Tidal%20triggering%20of%20earthquakes:%20Response%20to%20fault%20compliance?

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Tidal triggering of earthquakes Response to
fault compliance?

• Elizabeth S. Cochran
• IGPP, Scripps

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Acknowledgements

• Tidal triggering
• Sachiko Tanaka
• John Vidale
• Compliant faults
• Yuri Fialko
• Peter Shearer
• YongGang Li
• John Vidale

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Tides and Faults

• We determine what amplitude of stress loading is
  necessary to trigger an earthquake
• Tells us how sensitive faults are to stress
  changes.
• Given an applied stress load we can estimate how
  much more likely an earthquake is assuming a
  known background rate.
• We study the physical response of faults in the
  field to applied stresses and the underlying
  mechanics
• Folds back in the question of How are faults
  different from unbroken crust?

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Why tidal triggering?

• Correlation of seismicity with Earth tides has
  long been expected
• Earthquakes are triggered by stress changes from
  nearby earthquakes, water level changes in dams,
  etc.
• Laboratory friction experiments that use
  oscillatory stress input show an increase in
  events near the time of peak stress
• Evidence of tidal triggering has so far been
  sparse, but there have been some recent hints
• Tanaka et al., 2002, 2004 correlate seismicity
  with tides in Japan and other regions preceding
  large events.
• Tolstoy et al., 2002 correlate ocean tides with
  harmonic tremor and microearthquakes along the
  Juan de Fuca ridge

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Breathing of the seafloor Tidal correlations of
seismicity at Axial volcano Tolstoy et al., 2002
Microearthquakes and harmonic tremor recorded in
1994 at Axial volcano along the Juan de Fuca
ridge found to correlate with ocean levels
Peak at 2 cycles/day
Spectrum of harmonic tremor
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Global and local studies of tidal correlation
Tanaka et al., 2002, 2004

• Found hints of a correlation with Earth tides of
  seismicity on reverse and normal faults
• Looked at correlation only as a function of tidal
  phase, not amplitude
• Used shear stress or J1 (trace of stress tensor)
  component of stress only, not Coulomb stress

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Tidal Stress Calculation

• Calculations by Sachiko Tanaka
• Both solid Earth and ocean loading
• Elastic loading in PREM from model of sea surface
  topography
• Calibrated with sea surface level observed with
  satellites
• Stress can get very large near the coast due to
  ocean loading
• Solid Earth tide 0.005 MPa
• Ocean loading 0.05 MPa
• Ocean model JAO.99b

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Dataset

• Global Earthquakes CMT catalog
• Mw 5.5 or larger, since 1977
• 9,000 reverse, strike-slip, normal and oblique
  events

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Tidal Phase and Amplitude

• For each event we calculate
• Tidal phase (q) and stress amplitude at the
  earthquake origin time
• Coulomb stress calculated for m 0 (shear only),
  0.2, 0.4, 0.6, 0.8, 8 (normal only)
• tc ss msn
• Failure is encouraged when normal stress is
  decreased on the fault and shear stress is
  increased in the direction of slip Need to know
  the fault plane!

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Tidal Stress Oscillationswhen are tidal
stresses large?
Peak After
Average t
Earthquake q 45o
p
Peak Before
0
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Range of Peak Tidal Stress Amplitudes (tp)
Earthquakes should be more easily triggered when
the peak tidal stresses are high, so we order the
events from highest to lowest average peak tidal
stress (tp)
tp(1)
High stress to Low Stress
tp(2)
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Fault Plane Ambiguity
We can only identify the true fault plane on
which to calculate the Coulomb stress for thrust
fault earthquakes where the shallow-dipping plane
agrees with local strike and dip of subduction
We analyze 2027 shallow, thrust earthquakes
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Mechanisms of 19 shallow thrust events at the
time of largest peak stress (tp)
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Two Statistical Tests

• Schusters Test
• Requires independent datasets and tests for
  non-random distribution of the data across tidal
  phase
• Binomial Test
• Simple test assuming random distribution of
  events across tidal phase.

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Schusters Test

• Determine whether the distribution of observed
  tidal phases is non-random, however the majority
  of events do not have to be near 0o phase.
• We calculate the vector sum over the phase
  angles
• P-value gives a measure of random (near 1) or
  non-random (near 0) distribution

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Binomial Test
We distribute the events into two equal phase
range bins Bin 1 Tidal phase encouraging
stress (-90 lt q gt 90) Bin 2 Tidal phase
discouraging stress Null If events occur at
random (not influenced by the tides) there should
be equal probability of an earthquake being in
Bin 1 or Bin 2. However, if there are more
events in Bin 1 (encouraging tidal stress) than
Bin 2, we can compute the probability of having a
certain number of extra/excess events
Encouraging Phase (-90o to 90o)
Discouraging Phase (-180o to -90o 90o to 180o)
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5 level
Lowest probability (0.0027)
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Best Correlation (255 events, m0.4)Binomial
Probability0.0027 (99.997 would not see by
chance) P-value0.0076 (99.992 not a random
distribution)Sinusoidal fit to the data gives a
peak 0o phase. So, not only are there more
events during the encouraging stress phase, but
the number peaks near 0o phase!
255 Earthquakes with largest tp
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Revisiting the 255 most correlated earthquakes
161 out of 255 events are in Bin 1 (-90o lt q gt
90o) Nex 33.5 Nex() 13
161 / 255 63
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Excess Events (all earthquakes)
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Group events by tp

• We subdivide the events into four bins based on
  tp
• Bin A tp gt 0.02 MPa 19 eqs
• Bin B 0.01 lt tp gt 0.02 MPa 41 eqs
• Bin C 0.004 lt tp gt 0.01 MPa 155 eqs
• Bin D tp lt 0.004 1813 eqs
• The statistical significance on these bins is not
  as good as that of the entire dataset, but we
  attempt to see if a larger number of events are
  triggered when tidal stresses are highest

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Strong Tides Trigger More Earthquakes
Global Thrust California Strike-Slip
27,464 California Strike-Slip Events (Parkfield
and Calaveras)
Significant Triggering
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How does this compare to other studies of
non-tidally triggered earthquakes?

• Hardebeck et al., 1998 Stein, 1999 and others
• Aftershocks are triggered when coseismic stress
  changes are a few tenths of a bar (0.01 MPa)
• Aftershocks triggered by Landers rupture Stein,
  1999

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Do our results fit experimental rock physics
observations?
Several laboratory experiments have been
conducted to estimate the triggering of events
given a imposed stress load. We examine the
predictions of (1) Rate- and State- Dependent
Friction (2) Stress Corrosion
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Rate and State Friction
The rate of loading and the state of the fault
effect when a fault will rupture. Rate and state
friction predicts an increase in seismicity rate
given an increase in the stress load.
R/ro Actual / Background Rate Dt Load
Increase sn Normal Stress (10 MPa)
A Fault Viscosity Lab 0.003-0.006 Our
0.003
(Kanamori Brodsky, 2004 Beeler and Lockner,
2003 Dieterich, 1994)
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Stress Corrosion
Prior to rupture, acceleration of strain is due
to subcritical crack growth in a material. Stress
corrosion predicts weakening with increased load.
R/ro Actual / Background Rate Dt Load
Increase s Stress Drop (2 10 MPa) n
Material Property Lab 5 20 Our 16
(Kanamori Brodsky, 2004 Main, 1999)
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Least squares fit of data to rate- and state-
friction (A 0.003) and stress corrosion (n16)
Global Thrust California Strike-Slip
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Tidal Data vs. Experimental Results

• The data fit to both rate- and state- friction
  and stress corrosion suggest that faults are more
  responsive to small stress changes than predicted
  by laboratory theory.

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Detailing Fault Response

• Tidal triggering levels suggest faults are highly
  responsive to relatively weak stress loads.
• Faults are loci of strain?
• We need to study the
• structure of faults and
• their response to stress
• loading in greater detail.

After Fialko et al., 2002
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Fault ? Plane
Fault zone trapped wave studies show that faults
are not simple shear planes, but are zones of
damaged rock with reduced strength.
From Li et al., 2003
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• Faults are zones of damaged rock resulting from
  repeated rupture
• Influences fault rupture properties
• Localizes strain
• Changes the distribution and propagation of
  fluid flow in the crust

After Chester et al., 2003
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Further Questions

• Are all earthquakes types as responsive to tidal
  stresses?
• Regional studies suggest normal faults are also
  highly responsive, but no global studies have
  been done.
• Strike-slip earthquakes studied in California
  showed low levels of triggering by tides.
• Are earthquake depth and magnitude important?
• Are tidal stresses similar across the entire
  rupture plane of a large magnitude earthquake?
• Do details of the coastline affect how responsive
  a fault is to the large ocean tides?
• What fault properties result in the highly
  responsive nature of faults?