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Strain Energy Dissipation (Slow slip events (Potentially accommodate…
Strain Energy Dissipation
Zoo of phenomena
Great range of mechanisms by which faults get rid of elastically stored strain
Not just seismic plate motion & creep processes
Many phenomena in between
2 End-members Plate Motion
Upper crust - friction dominates plate motion
∴ no temperature sensitivity
At depth - temperature dominates
Temp activated mechanisms
i.e. High temps ∴ temp activated mechanisms = more effective
whole family of intermediate mechanisms
Range of Mechanisms
Seismic (seismograms)
Tremor - v low magnitude, long time
Low frequency equakes - 5 secs, less hectic seismic waves
V low frequency equakes
Earthquakes ~ tens secs
Geodetic (i.e. GPS)
shape/orientation/gravity
Slow slip
-
accommodate mm/cm over hours/days
Discovered geodetically frm GPS observations
Tremor
Low magnitude & long period
Sustained release of seismic energy
Slow slip events
Potentially accommodate ~mm/cm displacement
Over hours / days
Relatively fast motions (tho no as fast as equakes)
e.g. Alpine Fault, NZ - slow slip events before coseismic equake --> massive surface ruptures
At depths with temps ~500°C
Modelling slow slip events
Events lasting days / weeks
At depths with temps ~500°C
∴ cannot model with rate & state-dependent friction
Slip
modes
Stable slip
where (a - b) > 0 POSITIVE
unconsolidated sediments ∴ little resistance to sliding
Stick-slip
where (a - b) < 0 NEGATIVE
fault locked ∴ rate & state dependent friction
Slow slip
associated with high pore fluid pressure
Creep
temp activated creep
Where slow
slip occurs?
In transition zone
below locked zone
above aseismic slip (creeping) zone
Temp 300-750°C
Depths ~30km
Seismic moment & source duration
For various fault slip observations
Geodetic & seismic msmts
i.e. earthquakes, slow slip, icequakes, landslides, aftershocks
Various dissipative phenomena
(processes which dissipate strain energy)
Key point
seismic moment
& source duration
Aseismic events have moment on same order as big earthquakes
∴ dissipate just as much energy as seismic events
but over longer timescales
Slow Slip Mystery
No established mechanism
Temp activated creep = too slow / insufficient strain rate
Friction = irrelevant as temps too high (blueschist facies)
No physical basis for how slow slip events operate/work