Vertical Postseismic Deformation of the 2019 Ridgecrest Earthquake Sequence.

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]