Seismic Constraints on Slow Slip Events within the Cascadia Subduction Zone

Ana Cristina Aguiar
October 2007


Reanalysis of geodetic GPS time series from the Cascadia subduction zone have revealed at least 30 resolvable slow slip events along the megathrust, ranging from northern California to southern British Columbia, since 1997. Many of the smaller and more recent events are barely resolvable with GPS, but stand out clearly as tremor sequences. Since tremor bursts lasting less than 10-seconds are often visible across multiple stations, they offer the highest resolution for studying moment release through time. To test the hypothesis that tremor and transient deformation are two manifestations of the same faulting process, and to quantify the relative contribution of moment release during times of strain-transients versus other times, tremor bursts are systematically analyzed during the time period of June 2005 to February 2007. First, daily seismic files are consolidated from the Puget Basin of Washington State and SW British Columbia, where GPS density is highest. Seismic traces are included from the PNSN, the PBO borehole seismic network, and the EarthScope-funded CAFE experiment. Instrument gain is removed, and then the data is decimated to 10 sps, rectified, its envelope is computed using a Hilbert transform, and lastly the envelopes are averaged from regionally adjacent stations to provide a single metric indicative of tremor activity. Then tremor duration is compared to equivalent moment slip inversions of corresponding GPS- derived deformation to obtain a model that relates hours of tremor to moment magnitude, showing that moment is directly proportional to the hours of tremor. Finally, to locate tremor during the January 2007 event, cross-correlated envelopes of band-pass filtered instruments are used. The location is determined by minimizing the L2-norm of the vector containing the differences between the measured and predicted stations offsets for a 3D grid of possible locations. Although the scatter is high, particularly in the depth, it is found here that tremor during the 2007 event propagates in a northwesterly direction beneath the eastern Olympics Range over a 3-week period.

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