Earth's crustal deformation cycle is traditionally divided into coseismic, postseismic, and interseismic phases upon which transient motions from various sources may be superimposed. Here we present a new seismogeodetic methodology to define and identify the transition from the coseismic to the early postseismic phase. While this early period of postseismic deformation has not been well observed, it plays an important role in better understanding fault processes and crustal rheology because it is where the fastest evolution of fault slip occurs. Current methods often choose to rely on geodetic displacements with several hour to daily resolution to estimate static coseismic offsets. The choice of data span is often arbitrary and introduces some fraction of postseismic motion. Instead, we apply a more physics-based approach that is applicable to interleaved regional networks of high-rate Global Navigation Satellite System (GNSS) and seismic sensors. The start time of the coseismic phase is based on P wave arrivals and its end time on the total release of energy derived from seismic velocities integrated from strong-motion accelerations. In the absence of physical collocations, we interpolate the coseismic time window to the GNSS stations and estimate the static offsets from the high-rate displacements. We demonstrate our methodology by applying it to 10 earthquakes over a range of magnitudes and fault mechanisms. We observe that the presence of early postseismic motions within the widely used estimates of daily coseismic offsets can lead to an overprediction of coseismic moment and fault slip, up to several meters depending on the magnitude and mechanism of the event. Plain Language Summary Surface deformation following medium to large earthquakes, referred to as the postseismic phase, can start immediately after earthquake shaking and last for years. Only a small fraction of postseismic deformation produces seismic waves, and therefore cannot be detected by traditional seismic instruments. Instead, geodetic methods such as Global Navigation Satellite System (GNSS) are widely used for that purpose. Here we use a combination of strong-motion accelerometer data and GNSS-derived station displacements to define the exact time interval of earthquake shaking and identify the transition to early postseismic deformation. We looked at 10 different earthquakes, of different sizes and mechanisms, to test our method and to explore their early postseismic deformation. As expected, the time of earthquake shaking and its duration vary by location with respect to fault rupture, and hence cannot be taken as a single time interval throughout the affected region. We compared once per second GNSS-derived displacements with the more traditional twenty-four-hour positioning technique and found significant early postseismic deformation in the first hours or even minutes after earthquake shaking has ceased. Future studies should take these results into account as different data inputs can significantly affect earthquake models.
