A Twist of PLUM: Low-Magnitude Earthquakes and Ground-Motion-Based Early Warning

Jessie K. Saunders, Sarah E. Minson, Elizabeth S. Cochran, Julian Bunn, Annemarie S. Baltay, Debi Kilb, & Colin O'Rourke

Submitted August 13, 2021, SCEC Contribution #11360, 2021 SCEC Annual Meeting Poster #037

We examine the earthquake detection and alerting behavior of the Propagation of Local Undamped Motion (PLUM) earthquake early warning (EEW) algorithm as applied to real-time data in 2021 from U.S. West Coast earthquakes. PLUM uses observations of ground motions to directly forecast shaking intensity distributions, where PLUM’s detection and alerting parameters have been tailored to correspond with ShakeAlert EEW alerting criteria. We find that PLUM’s ability to detect earthquakes depends on the station distribution, where regions of high station density can enable detection of Mw<4.5 earthquakes. Low-magnitude (Mw<4.5) earthquakes can produce potentially damaging ground motions that are of interest to EEW but over a spatial footprint that is usually too small for EEW systems to provide timely alerts. As such, the ShakeAlert System sets magnitude-threshold criteria for issuing alerts to the public (e.g., Mw5.0+ for alerts delivered via the Wireless Emergency Alert system). Because PLUM does not produce magnitude estimates during its ground-motion prediction process, there is a concern that if PLUM is integrated into the ShakeAlert System, its detections of Mw<4.5 earthquakes will lead to a significant increase in over-alerting that may negatively impact public perception of EEW. However, out of 291 Mw3.0+ earthquakes from January 1 to August 5, 2021, that lie within the ShakeAlert reporting boundaries, only 20 had ground motions large enough to activate the PLUM algorithm, and of those, only 13 triggered an alert for modified Mercalli intensity (MMI) 4.0+ shaking due to the observed shaking distribution at the time the event detection was confirmed. Of these 13 earthquakes, four were for Mw>4.5 events, eight had over 1,000 “Did You Feel It?” reports, and all 13 had maximum-reported intensities of MMI>4.3. We discuss why PLUM earthquake detections do not necessarily lead to PLUM EEW alerts and highlight PLUM alert performance for several of these earthquakes, including the Mw6.0 Antelope Valley earthquake.

Key Words
earthquake early warning, ShakeAlert, PLUM

Citation
Saunders, J. K., Minson, S. E., Cochran, E. S., Bunn, J., Baltay, A. S., Kilb, D., & O'Rourke, C. (2021, 08). A Twist of PLUM: Low-Magnitude Earthquakes and Ground-Motion-Based Early Warning. Poster Presentation at 2021 SCEC Annual Meeting.


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