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Subduction Zone Seismicity (Fluid pressure & seismic behaviour (Nazca…
Subduction Zone Seismicity
Mega-earthquakes
Largest earthquakes on Earth = associated with destructive plate boundaries i.e. subduction zones
Varied
tectonic regimes
Seismogenic events vary throughout destructive p.b.s
Bending on overriding plate
extension & normal faulting
Backstop shortening
collision compresses overriding plate below forearc basin
Volcanic arc
magmatic seismicity
Plate interface
tensile & thrust faulting
Rate & state dependent friction laws
= used to model subduction zone processes
Friction coefficient evolves during shearing
--> μ is not a static quantity
i.e. μ changes on sliding surface
as two blocks move past earthother
(a - b)
Parameter
Dynamically evolves during slip events
Seismic behaviour of fault dictated by relative magnitudes of a & b
Stable
sliding
when (a - b) is zero or positive
**Shallow depths
( < 3 km)**
around accretionary wedge
no consolidation of sediments yet
∴ can expect fault to slide stably
Great depths
(> 19 km)
debated heavily
arguable temp activated creep
Stick-slip
domain
(a - b) is NEGATIVE
i.e. Dynamic ∆ μ > acceleration
When 3km < Z < 19km
Strike slip events characterise fault mvmts
Slip = seismic slip events
Stick = strain accumulation / locking in btwn slip events
n.b. depths are for strike slip fault
Expected Z earthquakes
Expect equakes 3 < Z < 19 km
Values are for strike slip
n.b. slightly differs for subduction zones
Subduction zone - thermal field distorted hence Z differs
Seismic Behaviour Subduction
Influenced by great range of factors
Fluid pressure & seismic behaviour
Nazca Plate study
Fracture zones facilitate water inflitration
Hydrothermal alteration dehydrates oceanic crust
Fluids expelled with increasing temperature
--> migrate along fractures into plate interface
Fluids trapped as permeability upper plate = low
Less fractured oceanic plate to North
∴ lower pore fluid pressure
Evidence less water content to N
Anticorrelation Poisson's ratio & locking degree
Higher locking degree
Lower Poisson's ratio
Behaviour differs
Poisson's
Ratio
V = - (strain in direction of load) / strain ⊥ load
Ratio of relative:
contraction strain ⊥ applied load
extension strain in direction of applied load
Metamorphic conditions ∆ during subduction
Temp & pressure increase as plate subducts
∴ Rock progresses through metamorphic facies
&
deformation behaviour changes
Shortsighted: just modelling w rate & state dependent friction
x Empirical - based on idealised law
x Mechanisms not accounted for
i.e. which tools rock uses to accommodate stress (on grain scale)