Earthquakes and Seismotectonics
Chapter 5
What Creates Earthquakes?
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The term “Earthquake” is ambiguous:
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Applies to general shaking of the ground and to the source of the shaking
We will talk about both, but are mainly concerned with the latter
Earthquakes occur due to
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Sudden motion on a fault
• Formation of a new fault
• Slip on an existing fault
• Movement of magma / explosion of a volcano
• Landslides
Meteorite impacts
Underground nuclear bomb tests / mine collapses
Offset
Earthquake Terminology
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Hypocenter (Focus): actual location of the earthquake at depth
Epicenter: location on the surface of the Earth above the hypocenter
Hanging Wall: top block of a fault (where a light would hang from)
Footwall: bottom block of a fault (where you would stand)
Types of Faults
• In general, faults come in three different types: Normal, Reverse,
and Strike-Slip
• Shallow angle (< 30°) reverse faults are called thrust faults
• Faults that have a mix of slip styles are called oblique slip faults
See: Fault
animations
online
Why are there different types of faults?
• Normal Faults: from stretching of or extending rock; points on
opposite sides of a fault are father apart after an earthquake
• Reverse Faults: from contracting or squishing rock; points on
opposite sides of the fault are closer together after an earthquake
• Strike-Slip: can form in either areas of stretching or squishing,
material slides laterally past each side of the fault.
– Described by sense of motion:
• Right-lateral (Dextral)
• Left-lateral (Sinistral)
Formation of Faults
• Faults and thus earthquakes form because of stress & strain
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Plate motion causes rocks to deform or bend
Stress and strain become localized
Eventually the strength of the rock is overcome
BAM!! The rock ruptures and snaps forward releasing the accumulated
stress/strain.
• The process is known as elastic rebound theory
Elastic strain:
strain that is recoverable
New cracks form and
link together
A through-going fault
forms and sliding occurs
causing a stress drop
Faults & Friction
• Like a brick sliding across a table, faults, too, are subject to friction
• Friction, on the micro-scale, is caused by asperities, bumps and
irregularities along a surface that resist sliding
• All other factors equal, faults with more cumulative slip may be
smoother and therefore have lower friction (e.g. the San Andreas
Fault has very low friction)
• Once a fault is formed it is a permanent scar that is weaker than the
surrounding rock
Stick Slip Behavior
- Without stick slip behavior, large earthquakes would not happen!
- Faults would constantly move (i.e. creep) and not build up significant stress
The Earthquake Cycle: A Simple View
[ Step 1 ]
- Plate motion continues
[ Initial Conditions ]
- Stress/strain is localized on fault
- Plate motion begins
- Fence is strained/deformed
- Fence is straight
- Deformation is recoverable (elastic)
[ Step 2 ]
- Plate motion continues
- Stress/strain exceeds rock strength
- The fault slips (ruptures)
- Fence is broken into two
undeformed pieces
Measuring Motion Across a Fault
M7.8 1906 Great San Francisco Earthquake
Locating Earthquakes
• Often we don’t see surface
rupture after an EQ
– Earthquakes occur deep in the
Earth.
• To locate EQ’s we can’t just
look at first arrivals of P-waves
– Time = 0 is unknown
– Seismic velocity is non-uniform
– Can only get a potential
epicentral area
• Instead we rely on the
difference in arrival times
– vs ≈ 0.55 vp
Locating Earthquakes
• Because P-waves travel fastest, they will always be recorded first
– The farther from the source, the more S-wave lag.
• If we calculate the difference in arrival times of S- and P-waves,
we can then calculate the distance to epicenter
– Called the S-P interval
S-P Intervals
• The S-P time only tells
distance, not direction
• A minimum of three
stations are needed to
calculate epicenter
location
– Called triangulation
Triangulation
• One station gives
infinite possible
epicentral locations
• Two stations give two
possible locations
• Three stations give
one location
•
Station #1
In practice there is
some error
• The epicenter is
located where these
circles from multiple
stations all intersect
Station #2
Station #3
Triangulation
• One station gives
infinite possible
epicentral locations
• Two stations give two
possible locations
• Three stations give
one location
•
In practice there is
some error
• The epicenter is
located where these
circles from multiple
stations all intersect
How is Earthquake Depth Determined?
• Seismologists determine hypocenter depth by:
– Determining the arrival of the pP ray
– Calculating the p-pP lag time and plugging it into an equation
• Hypocenter depth also effects S-P intervals, but this is
usually accounted for
– Most regions have earthquakes at a limited range of depth
Fault Plane Solutions
• Along with hypocenter location, seismograms can be
used to determine the type of fault that caused the EQ
• …But first we need to review how to quantify the
orientation of a plane!
Measuring Orientation: Strike and Dip
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In order to characterize geologic
structures, one must be able to quantify
the orientation of structures.
For Planar features we use:
• Strike: The orientation of the
intersection line between a horizontal
surface and the feature of interest.
Measured with a compass.
– E.g. north, N45W, 285, etc…
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Dip: The acute angle between the
feature of interest and a horizontal
plane.
– E.g. 0° = horizontal 90° = vertical
For linear features we use:
• Trend: the trend of the line if
you were looking down on the
feature from above
– E.g. north, NW, 320, 090,
etc…
• Plunge: Acute angle between
the line and a horizontal
– E.g. 46°, 75°, etc…
Fault Plane Solutions
• Consider a peg struck by a hammer…
– Only P-waves to the N-S
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Greatest amplitude directly ahead and behind…i.e. N-S
Amplitude decreases away from N-S direction
Dilatational first arrival to the S
How do we know if the
Contractional first arrival to the N
first arrival is dilatational
– Only S-waves to E-W
or contractional?
• same is true for S-waves…almost
• all first arrivals have the same sense of motion
– S-waves are of little to no help in determining the fault orientation
Faults Generate Contraction and Extension
• The hammer and peg example is too simple
• Both sides of a fault move
– Contraction and extension are both generated during slip
Geologists call this
• σ1
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maximum compressive
stress direction
Seismologists call this
• P-axis (sometimes C-axis)
• Pressure axis (compression
axis)
Geologists call this
• σ3
• minimum compressive
stress direction
Seismologists call this
• T-axis
• Tension axis
Extension
Contraction
Contraction
Extension
Fault in a Box
Focal Mechanisms
• Both sides of a fault move, so the radiation pattern is more complex.
• Seismologists use the pattern of first arrivals to determine several
properties of the causative fault
– strike, dip, and slip vector rake.
– we call these focal mechanisms, moment tensors, or beach balls
Contraction
Extension
Extension
Contraction
The Double Couple Mechanism
• Before an earthquake, rock is sheared
• The rock cannot rotate, so there must be other stresses involved.
The Double Couple Mechanism
• If two shear stresses are involved
– the rock can undergo shear strain without rotating
– called the double couple
• but this causes ambiguity in the focal mechanism solution…
The Auxiliary Plane
• Because of the double couple
– no rotation is allowed
• Focal mechanisms predict two potential fault planes
collectively called: nodal planes
– the fault plane
– the auxillary plane
Which Plane is the Fault?
• What are the two
potential fault
orientations?
• How do we know
which is the real
fault?
– Sometimes logic
combined with a
little Occam’s Razor
– Aftershocks &
Historical seismicity
– How else could we
determine the fault
plane?
Geology!!!
The Focal Sphere
• The process just outlined is fine for strike-slip events, but we
need a general method for any type of fault.
• To do this we use the focal sphere
– just like your favorite part of structural geology
• Stereonets!!!
Strike & Dip: The Stereonet Way
• Strike = 090
• Dip = 90⁰
• Dip Direction = N/A
Strike & Dip: The Stereonet Way
• Strike = 000
• Dip = 90⁰
• Dip Direction = N/A
Strike & Dip: The Stereonet Way
• Strike = 000
• Dip = 80⁰
• Dip Direction = East
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Strike & Dip: The Stereonet Way
• Strike = 000
• Dip = 60⁰
• Dip Direction = East
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Strike & Dip: The Stereonet Way
• Strike = 000
• Dip = 45⁰
• Dip Direction = East
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Strike & Dip: The Stereonet Way
• Strike = 000
• Dip = 30⁰
• Dip Direction = East
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Strike & Dip: The Stereonet Way
• Strike = 000
• Dip = 10⁰
• Dip Direction = East
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Strike & Dip: The Stereonet Way
• Strike = 045
• Dip = 45⁰
• Dip Direction = SE
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Strike & Dip: The Stereonet Way
• Strike = 135
• Dip = 80⁰
• Dip Direction = SW
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Strike & Dip: The Stereonet Way
• Strike = 280
• Dip = 60⁰
• Dip Direction = NE
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Beach Balls For Standard Fault Types
• For faults with pure dip-slip or pure strike-slip motion the
focal mechanisms are relatively straightforward
Focal Mechanisms For Oblique Slip
• Focal mechanisms can also
determine the direction of slip
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Called the slip vector rake, or just “rake”
180 ≥ rake ≥ -180
0 = left-lateral, 180/-180 right-lateral
90 = reverse slip -90 = normal slip
45 = ? 120 = ?
Calculating Focal Mechanisms
• Although it is impractical to put
seismometers deep in the ground,
we can still detect waves that are
radiated in all directions from a
hypocenter
• We can trace P-waves back to their
source using:
– inverse methods
– the ray parameter, p
• We can then calculate the take-off
angle
– relative to vertical
– this tells seismologists where to plot
each station on the focal sphere
(stereonet)
– can get azimuth to source from
triangulation
Calculating Focal Mechanisms
Odd Focal Mechanism?
• Really think about what the focal sphere represents…
– Why are certain parts are black and others white?
– This is all black?
• What could cause this?