金曜セミナー(9月18日)Heidi Houston氏

Friday Seminar(18th SEP)16:00-17:00


Conference Room #1 (5th Floor, Building #2)


Tidal influence on slow slip and tremor during and between major slow
slip events in Cascadia: A window into the physical environment deep in subduction zones


Heidi Houston, University of Washington, Seattle WA USA


 


Solid Earth and ocean loading tides generate small stresses that can
modulate slow slip and tremor on deep plate boundaries.
Analysis of the influence of tidal stress on 33,000 tremors located in a
part of northern Cascadia that ruptures repeatedly
in M6.5 to 6.8 slow earthquakes has yielded several new findings
(Houston, 2015). First, the influence of tides on tremor
increases during major Episodic Tremor and Slip events (ETS). Tidal
sensitivity is low for the first day or so of activity
at a spot, but increases progressively over a few days of slip at that
spot, constraining the pace of weakening during ETS.
Second, after the first day or so, the relationship between rate of
observed-to-expect tremors and the tidal stress change
is exponential. Third, an in-situ estimate of intrinsic friction on the
deep subduction interface can be made by considering
pore pressure to be proportional to mean rather than normal stress. This
analysis yields values of intrinsic friction far
below lab values (i.e., mu ~ 0.1). A strength-threshold model for
failure, in which slip and tremor occur when fault stress
exceeds the strength, can explain the evolving, increasing, exponential
sensitivity to tides.


By applying a similar analysis to the tremor that occurs between the
major ETSs, I constrain the evolution of tidal sensitivity
through the slow slip ‘seismic cycle’. About 40% of tremor in Cascadia
occurs between major ETSs. Preliminary results show
that tidal influence does evolve during the typical 12-15 month
intervals between ETSs in northern Cascadia. The level of
sensitivity to tidal stress is not as high as for tremor late during
ETS, but does increase moderately toward the end of
the interETS interval as the fault is reloaded and the next large ETS is
imminent. First-order features appear consistent
with an extension of the above-mentioned model of threshold failure
strength versus stress. The concept is that during the
inter-ETS period, strength rebuilds as the logarithm of time since the
last major ETS, while average shear or Coulomb stress
on the plate interface rebuilds roughly linearly due to plate
convergence. In this model, tidal influence is stronger when
stress is close to strength. Therefore, tidal stresses become more
effective in triggering tremor later in the cycle as the
linearly-growing stress approaches the logarithmically-growing strength.
This approach illuminates the competition between
healing on the plate interface and reloading with tectonic stress, and
can help constrain, and perhaps ultimately even monitor,
physical conditions on the deep subduction interface.