Transcript Stress Transfer Eart..

Stress Transfer Earthquakes
Andrei Popescu
What is Stress Transfer?
• Post-seismic (and co-seismic) slip induced
changes in the stress field surrounding an
earthquake
• Can trigger other events (such as
aftershocks)
• Influences both timing and slip distribution
of subsequent events
Static Stress Changes vs.
Dynamic Triggering
• Static stress changes: changes in the stress
field surrounding an earthquake associated
with permanent fault offset from main
rupture
• Dynamic triggering: changes in stress field
induced by the passage of large amplitude
seismic waves from a separate (sometimes
distant) event
Effects of Stress Transfer
• Short term: can trigger subsequent events
(aftershocks are the best example of this). This is
generally dependent on the conditions prior to the
event (how close to failure was a certain area
before the change in stress field)
• Long term: can affect timing of subsequent events,
bringing them either closer to failure, or farther
from it
• Slip distribution: extent of slip of a subsequent
event can be altered by stress changes caused by
the initial earthquake
Proposed Theories
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Coulomb Failure Stress
Viscoelasticity of upper mantle/lower crust
Pore fluid migration
Dynamic triggering
Aseismic creep
Coulomb Failure Stress
• CFS = |τ| + μ(σ + p) – S
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|τ| = magnitude of shear stress
μ = coefficient of friction (constant)
σ = normal stress
p = pore fluid pressure
S = cohesion (constant)
ΔCFS
• ΔCFS = Δ|τ| + μ(Δσ + Δp)
– If we assume a constant slip direction we get:
• ΔCFS = Δτslip + μ(Δσ + Δp)
• This equation is often simplified further:
– ΔCFS = Δτslip + μ’Δσ
– Where μ’ is a redefined “apparent” coefficient of
friction which takes into account pore pressure
– “This strategy is mostly and attempt to cover up our
lack of knowledge about the role of pore fluids”
ΔCFS and “Stress Shadows”
• After an earthquake event, the entire surrounding stress
field is subjected to changes which can be approximated
using ΔCFS as detailed above
• ΔCFS is resolved onto the fault plane and in the slip
direction of the subsequent earthquake
• ΔCFS > 0: the fault plane is loaded
• ΔCFS < 0: the fault plane is relaxed (a stress shadow)
• Stress shadows impose a time delay on subsequent events
which can be approximated by calculating the amount of
time needed for long-term tectonic loading to recover the
induced ΔCFS
North Anatolian Fault
1939 - 1992
• Ten M >= 6.7 events during this interval
• Calculations of ΔCFS reveal that 9 out of 10 of
the ruptures were brought closer to failure by the
preceding ruptures
• ΔCFS induced by these events is estimated to be
equivalent to 3-30 years of secular stressing
Results/Predictions
• 9 out of 10 of the epicenters were located in areas where
the preceding earthquakes had increased stress conditions
• “We identify several faults with an heightened
probability of failure. The port city of Izmit is most
vulnerable to an earthquake on the Sapanca fault
(Fig. 4g)…. We calculate a 30-yr probability during
1996-2026 for M>=6.7 shocks on the Geyve and
Sapanca fault segments to be 12%; this probability is
higher by a factor of 1.07 than the rate before the
these segments were stressed by the 1967
earthquake.”
1999…
References
• Harris, Ruth A. 2000. Earthquake stress triggers, stress shadows, and
seismic hazard. Current Science, Vol. 79, No. 9
• King, Geoffrey C.P., Stein, Ross S., Lin, Jian. 1994. Static stress
changes and the triggering of earthquakes. Bulletin of the
Seismological Society of America
• Lin, Jian, Freed, Andrew M. 2004. Time-dependent viscoelastic stress
transfer and earthquake triggering. Advances in Earth Sciences
Monograph, Vol. 2, pp. 21-38
• Kane, Deborah L., Kilb, Debi, Berg, Arthur S., Martynov, Vladislav
G., 2007. Quantifying the remote triggering capabilities of large
earthquakes using data from the ANZA seismic network catalog
(Southern California)
• Stein, Ross S., Barka, Aykut A., Dietrich, James H. 1997. Progressive
failure on the North Anatolian fault since 1939 by earthquake stress
triggering. Geophysical Journal International, Vol. 128, pp. 594-604