Is there a discrepancy between geological and geodetic slip rates along the San Andreas Fault System?

NO. Several previous inversions for slip rate along the San Andreas Fault System (SAFS), based on elastic half-space models, show a significant discrepancy between the geological and geodetic slip rates along a few major fault segments. In this study we use a more realistic model representing an elastic plate (schizosphere) over a viscoelastic half-space (plastosphere) to demonstrate that there is no significant discrepancy between long-term geologic and geodetic slip rates. The model includes ~ 50 major fault segments having steady slip from the base of the locked zone to the base of the elastic plate and episodic shallow slip based on known ruptures and geologic recurrence intervals. The slip rates are constrained by 1989 present-day velocity measurements from EarthScope GPS and high-resolution interseismic velocity data from L-band InSAR onboard ALOS. Five models with different rheological properties, including an elastic half-space, are tested in a slip-rate inversion. A model with a thick elastic plate (60 km) and half-space viscosity of 10¹⁹ Pa s is preferred because it produces the smallest misfit to both the geodetic data and the geological slip rates. We find that the geodetic slip rates from the 60 km thick plate model agree to within the bounds of the geological slip rates, while the rates from the half-space model disagree on certain important fault segments such as the Mojave and the North Coast segment of the San Andreas fault. In particular, along the Mojave segment the recovered geodetic slip rate is 24.7 mm/yr for the half-space model but our result comes closer to the preferred geological rates of 34 mm/yr using a 60 km thick plate model (27.5 mm/yr) and a 30 km thin plate model (34.4 mm/yr). The plate models have generally higher slip rates than the half-space model because most of the faults along the SAFS are late in the earthquake cycle so today they are moving slower than the long-term cycle-averaged velocity as governed by the viscoelastic relaxation process.