interseismic deformation with aseismic stress-dependent fault slip

transcript

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

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)

Pritchard & Simons, 2006

fault rheology is not (explicitly) included in after-slip model

1995 Antofagasta eq. (M8.1)

fault rheology is not included in after-slip model

2003 Tokachi-oki eq. (M8)

Baba et al., 2003

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

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

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

with fault loading:

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…

“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

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

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:

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

Ben-Zion, 2003

Lapusta et al., 2000

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…

= 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

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

Dlockedsection

steady slip

at depth

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

back-slip model

Dlockedsection

steady slip

at depth

does not include strains due to plate

bending, if incorporated, discrepancy

removed, total interseismic + coseismic =

subduction block motion…

a more realistic geometry

Ravi Kanda

elastic slab model

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

slip on the fault:

below the locked region

b>0 thrust slip

surface interseismic displacements:

xxxxxx

surface interseismic displacements:

xxxxxx

surface interseismic displacements motivation:

Baba et al., 2003

data from Sue Owen

slight curvaturetectonic?

surface interseismic displacements motivation:

Baba et al., 2003

data from Sue Owen

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

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

variation of coupling through an interseismic period

xxxxxx

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…

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, & …)

19732003

slip models from Yamanaka and Kikuchi (2002) vertically exaggerated

interseismic deformation with aseismic stress-dependent fault slip

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