuire, J. J., & Lux, A. (2022, 09). Testing Two New Features in ShakeAlert: Magnitude Weighting with Data Duration: 100km/5s Pause Radius for Ground Motions. Poster Presentation at 2022 SCEC Annual Meeting.

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