interseismic deformation with aseismic stress-dependent fault slip
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Transcript of interseismic deformation with aseismic stress-dependent fault slip
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interseismic deformation with aseismic stress-dependent fault slip
Eric A Hetland, Mark Simons, Ravi Kanda, Sue Owen
TO brown-bag – 03 April 2007
a very informal, and preliminary talk about how we are thinking about
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Hsu et al., 2006
post-seismic slip following subduction ruptures:
fault rheology is not (explicitly) included in after-slip model
2005 Nias-Simeulue eq. (M8.7)
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Pritchard & Simons, 2006
post-seismic slip following subduction ruptures:
fault rheology is not (explicitly) included in after-slip model
1995 Antofagasta eq. (M8.1)
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post-seismic slip following subduction ruptures:
fault rheology is not included in after-slip model
2003 Tokachi-oki eq. (M8)
Baba et al., 2003
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inter-seismic slip near regions of past subduction ruptures:
Suwa et al., 2006
model assumes fault slip during inter-seismic period is constant
Japan/southern Kurile trenches
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we want an internally consistent model we want an internally consistent model that can describe observations of both that can describe observations of both inter-seismic and post-seismic inter-seismic and post-seismic deformation…deformation…
for now we are building subduction zone models that include repeated ruptures, on assumed asperities, with stress-dependent aseismic slip on the non-asperity portions of the subduction interface during the interseismic period…
Baba et al., 2003 Suwa et al., 2006
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L
elastic half-space
long-termfault-slip
U’ cuts 1/2-space
with fault loading:
traction on the faultfinite fault plane in
1/2-space
slip on the fault(Burgers vector)
includes theoff-fault rheology
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with fault loading:
=0
e.g.; Rice, 1993; Liu and Rice, 2005.
Note: no seismic radiation damping (e.g., Rice, 1993) - there are no seismic waves & no problems with unbounded slip velocities in our models…
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“back-slip”introduced by J. Savage (Savage and Burford, 1973; Savage and Prescott, 1978; Savage, 1983) as a mathematically convenient fault loading mechanism in kinematic & quasi-kinematic models
+ =
Savage & Burford, 1973; Savage & Prescott, 1978
Suwa et al., 2006
red = lots of BSwhite = no BS
approximation only good approximation only good for spun-up systems:for spun-up systems:
rate of interseismic relaxation= rate of reloading
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tractionon fault
part of faultthat is allowed to
slip interseismically
part of fault withcoseismic slip
part of faultthat slipssteadily
interseismicslip on fault
imposedruptures at
times Tp
long-termfault-slip
we impose ruptures - we do not solve for them:
locked
-
’-
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tractionon fault
interseismicslip on fault
imposedruptures at
times Tp
part of faultthat is allowed to
slip interseismically
part of fault withcoseismic slip
part of faultthat slipssteadily
long-termfault-slip
we impose ruptures - we do not solve for them:
non-linear viscous(Montesi, 2004)
RS-friction(e.g. Marone et al., 1991)
linear viscous
need a fault rheology:
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Dieterich, 1979; Ruina 1983; Rice and Gu, 1983 (figure from Ben-Zion, 2003)
rate- and state-friction
(a-b)<0 “ruptures”, (a-b)>0 “aseismic slip”
is a state variable, assume it is constant = L/v
= N
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Ben-Zion, 2003
Lapusta et al., 2000
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we impose ruptures - we only solve for aseismic slip:
fault rheology:
bulk rheology:
given by for now, assume elastic half-space and use Okada, 1992
model works for 3D, non-planar faults, with multiple asperities, arbitrary rheologic
parameters, we allow both dip- and strike-slip co- and inter-seismic slip, and irregular
(imposed) rupture sequencescurrently, we can impose coseismic slip in non-locked regions of the fault, but we do not allow interseismic slip in the locked regions…
use boundary elements…
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= 30 GPa
’N = 300 MPa
D = 104 m
bo = 10 m
(a-b) = -1/10
-1 = 0.5 (a-b) = 0.05
-1 = 1.0 (a-b) = 0.10
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10D
D/2
Dlockedsection
steady slip
at depth
“thrust fault” in an elastic half-space, dipping 45 degrees
modification of ubiquitous subduction back-slip model, by allowing interseismic slip here
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10D
D/2
Dlockedsection
steady slip
at depth
“thrust fault” in an elastic half-space, dipping 45 degrees
interseismic surface
deformation is given by the locked
portions of the mega-thrust sliding as a normal fault at the plate rate (Savage, 1983)
a more realistic geometry
vert
ical
h
ori
zon
tal
back-slip model
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10D
D/2
Dlockedsection
steady slip
at depth
“thrust fault” in an elastic half-space, dipping 45 degrees
does not include strains due to plate
bending, if incorporated, discrepancy
removed, total interseismic + coseismic =
subduction block motion…
a more realistic geometry
Ravi Kanda
vert
ical
h
ori
zon
tal
elastic slab model
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“thrust fault” in an elastic half-space, dipping 45 degrees
10D
D/2
Dlockedsection
steady slip
at depth
in a spun-up model, total interseismic slip fills in the the areas above the co-seismic slip-profile
periodically impose this co-seismic slip
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slip on the fault:
below the locked region
b>0 thrust slip
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surface interseismic displacements:
xxxxxx
oo
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surface interseismic displacements:
xxxxxx
ooo
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surface interseismic displacements motivation:
2003 Tokachi-oki eq. (M8)
Baba et al., 2003
x
data from Sue Owen
slight curvaturetectonic?
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surface interseismic displacements motivation:
2003 Tokachi-oki eq. (M8)
Baba et al., 2003
x
data from Sue Owen
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determination of plate coupling:
Suwa et al., 2006
shown is back-slip rate vbs
invert GPS velocities for distributions of normal slip (vbs) on the mega-thrust
use back-slip model (Savage,
1983) to determine the “coupling coefficient”
• vbs = vT coupled (C=1)
• vbs = 0 uncoupled (C=0)
this assumes that the interseismic deformation is constant throughout the interseismic period
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10D
D/2
Dlockedsection
steady slip
at depthinvert GPS velocities for distributions of normal slip (vbs) on the mega-thrust
use back-slip model (Savage,
1983) to determine the “coupling coefficient”
• vbs = vT coupled (C=1)
• vbs = 0 uncoupled (C=0)
slip is not constant through the cycle
determination of plate coupling:
this assumes that the interseismic deformation is constant throughout the interseismic period
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variation of coupling through an interseismic period
xxxxxx
xxx
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variation of coupling through an interseismic period
xxxxxx
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variation of coupling through an interseismic period
xxxxxx
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Lapusta et al., 2000
this model only contains co-seismic slip in the locked regions, no interseismic slip-allowed in the locked regions…
contrary to dynamic calculations…
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two (of the many) remaining issues:
still learning to drive…
“lockedness” – we assume full slip in locked patches (asperities)
some directions currently aiming for:
include heterogeneous elastic structure by computing K(z;) from FE models…
include other bulk rheologies – K(z;): “simple” semi-analytic models & quite complicated FE models…
model the GPS data of inter- & post-seismic observations in Hokkaido (2D, 3D planar, respecting slab geometry, & …)
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gOcad
1968
19732003
slip models from Yamanaka and Kikuchi (2002) vertically exaggerated