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Evaluation of Earthquake Magnitude Estimation and Event Detection Thresholds for Real-Time GNSS Networks: Examples from Recent Events Captured by the Network of the Americas


 Several studies have shown that real-time (RT) Global Navigation Satellite Systems (GNSS) measurements can provide an estimate of an earthquake’s moment magnitude (Mw) using scaling laws that relate peak ground displacements (PGDs) and hypocentral distance with Mw. In this study, we use data from GNSS stations operated by UNAVCO as part of the National Science Foundation Geodetic Facility for the Advancement of Geoscience (GAGE) that comprises the Network of the Americas (NOTA) to show that precise point positioning (PPP) solutions distributed in RT during five recent earthquakes could be used to calculate Mw rapidly and reliably. We analyze solutions distributed by UNAVCO during the 8 September 2017 Mw 8.2 Tehuantepec, Mexico, 10 January 2018 Mw 7.5 Great Swan Island, Honduras, 23 January 2018 Mw 7.9 Gulf of Alaska earthquake, and the 4 July 2019 Mw 6.4 and 6 July 2019 Mw 7.1 Ridgecrest, California, earthquakes.
 We find that RT-GNSS Mw estimates consistent with Advanced National Seismic System Comprehensive Earthquake Catalog values are available tens of seconds to a few minutes after the event origin time. The speed with which an estimate is available is dependent on the proximity to the epicenter of the closest NOTA stations. The results demonstrate that RT-GNSS networks could be used to mitigate the problem of magnitude saturation observed in seismic-based earthquake early warning (EEW) systems. RT-GNSS effectively expands the spectrum of events for which a seismic EEW system can provide accurate warnings of impending ground shaking and provides an independent verification of seismically derived magnitudes. We also analyze the RT-PPP solutions from more than 800 RT-GNSS stations to determine the ambient-noise levels of each NOTA station and combine that with PGD magnitude scaling laws to construct regional network sensitivity maps for the NOTA network. Such maps may be used to determine “blind spots” or regions of lower sensitivity in RT-GNSS networks under consideration for EEW.