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Earthquake Cycle (3. Post- seismic (2. Fluid redistribution (fluids…
Earthquake Cycle
3. Post-
seismic
post-seismicity = relaxation/re-equilibration phase
1. Aftershocks
2. Fluid redistribution
fluids highly compressed during rupture/coseismicity
∴ shuffled around
3. Fault rocks produced at depth
Cataclasis/fracture of rocks @ Z
strain rate overcomes ability of temp-activated deformation to dissipate energy
4. Deformation acceleration
(due to fracture & grain size reduction)
Rupture Nucleation
= where fault
fails
= preseismic phase
slip accelerates with instability
eventually results in coseismic motions
2. Coseismic Deformation
1. Slip patch rupture
accomodates motions
Very shortlived
(few seconds)
2. Induces stress transfer
in system
other parts of system = forced to react
3. Slip patch propagates
up to surface (if surface ruptured)
downward to ~20 km z
Why rocks at depth fracture
Rapid stress input from coseismic motions
Problem:
ability of rock to dissipate strain energy via temp activated mechanisms = limited
∴ Fracture/cataclasis occurs coseismically
rapid reduction grain size
rock fragmented in place
Significance:
small grain size conducive to deformation
Fault
rocks
Produced at great depths
Temperatures = v. high
Normally rock = constantly deforming
in response to imposed stresses (plate motions)
During seismicity
more stress added
∴ Deformation accelerates significantly
Rock responds to rapid stress increase
via fracture
Grain size reduction
Small grain sizes = conducive to effective deformation
Increased surface area facilitates def processes
pressure solution-reprecipitation
viscous grain boundary sliding
∴ Post seismic deformation acceleration occurs @ depth
4. Interseismic period
@ Surface
Unconsolidated sediments slip aseismically
(Little / no resistance to slip)
@ Locked portion
Healed fault rock ∴ regains strength
hence locked state
@ Depth
Steady deep slip
Temp-activated strain accumulation @ depth
--> Hot temps at depth
Afterslip
nr surface
Granular flow @ surface
Unconsolidated fault rock = insufficient overburden
Slips aseismically ∝ log(time since equake elapsed)
Tohoku coseismic motions
~20 m/s coseismic displacement
megathrust equakes with 3-5 m displacement
Deformation Regime
Frictional
domain
Near cold surface
Plate boundary = locked
Seismic events accomodate relative plate motion
Frictional sliding releases accumulated elastic strain in seconds
Viscous
domain
At depth temperatures = high
Slow steady viscous creep of rocks
accomodates relative plate motion