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The U.S. Geological Survey (USGS)-operated ShakeAlert® system is the United States West Coast earthquake early warning system (Given et al., 2018). In this study we detail ShakeAlert's performance during some of the largest events seen by the system thus far. Statewide public alerting using ShakeAlert messages was authorized in California in October 2019. Over the next few years, public alerts were expanded into Oregon and then into Washington (U.S. Geological Survey, 2024). ShakeAlert source results are routinely compared to the USGS Comprehensive Catalog (ComCat; Guy et al., 2015; U.S. Geological Survey, Earthquake Hazards Program, 2017), which contains the earthquake location and magnitude determined using complete waveform data. M 4.5 and larger is the threshold used for public alerting and was deliberately set below the level where damage is likely to compensate for cases where the system underestimates the magnitude. Between 17 October 2019 and 1 September 2023, the ShakeAlert system created 95 events with maximum magnitude estimates of M =4.5, the public alerting threshold. 94 of the 95 events were due to real earthquakes. Seven were categorized "false" per ShakeAlert's internal definition that there was no matching catalog event within 100 km and 30 s of origin time; however, all but one of these were real earthquakes that were poorly located, primarily because they were at the edges of the seismic network. Three detected events were labeled "missed" because they were very poorly located (>100 km location error). In addition, the system did not produce solutions for four ComCat events M =4.5 (U.S. Geological Survey, Earthquake Hazards Program, 2017), which were all at the edge of the alerting and network boundaries. The ShakeAlert system has accurately detected the majority of earthquakes that have occurred within the operational region since completing the public rollout, and alerts from the system have been delivered to millions of cell phone users throughout the West Coast. [ABSTRACT FROM AUTHOR]

In this study, we explore whether the Parker and Baltay (2022) site response models for the Los Angeles (LA) basin region can improve ground-motion forecasts in the U.S. Geological Survey ShakeAlert earthquake early warning system (hereafter ShakeAlert). We implement the peak ground acceleration and peak ground velocity site response models of Parker and Baltay (2022) in ShakeAlert via the earthquake information to ground-motion (hereafter eqinfo2GM) module, which predicts ground motions from the estimated earthquake parameters of magnitude, rupture length, and location. The nonergodic site response models for the greater LA area were developed using ground motions from 414 M 3-7.3 earthquakes in southern California. We test nonergodic ground-motion forecasts for five earthquakes in the LA area: the 1994 M 6.7 Northridge earthquake, the 2008 M 5.4 Chino Hills earthquake, the 2019 M 7.1 Ridgecrest earthquake, the 2020 M 4.5 South El Monte earthquake, and a synthetic M 7.8 earthquake on the southern San Andreas fault from the ShakeOut scenario, which was the basis of a statewide emergency response exercise. From the test results, we find that with the nonergodic site response applied, ShakeAlert not only alerts larger areas but can also result in longer warning times in LA region. In addition, the modified Mercalli intensity (MMI) ground-motion predictions generated by the ShakeAlert eqinfo2GM module are improved in accuracy when compared with the corresponding ShakeMap ground-truth MMI when the nonergodic site response model is applied. [ABSTRACT FROM AUTHOR]

Complex seismic velocity structure near the earthquake source can affect rupture dynamics and strongly modify the seismic waveforms recorded near the fault. Fault-zone waves are commonly observed in continental crustal settings but are less clear in subduction zones due to the spatial separation between seismic stations and the plate boundary fault. We observed anomalously long duration S waves from earthquake clusters located near the interface of the subducted Gorda plate north of the Mendocino triple junction. In contrast, earthquakes located just a few kilometers below each cluster show impulsive S waves. A nodal array experiment was conducted around the Northern California Seismic Network station KCT for two months to investigate the origin of the complex S waves. Beamforming analysis shows that the S waves contain three arrivals that have different horizontal slownesses, which we term S1, S2, and S coda. Similar analysis on P waves also show two arrivals with different horizontal slownesses, which we term P1 and P2. P1 and S1 have larger horizontal slowness than P2 and S2, respectively, indicating that the phase pairs are body waves with different ray paths. Building upon a seismic refraction profile, we construct 1D velocity models and test different thicknesses and VP/VS ratios for the subducted oceanic crust. The arrival times and relative slownesses of P1/P2 and S1/S2 phases indicate that they are the direct and the Moho reflected phases, respectively. Their properties are consistent with a crustal thickness of ~6 km and a moderate VP/VS ratio (~1.8). The S coda is more difficult to characterize but has a clear dominant frequency that likely reflects the near-source velocity and attenuation structure. Our study indicates that waveforms from earthquakes near the interface of the subducted slab can be used to infer detailed structural information about the plate-boundary zone at seismogenic depths. [ABSTRACT FROM AUTHOR]