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The 2019 Ridgecrest conjugate Mw6.4 and Mw7.1 events resulted in several meters of strike‐slip and dip‐slip along an intricate rupture, extending from the surface down to 15 km. Now with >2 years of post‐rupture observations, we utilize these results to better understand vertical postseismic deformation from the Ridgecrest sequence and illuminate the emerging significance of vertical earthquake cycle deformation data. We determine the cumulative vertical displacement observed by the continuous GNSS network since Ridgecrest, which requires additional time series analyses to adequately resolve vertical deformation compared to the horizontal. Using a Maxwell‐type viscoelastic relaxation model, with a best fit time‐averaged asthenosphere viscosity of 4e17 Pa·s and a laterally heterogeneous lithosphere, we find that viscoelastic relaxation accounts for a majority of the cumulative vertical deformation at Ridgecrest and strongly controls far‐field observations in all north‐east‐up components. The viscoelastic model alone generally underpredicts deformation from GNSS and the remaining nonviscoelastic displacement is most prominent in the horizontal near‐field (−16 to 19 mm), revealing a deformation pattern matching the coseismic observations. This suggests that multiple deformation mechanisms are contributing to Ridgecrest's postseismic displacement, where afterslip likely dominates the near‐field while viscoelastic relaxation controls the far‐field. Similar deformation at individual GNSS stations has been observed for past earthquakes and additionally reveals long‐term transient viscosity over several years. Moreover, the greater temporal and spatial resolution of the GNSS array for Ridgecrest will help resolve the evolution of deformation for the entire network of observations as regional postseismic deformation persists for the next several years. Plain Language Summary: The 2019 Ridgecrest earthquake sequence is one the most well observed seismic events in California's history. We take advantage of the unprecedented amount of satellite observations and previous modeling efforts to better understand the vertical post‐earthquake signal resulting from Ridgecrest. We compare cumulative surface displacement since the earthquakes, derived from Global Navigation Satellite System (GNSS) point observations to a viscoelastic model, which allows us to quantify postseismic deformation due to viscoelastic (i.e., viscoelastic relaxation) and nonviscoelastic (i.e., afterslip and poroelastic rebound) contributions. We find that viscoelastic relaxation accounted for a majority of the currently observed vertical deformation since Ridgecrest and strongly controlled far‐field observations in all north‐east‐up components, emphasizing the importance of utilizing both horizontal and vertical observations when developing earthquake cycle models. The remaining observed deformation insinuates that multiple postseismic deformation mechanisms, and thus nonviscoelastic contributions, are also needed to reproduce vertical postseismic displacement, where afterslip is the most likely mechanism here. We additionally see similarities between Ridgecrest and past large regional earthquakes and emphasize the ability of our deformation model, derived from 3D surface observations, to provide important insight on crustal parameters and the characteristics of the seismic region for past, current, and future events. Key Points: Viscoelastic relaxation accounts for the majority of vertical postseismic displacement >1 year after Ridgecrest and controls the far‐fieldVertical observations provide important constraints on time‐averaged and transient upper mantle viscosityAdditional time series analysis techniques are needed to produce coherent vertical deformation patterns compared to the horizontal [ABSTRACT FROM AUTHOR]

Measuring crustal strain and seismic moment accumulation, is crucial for understanding the growth and distribution of seismic hazards along major fault systems. Here, we develop a methodology to integrate 4.5 years (2015–2019.5) of Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR) and continuous Global Navigation Satellite System (GNSS) time series to achieve 6 to 12‐day sampling of surface displacements at ∼500 m spatial resolution over the entire San Andreas fault system. Numerous interesting deformation signals are identified with this product (video link: https://www.youtube.com/watch?v=SxNLQKmHWpY). We decompose the line‐of‐sight InSAR displacements into three dimensions by combining the deformation azimuth from a GNSS‐derived interseismic fault model. We then construct strain rate maps using a smoothing interpolator with constraints from elasticity. The resulting deformation field reveals a wide array of crustal deformation processes including, on‐ and off‐fault secular and transient tectonic deformation, creep rates on all the major faults, and vertical signals associated with hydrological processes. The strain rate maps show significant off‐fault components that were not captured by GNSS‐only models. These results are important in assessing the seismic hazard in the region. Plain Language Summary: Seismic hazard models rely on accurate measurements of small motion over large areas on the Earth's crust. Traditional geodetic models based on Global Navigation Satellite System (GNSS) data cannot resolve small scale deformation patterns, mainly due to expensive and limited station deployment. Interferometric Synthetic Aperture Radar (InSAR) has become the emerging tool for mapping surface deformation, with its advantages of low‐cost and full‐coverage. Yet InSAR measurements, compared to GNSS, come with larger biases from the atmospheric noise, especially over length scales greater than 80 km. Here, we combined the two methods to resolve fine spatial scales and achieve high accuracy. Our results are presented as deformation time‐series over the entire San Andreas fault system (video link: https://www.youtube.com/watch?v=SxNLQKmHWpY). From these deformation time series, we have estimated fault creep rates and strain accumulation. One important finding is that there is significant off‐fault strain, though we suspect this is mainly due to hydrological processes. These results will advance our knowledge of the earthquake cycle, strain/moment accumulation, and the associated seismic hazards. Key Points: A practical approach is developed to integrate Sentinel‐1 Interferometric Synthetic Aperture Radar and Global Navigation Satellite System time‐series over the entire San Andreas fault systemThe product is used to estimate fault creep and three components of horizontal crustal strain which shows notable off‐fault portionChallenges remain in separating tectonic and hydrologic sources and whether hydrologic strain will increase seismic hazards [ABSTRACT FROM AUTHOR]

Rheologic variations in the Earth's crust (like elastic plate thickness [EPT] or crustal rigidity) modulate the rate at which seismic moment accumulates for potentially hazardous faults of the San Andreas Fault System (SAFS). To quantify rates of seismic moment accumulation, Global Navigation Satellite Systems, and Interferometric Synthetic Aperture Radar data were used to constrain surface deformation rates of a four‐dimensional viscoelastic deformation model that incorporates rheological variations spanning a 900 km section of the SAFS. Lateral variations in EPT, estimated from surface heat flow and seismic depth to the lithosphere‐asthenosphere boundary, were converted to lateral variations in rigidity and then used to solve for seismic moment accumulation rates on 32 fault segments. We find a cluster of elevated seismic moment rates (11–20 × 1015 Nm year−1 km−1) along the main SAFS trace spanning the historical Mw 7.9 1857 Fort Tejon earthquake rupture length; present‐day seismic moment magnitude on these segments ranges from Mw 7.2–7.6. We also find that the average plate thickness in the Salton Trough is reduced to only 60% of the regional average, which results in a ∼60% decrease in moment accumulation rate along the Imperial fault. Likewise, a 30% increase of average plate thickness results in at least a ∼30% increase in moment rate and even larger increases are identified in regions of complex plate heterogeneity. These results emphasize the importance of considering rheological variations when estimating seismic hazard, suggesting that meaningful changes in seismic moment accumulation are revealed when considering spatial variations in crustal rheology. Plain Language Summary: The earthquake potential of faults (i.e., moment magnitude) is determined by a fault's deep slip rate, the area of the fault, the time since the last major rupture, and the average crustal rigidity surrounding the fault. Here, we explore how earthquake potential is affected by variations in crustal rheology along the San Andreas Fault System (SAFS). We use measurements of surface heat flow and seismic velocity to estimate lateral variations in crustal rigidity and tectonic plate thickness. We then estimate seismic potential rate for 32 segments of the SAFS using surface deformation measurements from Global Navigation Satellite Systems and radar interferometry as constraints. We find high seismic potential rates along several segments of the San Andreas fault that also participated in the Mw 7.9 1857 Fort Tejon earthquake and have not ruptured in over 160 years. We also find that the change in earthquake potential scales almost linearly with plate thickness variations surrounding the fault. For example, the Salton Trough region has a plate thickness that is only 60% of the regional average, which lowers the earthquake potential along the Imperial fault by roughly 60%. These findings suggest that variations in crustal rheology have an important impact on earthquake magnitude forecasts. Key Points: Variable rheology was implemented in a four‐dimensional viscoelastic model to explore earthquake cycle deformation of the San Andreas Fault SystemElastic plate thickness variations are estimated from heat flow and depth to the lithosphere‐asthenosphere boundaryVariations in earthquake magnitude forecasts should be carefully considered when implementing variable crustal parameters [ABSTRACT FROM AUTHOR]

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. Key Points: Displacement fields from 19 years of GPS data in California are preferable to velocity fields in characterizing 3‐D transient motionsMapping of secular and transient motions at high temporal and spatial resolution is required for maintaining a 3‐D kinematic reference frameDisplacement fields derived from continuous GPS and InSAR provide 200‐m resolution with millimeter‐level velocity uncertainties [ABSTRACT FROM AUTHOR]

Moment accumulation rate (also referred to as moment deficit rate) is a fundamental quantity for evaluating seismic hazard. The conventional approach for evaluating moment accumulation rate of creeping faults is to invert for the slip distribution from geodetic measurements, although even with perfect data these slip-rate inversions are non-unique. In this study, we show that the slip-rate versus depth inversion is not needed because moment accumulation rate can be estimated directly from surface geodetic data. We propose an integral approach that uses dense geodetic observations from Interferometric Synthetic Aperture Radar (InSAR) and the Global Positioning System (GPS) to constrain the moment accumulation rate. The moment accumulation rate is related to the integral of the product of the along-strike velocity and the distance from the fault. We demonstrate our methods by studying the Creeping Section of the San Andreas fault observed by GPS and radar interferometry onboard the ERS and ALOS satellites. Along-strike variation of the moment accumulation rate is derived in order to investigate the degree of partial locking of the Creeping Section. The central Creeping Segment has a moment accumulation rate of 0.25-3.1 × 1015 Nm yr-1 km-1. The upper and lower bounds of the moment accumulation rates are derived based on the statistics of the noise. Our best-fitting model indicates that the central portion of the Creeping Section is accumulating seismic moment at rates that are about 5 per cent to 23 per cent of the fully locked Carrizo segment that will eventually be released seismically. A cumulative moment budget calculation with the historical earthquake catalogue (M > 5.5) since 1857 shows that the net moment deficit at present is equivalent to a Mw 6.3 earthquake. [ABSTRACT FROM AUTHOR]