Testing the ShakeAlert Earthquake Early Warning System Using Synthesized Earthquake Sequences

Creators: Böse, Maren¹; Andrews, Jennifer²; O'Rourke, Colin³; Kilb, Deborah⁴; Lux, Angela⁵; Bunn, Julian²; McGuire, Jeffrey⁶

1. ETH Zurich
2. California Institute of Technology
3. University of Washington
4. University of California, San Diego
5. University of California, Berkeley
6. United States Geological Survey

Abstract

We test the behavior of the United States (US) West Coast ShakeAlert earthquake early warning (EEW) system during temporally close earthquake pairs to understand current performance and limitations. We consider performance metrics based on source parameter and ground-motion forecast accuracy, as well as on alerting timeliness. We generate ground-motion times series for synthesized earthquake sequences from real data by combining the signals from pairs of well-recorded earthquakes (4.4≤M≤7.1) using time shifts ranging from −60 to +180 s. We examine fore- and aftershock sequences, near-simultaneous events in different source regions, and simulated out-of-network and offshore earthquakes. We find that the operational ShakeAlert algorithms Earthquake Point-source Integrated Code (EPIC) and Finite-Fault Rupture Detector (FinDer) and the Propagation of Local Undamped Motion (PLUM) method perform largely as expected: EPIC provides the best source location estimates and is often fastest but can underestimate magnitudes or, in extreme cases, miss large earthquakes; FinDer provides real-time line-source models and unsaturated magnitude estimates for large earthquakes but currently cannot process concurrent events and may mislocate offshore earthquakes; PLUM identifies pockets of strong ground motion, but can overestimate alert areas. Implications for system performance are: (1) spatially and temporally close events are difficult to identify separately; (2) challenging scenarios with foreshocks that are close in space and time can lead to missed alerts for large earthquakes; and (3) in these situations the algorithms can often estimate ground motion better than source parameters. To improve EEW, our work suggests revisiting the current algorithm weighting in ShakeAlert, to continue developments that focus on using ground-motion data to aggregate alerts from multiple algorithms, and to investigate methods to optimally leverage algorithm ground-motion estimates. For testing and certification of EEW performance in ShakeAlert and other EEW systems where applicable, we also suggest that 25 of our 73 scenarios become part of the baseline data set.

Copyright and License

Acknowledgement

This material is based upon work supported by the U.S. Geological Survey (USGS) under Grant/Cooperative Agreement Numbers G19AC00252 and G21AC10532 to ETH Zurich, G19AC00296, G21AC10561, and G19AC00125‐04 to Caltech, an Intergovernmental Personnel Act (IPA) to UC San Diego, and G21AC10525 to UC Berkeley. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This work benefited from helpful discussions with members of the ShakeAlert team. The authors would like to thank Minh Huynh, Deborah Smith, and Andrew Good for developing the codes to generate the warning time figures. The authors would also like to thank Grace A. Parker and Geneva Chong (USGS), two anonymous reviewers, and Editor‐In‐Chief Allison Bent for their thorough reviews.

Data Availability

The seismic waveform data for our building blocks (Table 1) was downloaded from the Southern California Earthquake Data Center (SCEDC, https://scedc.caltech.edu, last accessed March 2022). The ground‐motion waveforms for the 25 scenarios that we recommend becoming part of the baseline data set for testing and certification in ShakeAlert and potentially other earthquake early warning (EEW) systems can be downloaded as miniSEED files from the Southern California Earthquake Data Center at Caltech: https://scedc.caltech.edu/data/eewtesting.html#sim_seq (last accessed July 2022). We constructed our reference ShakeMaps in Figure 3 from U.S. Geological Survey (USGS) ShakeMaps downloaded from https://earthquake.usgs.gov/data/shakemap/ (last accessed November 2021). Additional information about the Earthworm Tankplayer software can be found at http://www.earthwormcentral.org/ (last accessed July 2022). The supplemental material includes predicted peak Modified Mercalli Intensity (MMI for data set number 3, and additional warning time and ground‐motion plots, as well as videos illustrating the alerting performance of Earthquake Point‐source Integrated Code (EPIC), Finite‐fault Rupture Detector (FinDer), Solution Aggregator (SA), and Propagation of Local Undamped Motion (PLUM) for the scenarios in Figure 3. The material also includes a table and map showing M 5+ earthquake pairs extracted from the Advanced National Seismic System (ANSS) ComCat (Guy et al., 2015; https://earthquake.usgs.gov/data/comcat/, last accessed January 2022) catalog [1975–2021]).

Supplemental Material

Supplementary data (PDF)

Supplementary videos s1-s4 (ZIP)

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