ABSTRACT

Geophysical studies almost exclusively reduce Global Navigation Satellite System (GNSS) observations to positions. Receiver velocities derived from single‐frequency high‐rate (1  Hz) carrier‐phase observations turn GNSS instruments into velocity meters of unlimited dynamic range, potentially in real time. This enables instantaneous investigation of large, rapid motions. The 125‐km‐deep Mw 7.1 Iniskin, Alaska, earthquake created only small permanent surface offsets between 1 and 2 cm but much larger dynamic displacements. We resolve S waves and basin resonance, including natural frequencies, using only single‐frequency GNSS data without any treatment of error sources along the signal propagation path. Using root mean squared error (rmse) over more than 2.5 min prior to the mainshock as a measure of noise, we determine median rmses for a 36 GNSS station network of 3.1±1.5, 2.0±0.4, and 4.8±1.4  mm/s in north, east, and vertical components, respectively. The instantaneous geodetic velocities for the Iniskin event fill observational gaps and allow re‐characterization of ground‐motion maps for the event. This application demonstrates the utility of geodetic and potentially consumer‐grade GNSS receivers for real‐time, instantaneous ground motion and site characterization, earthquake early warning, and structural monitoring.

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