Tectonic strain accumulation and release in California from 20+ years of continuous GNSS displacement observations with respect to a dynamic datum and consideration of transient deformation

GNSS networks at plate boundaries have proliferated in California beginning in the mid 1980's. The first-order goal was to quantify the magnitude and distribution of the crustal deformation through steady-state station velocity maps estimated from space geodetic observations to model the accumulation and release of elastic strain and its relationship to seismic hazards. An outstanding question was whether the geodetic, paleoseismic and geologic velocities agreed, implying that they do not change over multiple crustal deformation cycles. Recent studies of plate boundaries at different phases of their earthquake cycles indicate possible changes in interseismic velocities. The presence of time-varying tectonic motions such as postseismic and other transients, and non-tectonic effects such as natural and anthropogenic subsidence complicate these studies and assumptions and calls into question the current geodetic methodology to characterize fault motions and tectonic strain accumulation, release as steady-state processes. Over the GNSS era, California has experienced seventeen detectable earthquakes, the largest of them triggering significant and long-lasting postseismic deformation. We also observe significant areas of non-tectonic deformation. All of these processes evidence the unsteady, time-dependent properties of California's fault zones. To address these complications, we present a methodology that is based on the dynamic analysis of displacements for 950 stations in California, in the period 1999-2017, that straddle its complex system of faults. We examine time-dependent motions after correcting for the steady-state horizontal interseismic fault model of Zeng et al. (2017) for the Western United States through derivation of weekly residual strain rate maps. We identify the temporal and spatial evolution of the crust due to postseismic, magmatic and non-tectonic motions. A complementary analysis addresses vertical motions without the adoption of any underlying vertical model. In a related abstract, we discuss the use of our methodology to define and maintain a dynamic geodetic datum for precise surveying and kinematic spatial referencing in California.