Geodetic monitoring of subduction zones

• Some idea of the kinematics of the subduction interface can be inferred from surface deformation measured from geodetic techniques. (Models are non unique in general)

What other data, or modeling techniques, are needed to understand better the physical parameters (lithology, temperature, stress, fluids…) controlling the mechanics and seismic behavior of the plate interface?

• Providing there is some independent control on the fault’s geometry, Geodesy can provide information on the distribution of slip rate on the fault plane. The dominant feature being the transition from a LFZ to a creeping zone.

• Geodetic monitoring allows tracking the spatio-temporal evolution.

NB: Vertical displacements are key

The Sumatra Subduction case

(Mohamed Chlieh, Danny Natawidjadja et al, work in progress)

GPS and coral records of uplift rates indicate that the plate interface along the Sumatra Subduction zone is locked over a width varying between 120 and 160km.

• Generally surface strain along subduction zone can be reasonably well adjusted assuming a transition from a fully locked to a fully unlocked zone.

(Chlieh et al, 2004)

(Chlieh et al, 2004)

Displacement at AREQ relative to ‘stable South America’, before and after the 2001 Mw 8.4 peru Earthquake.

Before the earthquake the subduction interface was locked over a with of about 100km comparable to the zone with large co-seismic displacements.

Postesimic record suggests a combination of afterslip and broader scale and longer term postseismic relaxation.

(Perfettini, Avouac and Ruegg, submitted)

• Geodesy indicates that megathrusts (subduction zones or major intracontinental thrust faults) are ‘most of the time’ locked over a width systematically too large to balance long term geodetic moment release rate (inferred from slip rates and width of LFZ) and seismic moment release rate over the long term. Seismic coupling is typically estimated to be of the order of 50% or less.

• Either significant fraction of the slip along the LFZ must occur during transient aseismic events (unless historical are globally short of giants events)

• …or there are significant variations of the width of the LFZ during the seismic cycle.

The long paleogeodetic records form Sumatra will for example help adress this question

But what physical parameters are determining whether a fault patch is Locked, creeping and may produce aseismic transient?

Experimental Rock mechanics

• Frictional sliding can be stable or unstable, depending on the lithology, water content, confining pressure and temperature.

• For quartz and granite the transition occurs around 300°C, probably in relation to thermally activated ductility.

• Some clay minerals and serpentinite undergo stable sliding at low temperature but (may) undergo unstable sliding at higher temperature.

• Olivine? There are no data on frictional properties of olivine. Far less ductile than quartzofeldspathic rocks (the transition to fully plastic flow occurs at a temperature of the order of 700°C). Presumably the transition from unstable to stable sliding occurs at a temperature much higher than for Quartz, hence above 300°C.

• Fluid contents favors ductility through the effect of temperature on crystalline plasticity and on pressure-solution deformation (dissolution-recrystallisation)

(Dieterich, Tullis, Marone, Blanpied, Lockner,Kholstedt, Byerlee, …)

Himalayan case :

- The megathrust at front of the Himalaya is locked down to the mid-crustal decollement, over a width of about 100-120km.

(Cattin and Avouac, 2000)

Aseismic slip becomes dominant for temperatures above ~ 300°C-400°C.

This is consistent with what we kniow about rheology of quartzo-feldpathic rocks from experimental rock mechanics

The Himalayan case

The transition from the LFZ to aseismic sliding corresponds to a zone of high conductivity requiring a well connected fluid phase.

Fluid released from dehydration of the footwall?

For intracontinental thrust

In an intracontinental setting, the downdip end of the LFZ corresponds to a temperature between 250°C and 400°C hence potentially to the transition from stick-slip to stable sliding of Quartzo-feldspathic rocks. Fluids released from metamorphic reactions may also play a role.

Stress transfer during the seismic cycle

F : Driving Force (assumed constant)Ffr : Co-seismic drop of frictional resistanceF: Viscous resistance

F >> Ffr

F Ffr

Two Problems :• This model predicts a nearly

100% seismic coupling…So there must be something wrong!

• Why is the subduction interface creeping steadily at depth greater than about 50km?

(Perfettini, Avouac and Ruegg, submitted)

Dee afterslip triggered by co-seismic redistribution of stress in zone with rate-strengthening friction is a viable mechanism to explain afterslip and the time evolution of aftershocks.

(Mohamed Chlieh, Danny Natawidjadja,… work in progress)

(Hyndman, Wang,…Oleskevich et al, 1999)

When the subduction interface intersects the Moho at a temperature above 300-400C, the rheology of the quartzo-feldspathic crust is probably the controlling factor of the width of the LFZ.

The current paradigm. A global analysis based on very approximative estimates of the depth of the LFZ and thermal structure along a number of major fault zone.

(Oleskevich et al, 1999)

So when the subduction interface intersects the Moho at a temperature below 300-400C, the position of the Moho is probably the controlling factor of the width of the LFZ.

In both cases the subduction interface would always be creeping when it hgets into the mantle, although temperatures are far too low for ductile flow of olivine.

(Oleskevich et al, 1999)

(Simoes et al, 2004)

Pb: A more detailed analysis show that in some (many?) cases the LFZ extends well below the forearc Moho (Sumatra, Japan, …)

(Simoes et al, 2004)

The temperature at the downdip end of the LFZ, is generally poorly constrained but most probably lower than about 400C.

Some questions we may try to adress within the TO

• What controls the transition from fully locked to a fully unlocked plate interface. Is it lithology, temperature, pressure, fluids?

• Is this transition stable with time?

• How does coseismic slip distribution during the very large earthquakes compare with the LFZ?