Real‐time, reliable magnitudes for large earthquakes from 1 Hz GPS precise point positioning: The 2011 Tohoku‐Oki (Japan) earthquake

Abstract

The early warning issued after the onset of the M w 9.0 Tohoku‐Oki earthquake significantly underestimated its magnitude, saturating, 120 seconds after the earthquake began, at M w 8.1. Here we investigate whether real‐time deformation data from Japan's dense network of continuously‐recording Global Positioning System (GPS) stations could have been used to provide a more reliable rapid estimate of the earthquake's magnitude, and ultimately a more robust tsunami forecast. We use precise point positioning in real‐time mode with broadcast clock and orbital corrections to give station positions every 1 s. We then carry out a simple static inversion on a subset of stations to determine the portion of the fault that slipped and the earthquake magnitude. Unlike most previous methods, our method produces estimates of seismic moment before the earthquake rupture has completed. We find that the deformation data allow a robust magnitude estimate just ∼100 s after the earthquake onset. We also investigated the density of stations required for a robust moment magnitude estimate. Fewer than 1 station every 100 km are needed. We recommend that GPS data be incorporated into earthquake early warning systems for regions at threat from large magnitude earthquakes and tsunamis.