- Liu and Rice (2005), Aseismic slip transients emerge spontaneously in three-dimensional rate and state modeling of subduction earthquake sequences, JGR

- Liu and Rice (2007), Spontaneous and triggered aseismic deformation transients in a subduction fault model, JGR

Potential role of fluids in modulating stable and unstable slip

Nadaya CubasGE 277/169b

Common features:

-slip velocity 10 to 100* Vplate (10-9 to 10-7 m/s)

-transients migrate along trench at speed from km/day to km/year

-limited to a depth range near and below end of the seismogenic zone.

-quasiperiodicity of aseismic transients observed in some shallow subduction zones from few months up to several (10) years

-Associated with non-volcanic tremors

-Evidence for high Pore Fluid Pressure

Slow slip events

Liu and Rice 2005:Slow slip events can arise spontaneously

•rate and state formulation, with T (depth) dependence (a-b) parameter

•for certain (low) effective normal stress with the simplest representation of metamorphic fluid release (typically 50 MPa or 100 MPa) in the seismogenic zone and downdip region.

Slow slip events

Model

Dietrich-Ruina

3D thrust fault with rate- and state- dependent friction law

Transition weakening/strengthening

(2) P lower than lithostatic – constant with time, n eff from 50 to 150MPa (100MPa)

(3) Lf = 120mm

Parameters:

(1) Temperature (depth) dependence of material parameter (a-b), for granite gouge under hydrothermal conditions

Non uniform slip mode, Complex slip, with strongly locked zones

Results: in the seismogenic zonewith small perturbations of (a-b) parameters

Vplate downdiplocked

Transient,Vtransient = 3km/year

10*Vplate

shallow aseismic slip during interseismic

Results: near and down dip the transition zoneAseismic transients

Common characteristics compared to GPS observations:

1.Transients have typical aseismic slip rates averaging around 10 to 102 times Vpl.

2.Transients migrate along strike at a speed of several to tens of km/yr in simulations.

1.Transients are limited to depths around the downdip end of seismogenic zone defined in the model. They generally cannot rupture into the updip portion because the fault there is very firmly locked. Nor can slip be enhanced in the velocity strengthening region that is further downdip.

CCL: Can arise spontaneously near neutral stability

Transition of frictional properties from velocity weakening to velocity strengthening, combined with great earthquake sequences that leave zones of heterogenous slip and

stress alteration along strike, seem to be the cause of the aseismic slip event occurrence. They may reflect episodic relaxation, during the interseismic period, of stress concentrated

around the transition zone.

Common characteristics compared to GPS observations:

CCL: Can arise spontaneously near neutral stability

Transition of frictional properties from velocity weakening to velocity strengthening, combined with great earthquake sequences that leave zones of heterogenous slip and

stress alteration along strike, seem to be the cause of the aseismic slip event occurrence. They may reflect episodic relaxation, during the interseismic period, of stress concentrated

around the transition zone.

However: Long transient: tens of years rather than the several weeks span observed in some natural events

- Large characteristic slip distance L, with smaller L : speed increase

- The speed with which transient slipping zones propagate along the strike increases with the reduction of ; e.g., that speed increases by a factor of 3 when the assumed is reduced by a factor of 2, from 200 MPa to 100.

Liu and Rice 2007:

•rate and state formulation, with T (depth) dependence (a-b) parameter

•2D to test smaller L

•Lithostatic pore pressure

Reproduce oscillatory behavior, ranging from

3 months beneath the Bungo Channel, southwest Japan11-19 months along the Cascadia 2–3 years near Gisborne, along the Hikurangi subduction zone in New Zealand4 years in Guerrero, Mexico6 years beneath the Bungo Channel 10 years or more, Alaska, Sunda Megathrust between the Mentawai

Slow slip events

Evidences for HPP:1.petrological analysis of phase equilibrium:

dehydration conditions would be met for shallowdipping subduction zones around 350C and above, whichmeans near and downdip of the frictional stability transition

Fluids released from such dehydration reactions can significantly increase the porepressure

Here, add one slide about HPP shallow NE japan – thermal pressurization presented before.

Model

2D thrust fault with rate- and state- dependent friction law

a) Realistic model:- effective normal stress is high (finite) in the seismogenic zone,- much lower from stability transition and further downdip.

b) Simplified model:-RW patch completely locked (eff infinitely high) -W extending up dip of the stability transition-Whole down-dip region at a much lower, uniform eff, due to dehydration.

(2) P near lithostatic

(3) Lf = 20m

(1) Temperature (depth) dependence of material parameter (a-b), for granite gouge under hydrothermal conditions

Parameters:

h*critical cell size [Rice, 1993; Lapusta et al., 2000]Stable slip patch size for steady sliding with rate-weakening friction

Results: with locked seismogenic zone

Oscillation depends on W/h*

W: (a-b) < 0 but very high pore pressure: eff very low

Shear modulus

Poisson’s ratio

With L = m, eff = 2-3 Mpa:14 months period

However, oscillatory slip rate amplitude extremely low case b and c: lower than Vplate

Results: with locked seismogenic zone

Oscillation depends on W/h*

Rate weakening zone as being at a finitely higher eff so that slip is allowed to penetrate naturally into it by a small amount show that the amplitude of the oscillatory slips increases substantially.

Average period: 2 yearsSlip rate: 2 to 4 * VplateAccumulated slip: 1 to 2 cm

Results: with unlocked seismogenic zone

Higher amplitude

Can arise spontaneously near neutral stability

Or

transient sequences can be triggered

•from the stress field alteration due to nearby earthquakes, •or the pore pressure variations due to episodes of metamorphic fluid release along the subducting slab, •or along-strike variations in seismic slips.

aseismic transients might act as a spatial-temporal connection between distant seismicity clusters in subduction zones

Transient aseismic slip

NFTF

NF

TF

+ Repetition in 2006

Transient sequences can be triggered by slab seismicity

Transients response will dependent on:

Perturbation Introduction Time (Ttp /Tr0 ) : Accelerate or delay next thrust EQ

Relative Location Along the Thrust Fault:Slightly varies interseismic time

Magnitude of the Stress Perturbation:Pore pressure rise or drop can trigger transient

>> And have direct consequences on te timing of future thrust EQ

CclOur studies can produce transients in abundance, of a wide range of recurrence intervals, including transients of relatively short periods (1 year) in cases for which there is a zone of near-lithostatic fluid pressurization around and downdip of the frictional stability transition, a situation independently supported by tremor triggering and petrology.

However:However, with the simple model of a completely locked seismogenic zone updip of width W, the amount of deformation at the earth’s surface associated with the spontaneous short-term transients is at least an order of magnitude smaller than observed in Cascadia. Further, even with a relatively more realistic model of an unlocked seismogenic zone as in section 3.2, most of the aseismic moment in our modeling is contributed over a zone extending updip and moderately downdip from the stability transition, whereas a much broader downdip zone is thought to be activated during the transient events in the northern Cascadia and Guerrerosubduction zones [Dragert et al., 2001; Hirn and Laigle, 2004; Larson et al., 2004

Have we made the downdip rate-strengthening zone too stable?

Segall et al. (2010), Dilatant strengthening as a mechanism for slow slip event, JGR