Present and Past Crustal Deformation in the Western United States: Why Are They Different?

The present-day crustal deformation in the western United States delineated by recent Global Positioning System (GPS) measurements differs significantly from that reconstructed from the relative motions of crustal blocks in the past few million years. We show that the difference can be largely reconciled by the stick-slip behavior of the San Andreas Fault (SAF). The geodynamic model constrained by the GPS data indicates strong traction (~25 MPa) on the SAF, attributable to interseismic locking on the fault. The same model with lower traction on the SAF (~17-13 MPa), expected when the traction is averaged over seismic cycles, reproduces much of the long-term crustal deformation.