New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 32

adjacent to a plate boundary can be punctuated by abrupt changes in sign, indicating
non-seismic slip of portions of megathrust faults. These “slip reversals” were first
observed at multiple stations in the Cascadia region within the past decade and have
been termed either “slow slip” or “slow earthquakes.” In Cascadia the slow slip
events occur about every 14 months (Miller et al., 2002) and are thought to involve
intermittent shearing displacement of the down-dip region of the megathrust in a
transition zone from unstable to stable sliding, partially relaxing strain in the upper
plate. It is plausible that this is a primary mechanism that helps load and initiate large
earthquake ruptures on the shallower unstable sliding regime of the plate boundary.
Figure 2.10 Comparison of GPS observations of upper plate displacement and seismic
tremor activity levels for the Cascadia subduction zone. Cyan dots represent daily
location solutions for the East-West component of the Victoria GPS station, with the
overall eastward trend (green) representing the upper plate deformation caused by
convergence between the frictionally locked (not slipping) shallow megathrust fault
between the Juan de Fuca and North American plates. Every ~14 months the trend of the
GPS locations reverses direction for ~2 weeks away from the secular trend, which is
inferred to result from the deeper portion of the megathrust fault slipping slowly,
relaxing some strain in the upper plate even while the shallow portion of the fault is still
not slipping. Blue lines represent hours of non-volcanic tremor in each 10-day window
recorded in the same region. There is a positive correlation between the times when the
GPS displacement has a reversal and periods of strong tremor activity. SOURCE:
Reprinted from Rubinstein et al. (2010) with permission from Springer
Science+Business Media. Modified and extended from Rogers and Dragert (2003) with
permission from AAAS.
Just as slow earthquakes were first being recognized, seismologists made an
additional discovery, classifying a new kind of seismic signal: non-volcanic tremor