An improved understanding of how energy and seismic moment accumulate in the crust and upper mantle along major plate boundaries is essential for forecasting the size and timing of major earthquakes (Field et al., 2015;Rollins et al., 2020;Smith-Konter & Sandwell, 2009;Weiss et al., 2020). Recent studies have shown that most damaging earthquakes occur in areas where the crustal strain rate exceeds 100 nanostrain/yr (e.g., Bayona et al., 2021;Elliott et al., 2016;Zeng et al., 2018). Many of these areas are heavily populated and have been struck by major destructive earthquakes in the past (Ward et al., 2021). Moreover, one of the largest uncertainties in earthquake hazard models (e.g., California's UCERF-3 model (Field et al., 2014(Field et al., , 2015 is the amount of plate boundary deformation that is accommodated by off-fault strain and whether this strain is accumulating as elastic or plastic deformation. Therefore, accurate strain rate measurements are needed to improve earthquake forecasts. Achieving an ideal 100-nanostrain/yr accuracy in California at an ideal 10-km resolution (i.e., the typical fault locking depth) requires a horizontal velocity model that has an accuracy of 1 mm/yr. Besides, moderate earthquakes, fault creep, and other