What controls the spatial distribution of remote aftershocks?

1
Earthquake interaction

• The domino effect
• Stress transfer and the Coulomb Failure Function
• Aftershocks
• Dynamic triggering
• Volcano-seismic coupling

2
Earthquake interaction The domino effect
Example from California
Figure from www.earthquakecountry.info
3
Earthquake interaction The domino effect
Example from the North Anatolia Fault (NAF)
Figure from Stein et al., 1997
4
Earthquake interaction The Coulomb Failure
Function
Slip on faults modifies the stress field
Animation from the USGS site
5
Earthquake interaction The Coulomb Failure
Function
A function that measures the enhancement of the
failure on a given plane due to a stress
perturbation is the Coulomb Failure Function
(?CFF) where ?S is the shear stress (-
positive in the direction of slip) ?N is the
normal stress (- positive in compression) M is
the coefficient of friction Failure on the plane
in question is enhanced if ?CFF is positive, and
is delayed if it is negative.
6
Earthquake interaction The Coulomb Failure
Function
The figures above show the change in the
fault-parallel shear stress and
fault-perpendicular normal stress, due to
right-lateral slip along a dislocation embedded
in an infinite elastic medium
7
Earthquake interaction The Coulomb Failure
Function
8
Earthquake interaction The Coulomb Failure
Function
The area affected by the stress perturbation
scales with the rupture dimensions.
The change in CFF due to the eight largest
earthquakes of the 20th century.
Alaska, 1964, Mw9.2
Chile, 1969, Mw9.5
Figure from legacy.ingv.it/roma/attivita/fisicai
nterno/modelli/struttureattive
9
Earthquake interaction The Coulomb Failure
Function
Example from NAF
Animations from the USGS site
10
Earthquake interaction Stress shadows
The 1906 Great California stress shadow
Stein, 2002
So the CFF concept works not only for positive,
but also for negative stress change.
11
Earthquake interaction Multiple stress transfers
- The Landers and Hector Mine example
Maps of static stress changes suggest that the
Landers earthquake did not increase the static
stress at the site of the Hector Mine rupture,
and that Hector Mine ruptured within a stress
shadow.
Kilb, 2003
12
Earthquake interaction Multiple stress transfers
- The Landers and Hector Mine example
This map shows the change in CFF caused by the
Landers quake on optimally oriented planes at 6km
depth. The arrows point to the northern and
southern ends of the mapped surface rupture.
Figure downloaded from www.seismo.unr.edu/htdocs/W
GB/Recent.old/HectorMine
13
Earthquake interaction Multiple stress transfers
- The Landers and Hector Mine example

• Most Landers aftershocks in the rupture region
  of the Hector Mine were not directly triggered by
  the Landers quake, but are secondary aftershocks
  triggered by the M 5.4 Pisgah aftershock.
• The Hector Mine quake is, therefore, likely to
  be an aftershock of the Pisgah aftershock and its
  aftershocks.

Felzer et al., 2002
14
Earthquake interaction Aftershock triggering
Maps of ?CFF calculated following major
earthquakes show a strong tendency for
aftershocks to occur in regions of positive
?CFF. The Landers earthquake (CA)
King and Cocco (2000) Stein et al., 1992.
15
Earthquake interaction Aftershock triggering
The Homestead earthquake (CA)
King and Cocco (2000).
16
Earthquake interaction Remote aftershock
triggering
Ziv, 2006
17
Earthquake interaction Remote aftershock
triggering
The Mw7.4 Izmit (Turkey)
Mw5.8 Two weeks later
18
Earthquake interaction Remote aftershock
triggering
The decay of M7.4 Izmit aftershocks throughout
Greece is very similar to the decay of M5.8
Athens aftershocks in Athens area (just multiply
the vertical axis by 2).
19
Earthquake interaction Dynamic triggering

• The magnitude of static stress changes decay as
  disatnce-3.
• The magnitude of the peak dynamic stress changes
  decay as distance-1.
• At great distances from the rupture, the peak
  dynamic stresses are much larger than the static
  stresss.

Figure from Kilb et al., 2000
20
Earthquake interaction Dynamic triggering
Instantaneous triggering
No triggering
Stress
Time
Time
21
Earthquake interaction Dynamic triggering
Indeed, distant aftershocks are observed during
the passage of the seismic waves emitted from the
mainshock rupture.
Izmit aftershocks in Greece.
Brodsky et al., 2000
22
Earthquake interaction Dynamic triggering

• Dynamic stress changes trigger aftershocks that
  rupture during the passage of the seismic waves.
• But the vast majority aftershocks occur during
  the days, weeks and months after the mainshock.
• Dynamic stress changes cannot trigger delayed
  aftershocks, i.e. those aftreshocks that rupture
  long after the passage of the seismic waves
  emitted by the mainshock.
• It is, therefore, unclear what gives rise to
  delayed aftershocks in regions that are located
  very far from the mainshock.

23
Earthquake interaction Volcano-seismic coupling
- the Apennines and Vesuvius example
How normal faulting in the Apennines may promote
diking and volcanic eruptions in the Vesuvius
magmatic system, and vice versa.
Nostro et al. (1998)
24
Earthquake interaction Volcano-seismic coupling
- the Apennines and Vesuvius example
Coulomb Failure Function calculations
Nostro et al. (1998)
25
Earthquake interaction Volcano-seismic coupling
- the Apennines and Vesuvius example
Volcano-seismic coupling?
Nostro et al. (1998)
26
Further reading

• Scholz, C. H., The mechanics of earthquakes and
  faulting, New-York Cambridge Univ. Press., 439
  p., 1990.
• Harris, R. A., Introduction to special section
  Stress triggers, stress shadows, and implications
  for seismic hazard, J. Geophys. Res., 103,
  24,347-24,358, 1998.
• Freed, A. M., Earthquake triggering by static,
  dynamic and postseismic stress transfer, Annu.
  Rev. Earth Planet. Sci., 33, 335-367, 2005.