2.2 Data Used to Study the First Three Months of Post Seismic
Deformation at Continuous Sites
We used the continuous GPS sites along the Alaska subduction zone to
estimate the displacement due to the first three months of post-seismic
deformation. We fit a parametric model to each site time series to
isolate the postseismic displacements from the interseismic, coseismic
and seasonal deformation. We fit the model to the time series starting
from January 2018 after the M7.8 earthquake in the Gulf of Alaska
(Ruppert et al., 2018) and ending at 3.3 months after the Chignik
earthquake. The surface displacements of the sites we study due to the
November 30, 2018 Mw7.1 Anchorage earthquake are very small, and were
not corrected or considered in our analysis. The model includes terms
for the linear interseismic velocity, annual and semi-annual seasonal
displacements, and time dependent terms for the Simeonof, Sand Point,
and Chignik earthquakes. For the Simeonof earthquake, we estimated the
coseismic offset plus a logarithmic relaxation with a relaxation time of
0.025 years to account for the postseismic deformation. For the Sand
Point earthquake, we estimated the coseismic displacement only, as there
is no evidence for a measurable postseismic transient (including such a
term does not change other model parameter values or improve the fit).
For the Chignik earthquake, we estimated the coseismic displacements
plus a logarithmic relaxation with a relaxation time of 0.005 years to
account for the postseismic deformation.
We compute the postseismic displacements for Chignik by evaluating only
the term for the post-Chignik relaxation at two different epochs, three
weeks and three months after the earthquake. The vertical data were not
used in our primary models because it has a relatively small signal,
larger noise, and it is more difficult to model time-varying
displacements caused by glacial isostatic adjustment and other signals.
However, we did explore how postseismic models fit the vertical data.