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Abstract:
During the lifetime of a convergent plate margin stress transfer across
the plate interface (a megathrust) can be expected to vary at multiple
timescales. At short time scales (years to decades), a subduction
megathrust interface appears coupled (accumulating shear stress) at
shallow depth (seismogenic zone <350°C) in a laterally
heterogeneous fashion. Highly coupled areas are prerequisite to areas of
large slip (asperities) during future earthquakes but the correlation is
rarely unequivocal suggesting that the coupling pattern is transient
during the interseismic period. As temperature, structure and material
properties are unlike to change at short time scales as well as at short
distance along strike, fluid pressure change is invoked as the prime
agent of lateral and time-variable stress transfer at short time
(seismic cycle) scale and beyond. On longer time scales (up to Wilson
cycles), additional agents of time-variable stress change are discussed.
Shear tests using velocity weakening rock analogue material suggest
that in a conditionally stable regime the effective normal load controls
both the geodetic and the seismic coupling (fraction of convergence
velocity accommodated by interseismic backslip/seismic slip).
Accordingly seismic coupling decreases from 80% to 20% as the pore fluid
pressure increases from hydrostatic to near-lithostatic. Moreover, the
experiments demonstrate that at sub-seismic cycle scale the geodetic
coupling (locking) is not only proportional to effective normal load but
also to relative shear stress. For areas of near complete stress drop
locking might systematically decrease over the interseismic period from
>80-95 % shortly after an earthquake to backslip at significant
fractions of plate convergence rate (<5-45 % locking) later in the
seismic cycle. If we allow pore fluid pressures to change at sub-seismic
cycle scale a single location along a megathrust may thus appear fully
locked after an earthquake while fully unlocked before an earthquake.
The mechanisms and timescales of fluid pressure changes along a
megathrust are yet to be explored but a valid hypothesis seems to be
that non-volcanic tremor and slow slip below the seismogenic zone
represent short term episodes of metamorphic fluid infiltration into the
shallow megathrust. A megathrust fault valve mechanism clocked by the
greatest earthquakes then accounts for cyclic fluid pressure build up
and drainage at sub-seismic cycle scale. As pore pressure dynamics are
controlled primarily by permeability which in turn is controlled by
structure and material properties, then more long term coupling
transients associated with structural evolution of the plate margin can
be implied. Fluid controlled transients might interfere with transients
and secular trends resulting from changes in material strength and plate
tectonic forces over the Wilson cycle resulting in a multispectral
stress-transfer pattern associated with convergent margin evolution.
Because of the viscous damping effect of the underlying asthenosphere,however, only longterm transients (periods >1-10 ka) are transmitted
into the engaged plates. We therefore speculate that the multispectral
nature of stress transfer across a megathrust filtered through the
asthenosphere explains transient fault activity in some intraplate settings.
