The ShakeAlert earthquake early warning system is designed to automatically identify and characterize the initiation and rupture evolution of large earthquakes, estimate the intensity of ground shaking that will result, and deliver alerts to people and systems that may experience shaking, prior to the occurrence of shaking at their location. It is configured to issue alerts to locations within the West Coast of the United States. In 2018, ShakeAlert 2.0 went live in a regional public test in the first phase of a general public rollout. The ShakeAlert system is now providing alerts to more than 60 institutional partners in the three states of the western United States where most of the nation’s earthquake risk is concentrated: California, Oregon, and Washington. The ShakeAlert 2.0 product for public alerting is a message containing a polygon enclosing a region predicted to experience modified Mercalli intensity (MMI) threshold levels that depend on the delivery method. Wireless Emergency Alerts are delivered for M 5+ earthquakes with expected shaking of MMI≥IV. For cell phone apps, the thresholds are M 4.5+ and MMI≥III. A polygon format alert is the easiest description for selective rebroadcasting mechanisms (e.g., cell towers) and is a requirement for some mass notification systems such as the Federal Emergency Management Agency’s Integrated Public Alert and Warning System. ShakeAlert 2.0 was tested using historic waveform data consisting of 60 M 3.5+ and 25 M 5.0+ earthquakes, in addition to other anomalous waveforms such as calibration signals. For the historic event test, the average M 5+ false alert and missed event rates for ShakeAlert 2.0 are 8% and 16%. The M 3.5+ false alert and missed event rates are 10% and 36.7%. Real-time performance metrics are also presented to assess how the system behaves in regions that are well-instrumented, sparsely instrumented, and for offshore earthquakes.

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The University of California Berkeley's (UCB) Earthquake Alert Systems (ElarmS) is a network-based earthquake early warning (EEW) algorithm that was one of the original algorithms developed for the U.S. west-coast-wide ShakeAlert EEW system. Here, we describe the latest update to the algorithm, ElarmS v.3.0 (ElarmS-3 or E3). A new teleseismic filter has been developed for E3 that analyzes the frequency content of incoming signals to better differentiate between teleseismic and local earthquakes. A series of trigger filters, including amplitude-based checks and a horizontal-to-vertical ratio check, have also been added to E3 to improve the quality of triggers that are used to create events. Because of its excellent performance, E3 is now the basis for EPIC, the only ShakeAlert point-source algorithm going forward. We can therefore also use the performance of E3 described here to assess the likely performance of ShakeAlert in the coming public rollout. We should expect false events with magnitudes between M 5 and 6 less than once per year. False events with M ≥ 6 will be even less frequent, with none having been observed in testing. We do not expect to miss any M ≥ 6 onshore earthquakes, though the system may miss some large offshore events and may miss one onshore earthquake between M 5 and 6 per year. Finally, in the metropolitan regions where the station density is on the order of 10 km, we expect users 20, 30, and 40 km from an earthquake epicenter to get 3, 6, and 9 s warning, respectively, before the S-wave shaking begins. Electronic Supplement: Screenshot of the Earthquake Alert Systems (ElarmS) review tool, and example histograms and tables of algorithm performance created by the review tool.

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