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Strike slip faults
Strike slip faults
Earthquakes on strike slip faults
San Andreas fault
Fault segments & slip patches
Southern California earthquakes
San Diego, central California,
Mojave desert
Transform (sliding) Boundaries
• fault zones between plates
• neither destruction nor construction occurs
(usually)
• relative motions are parallel to faults (usually)
• small regions may be collisional or extensional
• transform boundaries connect two diverging
boundaries, two converging, or one of each
diverging and converging
• earthquakes are generally shallow
• transform faults that cease to be plate
boundaries are called fracture zones.
NORTH AMERICA 80 MA & 20MA - PRESENT
DEVELOPMENT OF SAN ANDREAS FAULT
Triple Junctions
•  there are 16 possible arrangements of triple junctions
•  see Kearey &Vine (1990), page 100
•  some are always stable, some are always unstable and cannot
form
•  some are unstable, but will decay to a stable configuration
•  an example of the effect of changing triple junctions can be seen
in California
Oligocene
Farallon
Plate
Mendocino FZ
Pacific
Plate Murray FZ
NA
•  Farallon plate separated from
Pacific plate
•  subduction of Farallon beneath
NA was faster than supply of
new ocean crust
•  as a result, the ridge system
moved east, toward NA.
Triple Junctions
28 Ma
F
NA
Pacific
Plate
F
28 Ma
F
FFT
RTF
Pacific
Plate
F
NA
•  momentarily, a quadruple point
existed, FRTT
•  quadruple point decayed to two
triple points almost immediately
(FFT, RTF)
•  both of these are stable triple
junction configurations
(see K&V, p.101) for the velocity
space solutions
•  Mendocino triple junction
migrated north along NA
boundary
•  Murray triple junction migrated
south to the ridge-transform
contact.
Triple Junctions
18 Ma
F
Sanas Ft.
re
And
Pacific
Plate
F
NA
•  by 18 Ma, the trench had
overridden the entire
Mendocino-Murray
ridge section
•  both triple junctions now
revert to RTF configurations
•  the situation is stable
•  it has survived for 18 Ma.
TRANSFORM FAULT EARTHQUAKES
SAN ANDREAS FAULT
Cape Mendocino to Gulf of California
Coast line in blue. San Andreas fault heavy dark line.Very complex system
of faults especially in southern California.
TRANSFORM EARTHQUAKES
LOCKED SECTIONS
1857 Fort Tejon and 1906 San Francisco
Two sections of the fault locked
for long periods of time.
Failed in 1857 and 1906 creating
two major ruptures
1857 Fort Tejon 360 km long
1906 San Francisco 430 km long.
Two very large earthquakes
TRANSFORM EARTHQUAKES
1906 SAN FRANCISCO
Locked section moved
 Average 6m offset along 430 km
of fault.
 60 seconds of shaking.
 Magnitude 7.8 earthquake
 Most damage due to fire and
disease.
 Important for developing elastic
rebound theory for earthquakes
TRANSFORM EARTHQUAKES
1989 LOMA PRIETA
During World Series
 Bend of San Andreas, where two
faults split off from main fault,
ruptured.
 Earthquake did not rupture the
ground.
 Moved 1.9 m horizontally and 1.3
m vertically. Magnitude 6.9
 Extensive damage in San
Francisco.
TRANSFORM EARTHQUAKES
LOMA PRIETA 1989
Damage in Marina District San Francisco
 Water saturated sediments were shaken became a slurry and flowed.
 Foundations collapsed especially soft buildings.
 Bottom floors used for garaging cars collapsed.
TRANSFORM EARTHQUAKES
1989 LOMA PRIETA
Interstate 880 collapse
 Portion of the elevated 880 built on soft mud.
 The soft mud resonated with the earthquake waves and the mud liquified.
 The hard rock had much smaller accelerations
 TRANSFORM EARTHQUAKES
1989 LOMA PRIETA
Interstate 880 collapse
 The bridge had roughly the
same frequency as the
amplified surface waves and
shook severely.
 The support columns failed
at the joints. There were 20
#18 steel bars in the columns
but they were discontinuous
at the joints.
 With repeated shaking the
joints failed and the bridge
collapsed.
HOW FAULTS WORK
Elastic rebound theory
 Active fault with road as reference.
 Elastic deformation occurs either side
of the fault but because of friction at the
fault, no movement
 Elastic deformation exceeded strength
of rock - fault ruptures and two side
snap away from each other.
 Current models add propagation of
fault break. Like a ripple on a carpet.
PALEOSEISMOLOGY
Tectonic Geomorphology - examples
  General topography
along the central San
Andreas
  Sag ponds - down
dropped areas along
the fault.
  Sediments deposited
along fault preserve
prehistoric record of
faulting.
PALEOSEISMOLOGY
Sag ponds
Soda Lake Road at
the southern end of
Carrizo Plain
National Monument.
PALEOSEISMOLOGY
Sag ponds
San Andreas
Lake and
Crystal Springs
reservoir from
the air, looking
southeast.
PALEOSEISMOLOGY
Fault scarp
San Andreas fault from
the air
PALEOSEISMOLOGY
Fault scarp
Offset of rows in
plowed field
A right-lateral fault
trace crosses a
plowed field during
the Imperial Valley
earthquake.
The agricultural
industry suffered heavy
losses from damage to
canals, irrigation
ditches, and
subsurface drain tiles
disturbed by the
movement along the
Imperial fault.
PALEOSEISMOLOGY
Plastic deformation
Left-Lateral Strike-Slip
Faults in the cultivated
field west of El
Progresso, Guatemala,
February 4, 1976.
The thick, saturated,
unconsolidated
deposits have yielded
by plastic deformation
rather than rupture
along the left- lateral
strike-slip fault.
This EQ resulted in the
deaths of 23,000
people and $1.1 billion
dollars in property
damage.
PALEOSEISMOLOGY
Tectonic geomorphology - summary
Scarp
Beheaded stream
Shutter ridges
Offset streams
Sag ponds
Linear ridges
Mole tracks
Water gaps
Wind gaps
THRUST FAULT EARTHQUAKES
BEND IN SAN ANDREAS
Complex plate pattern south of bend
Left stepping right lateral fault
creates compression at the bend
Leads to many little plates south of
the bend.
Compression creates the
Transverse Ranges and the San
Gabriel Mts.
Thrust faults under Los Angeles
THRUST FAULT EARTHQUAKES
BEND IN SAN ANDREAS
SAN ANDREAS FAULT
BLIND THRUST FAULTS
San Fernando 1971, Northridge 1994
SEISMIC GAPS
Cross section of seismicity, San Andreas 1969 to 1989
 Dense cluster in central creeping section of the fault.
 1989 Loma Prieto earthquake and after shocks filled in a gap
 Gap exists south of San Francisco.
Probability of EQ magnitude along SAF
Prediction of magnitude and probability of occurring before 2030.
Forecasts based on historic records, dated trench wall offsets and GPS
The Parkfield Experiment
is a comprehensive, long-term
earthquake research project
on the San Andreas fault.
The experiment's purpose is to better
understand the physics of earthquakes what actually happens on the fault and
in the surrounding region before, during
and after an earthquake
Prediction by USGS in April, 1985: "A 90% probability of an earthquake with
Magnitude 5.5 to 6.0 occurring sometime
between 1985 and 1993."
The San Andreas fault defines an
approximately 1300 km portion of the
boundary between the Pacific and
North American plates.
Along its length, the fault undergoes
horizontal strike-slip motion that
accommodates most of the relative
motion between the plates.
To the north, a complex of transform
faults and spreading centers
accommodates the motion of the
Gorda and Juan de Fuca plates.
To the south, a similar complex of
spreading centers and transform faults
accommodate the displacement in the
Gulf of California.
Foreshocks and Aftershocks
The size of the aftershocks depends on the size of the initial large earthquake
Press et al., 2004
Two Big Questions:
*What causes earthquakes to start in specific locations?
*What causes earthquakes to stop in specific locations?
Three Big Possible Answers:
*Fault Zone Rheology
Friction, Deformation Styles
*Fault Zone Stress Conditions
Tectonics, Stress Triggers/Shadows, Pore-pressure
*Fault Zone Geometry
Fault Continuities and Discontinuities
March 2006 WGCEP Workshop
 The San Andreas fault in central
California.
 A "creeping" section (green)
separates locked stretches north
of San Juan Bautista and South of
Cholame.
 The Parkfield section (red) is a
transition zone between the
creeping and southern locked
section.
 Stippled area marks the surface
rupture of the 1857 EQ
  Most of the northern section of the
fault is also currently locked, with no
detectable movement and few
earthquakes since 1906.
  Between these locked sections, the
San Andreas fault creeps (slips
aseismically).
  From San Juan Bautista to Parkfield,
the creeping section produces
numerous small (mostly M=5 and
smaller) earthquakes but no large
ones.
 Between Parkfield and Gold Hill
defines a transition zone on the SAF
between the creeping and locked
behavior of the fault
Waveforms recorded on
regional seismographs are
strikingly similar for the 1922,
1934 and 1966 earthquakes,
These earthquakes may have
involved repeated rupture of
the same area on the fault.
Recordings of the east-west component of
motion from the 1922 earthquake (shown
in black) and the 1934 and 1966 events at
Parkfield (shown in red) are strikingly similar,
suggesting virtually identical ruptures.
Suggest that there may be
some predictability in the
occurrence of earthquakes, at
least at Parkfield??
 Do earthquakes occur
completely randomly, or do they
have a pattern that tends to
repeat?
  If they're random, there's no
hope for earthquake prediction.
Regular repetitions of the same
rupture event, (characteristic
earthquake) may be occurring
at Parkfield.
  This repetition was part of the
basis for developing the Parkfield
experiment.
Vertical component seismograms from
clustered micro-earthquakes on the San
Andreas fault at Parkfield. Three types of
events (numbers on left) are identified on
the basis of subtle differences in the
waveform.
  Adding to the sense of
repetition, similar-size foreshocks
occurred 17 minutes before both
the 1934 and 1966 Parkfield
earthquakes.
The seismicity at Parkfield:
Since 1857, six similar, M~6
earthquakes have occurred on the
San Andreas fault near Parkfield
with apparent regularity -- one
approximately every 22 years.
Little is known about the first three
shocks
Available data suggest that all six
earthquakes may have been
"characteristic”
The Eqs occurred with some
regularity (mean repetition time of
about 22 year) and may have
repeatedly ruptured the same area
on the fault.
2004 Parkfield Earthquake
Are these six earthquakes
"characteristic" with a mean
repetition time of about 22 year?
September 28, 2004 Parkfield
Earthquake
 Ruptured the same segment of
the fault that broke in 1966.
 8 km depth.
 Northwest rupture primarily
along the San Andreas fault.
 Strong shaking lasted for about
10 seconds.
 This earthquake is the seventh in
a series of repeating earthquakes
on this stretch of the fault.
M>2 aftershocks following the
September 28, 2004 M6.0 earthquake
 The previous events were in
1857, 1881, 1901, 1922, 1934, and
1966.
Lu Dongao/Xinhua
Parkfield recurrence σ1 represents the failure stress of the fault. Most
characteristic earthquakes occur at al; the 1934 shock occurred at a2.
A constant loading rate of 2.8 cm per year and a coseismic slip of 60 cm for the
Parkfield earthquake sequences in 1881, 1901, 1922,1934, and 1966 are assumed.
Lu Dongao/Xinhua
b) Series of earthquake sequences at Parkfield since 1850. The line represents the
linear regression of the time of the sequence obtained without the 1934
sequence, The anticipated time of the seventh Parkfield sequence for the
regression is January 1988.
c) Shocks of ML greater than 4 since 1930 have tended to occur when the stress
exceeded σ2.
Parkfield
(1) Will the strain release during the next
earthquake be approximately the inverse,
both in amount and distribution, of the strain
accumulation since the 1966 shock?
The answer is crucial to the basic
assumptions underlying earthquake
recurrence models, such as the timepredictable and Parkfield recurrence
models, which are the foundation of longterm prediction efforts.
(2) Are there changes in the details of the
deformation field that might permit a
refined estimate of the time of the next
earthquake?
The answer to this question will have a major
impact on efforts toward medium- and
short-term prediction.
Predicting earthquakes requires an
understanding of the underlying
physics, which calls for novel
multidisciplinary approaches at a
level never yet undertaken.
We still have very limited precise
quantitative measurements of the
many parameters involved.
Video camera looking along San Andreas
fault at Parkfield, CA.
The physical phenomena
underlying earthquakes are much
more intricate and interwoven and
we do not have a fundamental
equation for the crustal
organization.
Two Big Questions:
*Where will earthquakes start?
* Where will earthquakes stop?
Possible Answers:
Both initiation and termination are likely caused by
*Fault Zone Rheology
Friction, Deformation Styles
*Fault Zone Stress Conditions
Tectonics, Stress Triggers/Shadows, Pore-pressure state
*Fault Zone Geometry
Fault Continuities and Discontinuities)
It is probably best to assume that earthquakes can start anywhere (on faults).
Large earthquakes can be stopped by
1) Encountering large creeping sections of a fault
2) A Stress shadow from a recent large earthquake on the same fault
3) Big changes in fault geometry. (e.g., inter-fault distances of >5 km)
Earthquake Cycle and Geodetic observations
  Modeled as elastic lithosphere overlying a viscoelastic asthenosphere.
  Displacement on a single fault that slip periodically in large earthquakes.
  The fault that cuts through entire elastic lithosphere.
  The lower part of the fault creeps at continuously at a slip rate equal to
plate rate. Flow rate constant in asthenosphere.
  The stresses on the fault are generated by far field load (tectonic
motions) and time-dependent loading of the lithosphere due to
viscoelastic flow in the asthenosphere.
Earthquake Cycle and Geodetic observations
Stage 1
Interseismic deformation. Far field deformation present on the Earth's
surface at about a fault depth away from the locked zone
Earthquake Cycle and Geodetic observations
Stage 2
Coseismic deformation, showing discrete slip along fault, note far field
base line has elastically recovered along a horizontal datum
Earthquake Cycle and Geodetic observations
Stage 3
Possible post seismic transient response - viscoelastic relaxation post
earthquake. Sometimes called “afterslip”.
Fault plane between two
crustal blocks
Rupture (slip) along fault plane
causes earthquake
Stress builds up and
strain accumulates
Elastic rebound
after earthquake
The Physics of Friction
Why friction?
Because slip on faults is resisted by frictional forces.
•  Earthquake cycles,
•  Earthquake depth distribution,
•  Earthquake nucleation,
•  The mechanics of aftershocks,
Question:
Given that all objects shown below are of equal mass
and identical shape, in which case the frictional force is greater?
Question: Who sketched this figure?
Da Vinci law and the paradox
Leonardo Da Vinci (1452-1519) showed that the friction force
is independent of the geometrical area of contact.
The paradox:
Intuitively one would expect the friction force
to scale proportionally to the contact area.
Amontons’ laws (1699)
- verified by Coulomb later in 1781 Amontons' first law:
The frictional force is independent of the geometrical contact area.
Amontons' second law:
Friction, FS, is proportional to the normal force, FN:
FS = µFN
€
Bowden and Tabor (1950, 1964)
A way out of Da Vinci’s paradox has been suggested
by Bowden and Tabor, who distinguished between the
real contact area and the geometric contact area.
The real contact area is only a small fraction of the
geometrical contact area.
Figure from:
Scholz (1990)
FN = pAr ,
where p is the penetration hardness.
€
FS = sAr ,
where s is the shear strength.
Thus:
€
FS p
µ≡
=
.
FN s
Since both p and s are material constants, so is µ.
€
This explains Da Vinci and Amontons’ laws.
Byerlee’s law
For σ N < 200MPa : µ = 0.85
For σ N > 200MPa : µ = 0.60
€
Byerlee (1978)
Static versus kinetic friction
The force required to start the motion of one object relative to another
is greater than the force required to keep that object in motion.
µstatic
µdynamic
€
€
µstatic > µdynamic
Ohnaka (2003)
Velocity stepping - Dieterich
Dieterich and
Kilgore, 1994
•  A sudden increase in the piston's velocity gives rise to
a sudden increase in the friction, and vice versa.
•  The return of friction to steady-state occurs over
a characteristic sliding distance.
•  Steady-state friction is velocity dependent.
Slide-hold-slide - Dieterich
Dieterich and Kilgore, 1994
Static (or peak) friction increases with hold time.
Slide-hold-slide - Dieterich
•  The increase in static friction is
proportional to the logarithm of
the hold duration.
Dieterich (1972)
Monitoring the real contact area during slip - Dieterich and Kilgore
Change in true contact area with hold time
Dieterich and Kilgore (1994)
•  The dimensions of existing contacts are increasing.
•  New contacts are formed.
Dieterich and Kilgore (1994)
•  The real contact area, and thus also
the static friction increase
proportionally to the logarithm of
hold time.
Summary of experimental results
•  Static friction increases with the logarithm of hold time.
•  True contact area increases with the logarithm of hold time.
•  True contact area increases proportionally to the normal load.
•  A sudden increase in the piston's velocity gives rise
to a sudden increase in the friction, and vice versa.
•  The return of friction to steady-state occurs over
a characteristic sliding distance.
•  Steady-state friction is velocity dependent.
•  The coefficient of friction response to changes in the normal stresses
is partly instantaneous (linear elastic), and partly delayed
(linear followed by non-linear).
The constitutive law of Dieterich and Ruina
* ⎞
⎛
⎛
⎞
τ
V
θ
V
∗
= µ = µ + Aln⎜ * ⎟ + Bln⎜
⎟
⎝ V ⎠
σ
⎝ DC ⎠
and
dθ
θV αθ dσ /dt
= 1−
−
,
dt
DC B σ
€
were:
•  V and θ are sliding speed and contact state, respectively.
•  A, B and α are non-dimensional empirical parameters.
•  Dc is a characteristic sliding distance.
•  The * stands for a reference value.
The set of constitutive equations is non-linear.
Simultaneous solution of non-linear set of equations may be obtained
numerically (but not analytically).
Yet, analytical expressions may be derived for some special cases.
•  The change in sliding speed, ΔV, due to a stress step of Δτ:
(
)
ΔV = exp Δτ Aσ .
•  Steady-state friction:
* ⎞
⎛
⎛
⎞
V
θ
V
µss€= µ* + (A − B)ln⎜ ss* ⎟ = µ* + (B − A)ln⎜ ss ⎟ .
⎝ V ⎠
⎝ Dc ⎠
•  Static friction following hold-time, Δthold:
µstatic ∝ (B − A)ln(θ 0 + Δt hold ) .
€