Our understanding of plate boundary deformation has been enhanced by transient signals observed against the backdrop of time-independent secular motions. We make use of a new analysis of displacement time series from about 1,000 continuous Global Positioning System (GPS) stations in California from 1999 to 2018 to distinguish tectonic and nontectonic transients from secular motion. A primary objective is to define a high-resolution three-dimensional reference frame (datum) for California that can be rapidly maintained with geodetic data to accommodate both secular and time-dependent motions. To this end, we compare the displacements to those predicted by a horizontal secular fault slip model for the region and construct displacement and strain rate fields. Over the past 19 years, California has experienced 19 geodetically detectable earthquakes and widespread postseismic deformation. We observe postseismic strain rate variations as large as 1,000 nstrain/year with moment releases equivalent up to an Mw6.8 earthquake. We find significant secular differences up to 10 mm/year with the fault slip model, from the Mendocino Triple Junction to the southern Cascadia subduction zone, the northern Basin and Range, and the Santa Barbara channel. Secular vertical uplift is observed across the Transverse Ranges, Coastal Ranges, Sierra Nevada, as well as large-scale postseismic uplift after the 1999 Mw7.1 Hector Mine and 2010 Mw7.2 El Mayor-Cucapah earthquakes. We also identify areas of vertical land motions due to anthropogenic, natural, and magmatic processes. Finally, we demonstrate the utility of the kinematic datum by improving the accuracy of high-spatial-resolution 12-day repeat-cycle Sentinel-1 Interferometric Synthetic Aperture Radar displacement and velocity maps.

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.

Tsunamis are a devastating natural, high-fatality hazard (Bryant, 2008). Because most tsunamis are generated by earthquakes, the first indication of a potentially life-threatening tsunami is the earthquake itself. Effective tsunami warning systems must therefore detect, locate, and estimate the magnitude of the causative earthquake to infer tsunamigenic potential, and warn coastal populations as soon as possible after initiation of fault rupture. Rapid characterization of the earthquake focal mechanism also aids in inferring tsunamigenic potential Melgar et al., 2016). Most tsunamis originate from Earth's subduction zones due to thrust faulting although earthquakes with other source mechanisms have also generated tsunamis (Elbanna et al., 2021;Scott, 2021). For example, the 2012 𝐴𝐴 𝐴𝐴𝑤𝑤 8.6 predominantly strike-slip intraplate event off Sumatra, Indonesia (Satriano et al., 2012), generated a tsunami that was recorded at sea-level stations as far as 4,800 km from the epicenter and by ocean bottom pressure sensors (i.e., DART buoys) in the Indian Ocean (Wang et al., 2012). Similarly, the 2009 𝐴𝐴 𝐴𝐴𝑤𝑤 8.1 Samoa earthquake was a normal faulting, outer-rise type event that produced a sizable tsunami with 189 fatalities (Okal et al., 2010).Current warning systems are well-developed for basin-wide and regional tsunamis. For earthquakes over 𝐴𝐴 𝐴𝐴𝑤𝑤 8.0, they rely mainly on long period (>∼300 s) seismic data recorded by broadband seismometers at distances greater than ∼500 km from the epicenter. However, for large tsunamigenic events, ground motions can exceed the dynamic range of a seismometer and result in a clipped record if measured too close to the seismic rupture. Therefore, tsunami warnings to the coastal communities located closest to the earthquake rupture may not be issued in a sufficiently timely manner. Another serious challenge for tsunami warning is the identification of tsunami