Seismic Constraints on Slow Slip Events within the Cascadia Subduction Zone
Ana Cristina Aguiar
October 2007
Abstract
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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