Imaging of aseismic fault slip transients recorded by dense geodetic networks

The large-scale continuous GPS networks that have emerged in the last decade have documented a wide-range of aseismic deformation transients that resulted from physical processes such as aseismic fault slip and magma intrusion. In particular, a new class of (M similar to 7) slow earthquakes with durations ranging from days to months have been observed with GPS arrays located above the downdip portion of subduction zone thrust interfaces. Interpretation of the displacement time-series resulting from these events is not straightforward owing to the contaminating effects of multiple contributing signals such as fault-slip, local benchmark motion, seasonal effects, and reference frame errors. We have developed a time-dependent inversion algorithm based on the extended Kalman filter which can separate the various signals and allow the space-time evolution of these slow-slip transients to be studied in detail. We applied the inversion algorithm to the 1999 Cascadia slow earthquake. This event had two primary episodes of moment-release separated by a two week period in which relatively little moment-release occurred. The Cascadia event and other slow earthquakes share numerous similarities with both ordinary earthquakes and afterslip transients suggesting that they may represent fault slip under a conditionally stable regime.