SHIFT HAPPENS: QUANTIFYING THE TIME-DEPENDENCE OF PLATE BOUNDARY DEFORMATION USING CONTINUOUS GPS

Recently installed networks of continuously operating GPS stations are revealing an exquisitely rich time-dependence of crustal motion within broad continental plate boundary zones. The precision achievable with continuous GPS (CGPS) is now better than 1 mm daily positioning, and 0.1 mm/yr velocity. CGPS is presently recording and monitoring a variety of transient and episodic strain phenomena with durations/periods of days to decades, in addition to secular strain accumulation and rigid microplate motion. Post-earthquake strain accelerations, slow strain events not associated with large magnitude earthquakes, and other spatially coherent time-variable signals are the targets of many focused investigations. When compared with crustal strain rates estimated using the methods of structural geology, tectonic geomorphology, and paleoseismology, GPS measurements can also contribute appreciably to inferences on the temporal-behavior of tectonic processes acting over significantly longer periods. The spectrum of time-dependent plate boundary deformations will continue to become more completely understood as the continuous GPS technique proliferates and matures. Time-variability in deformation rates may prove to be the norm rather than the exception for many tectonic settings. Results from existing CGPS networks are already raising fundamental questions regarding how geodetic deformation, which can sense present-day lower crustal and upper mantle strain accumulation, relates to the long-time scale permanent strain recorded by surface geology, with important implications for lithospheric dynamics, and earthquake recurrence. In this presentation, we will review examples of geodetic velocity determinations and associated tectonic interpretations that challenge our present understanding of plate boundary zone processes.