Episodic Tremor and Slip of Cascadia

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Episodic Tremor and Slip of Cascadia

• Tami Ben-Zvi and Josh Carmichael
• Department of Earth and Space Sciences
• University of Washington, Seattle WA

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ETS Basic Observations

• Slow-slip events GPS measured, cyclic
  deformation transients ? subduction zone
  interfacial slip
• Tremor and slip activity approximately periodic
  Repeat over 1316 month intervals
• Seismic chatter and slip correlate temporally
• Intermittent periods almost seismically quiet

Rogers Dragert, 2003
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Observables from Waveforms

• Waveforms emergent no clear P or S wave
  arrivals, non-impulsive
• Bulk of energy content in the low frequency
  range 1-5 Hz
• Hypocenter locations not coincident with plate
  interfacewide source depth distribution
• Signals travel at shear wave speed

Rogers Dragert, 2003
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Spatial Temporal Correlation Between Slip and
Tremor Events

• Spatial correlation source located near plate
  interface.
• Temporal correlation tremors detected near slip
  events, fewer or no tremors detected between slip
  events.

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Dynamics Suggestions

• Fluid migration accelerated by slip ?
• Pore-pressure increase followed by volumetric
  strain energy release in small earthquakes?
• Localized expansion from serpentinization
  inducing microfractures?
• J.R.Rice No fluids needed! Depth and rate
  dependent friction, with pore pressure, predicts
  spontaneous aseismic slip on subducting interface

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The Depth and Rate Dependent Slip Model
Pictorial summary
Taken from Lui and Rice, 2005 (Journal Geophys.
Research Volume 10)
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The Basic Physics
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Simulation Results
Type I transients
Type II transients
Reduce locking nucleation
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Characteristics

• Occurrence is on the order of 100 years of last
  seismic event
• TI Coalescence of two transient slip fronts
  after 30 years.
• TII Stress transfer creates multiple
  transients displays nucleation, and relocking
  slip velocity O(10Vplate)
• Reduced locking may create large-slip nucleation
  sites

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Comparisons

• Slip velocities 10-7-10-9 m/sec calculated in
  model comparable from Cascadia (Dragert 2001)
• Depth range transient activity occurs toward
  down-dip end of seismogenic zone
• Transients migrate along strike at O(10km/sec)
  year. Smaller Lf or larger pore pressure ?
  faster transient migration speeds.

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Fluid related source modelFluid-flow-induced
oscillations

• Model Assumptions
• Single phased, constant density
• fluid
• 1D/2D motion
• Elastic conduit w/width change in time
• Conservation of mass and momentum of fluid gt 3rd
  order of non-linear diff. equations

Julian, 1994
Cyclic Behavior Bernoullis theorem p(rv2)/2
const Big vgtsmall pgtwalls move closergtp builds
up
Chaotic behavior period doubling cascade -
almost noisy looking time series.
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Fluid related source modelExcitation of
fluid-filled cracks
Model by Chouet (1986, 1988) Crack wall
displacement caused by sustained fluid pressure
disturbance. Oscillation modes 2L/n, 2W/n
n2,3,4, Unsteady shock wave near crack
caninduce pressure disturbance and oscillations.
Konstantinou, 2002
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Summary Possible Mechanisms