1. Earthquakes and Seismotectonics Chapter 5

2. What Creates Earthquakes?
The term “Earthquake” is ambiguous:
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
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

3. Earthquake Terminology
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)

4. 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

5. 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)

6. Formation of Faults
Faults and thus earthquakes form because of stress & strain
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
A through-going fault
forms and sliding occurs
causing a stress drop
Elastic strain:
strain that is recoverable
New cracks form and
link together

7. 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

8. 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

9. The Earthquake Cycle: A Simple View
[ Step 2 ]
- Plate motion continues
- Stress/strain exceeds rock strength
- The fault slips (ruptures)
- Fence is broken into two undeformed pieces
[ Step 1 ]
- Plate motion continues
- Stress/strain is localized on fault
- Fence is strained/deformed
- Deformation is recoverable (elastic)
[ Initial Conditions ]
- Plate motion begins
- Fence is straight

10. Measuring Motion Across a Fault
M Great San Francisco Earthquake

11. 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

12. 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

13. 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

14. 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
Station #1
Station #2
Station #3

15. 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

16. 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

17. 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!

18. Measuring Orientation: Strike and Dip
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…
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…

19. Fault Plane Solutions Consider a peg struck by a hammer…
Only P-waves to the N-S
Greatest amplitude directly ahead and behind…i.e. N-S
Amplitude decreases away from N-S direction
Dilatational first arrival to the S
Contractional first arrival to the N
Only S-waves to E-W
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
How do we know if the first arrival is dilatational or contractional?

20. 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
maximum compressive stress direction
Seismologists call this
P-axis (sometimes C-axis)
Pressure axis (compression axis) σ3
minimum compressive stress direction
T-axis
Tension axis
Extension
Contraction
Contraction
Extension
Fault in a Box

21. 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

22. The Double Couple Mechanism
Before an earthquake, rock is sheared
The rock cannot rotate, so there must be other stresses involved.

23. 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…

24. 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

25. 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?

26. Even geophysicists need to look at rocks and geologic maps
Geology!!!

27. 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!!!

28. Strike & Dip: The Stereonet Way
Dip Direction = N/A

29. Strike & Dip: The Stereonet Way
Dip Direction = N/A

30. Strike & Dip: The Stereonet Way
Dip Direction = East +

31. Strike & Dip: The Stereonet Way
Dip Direction = East +

32. Strike & Dip: The Stereonet Way
Dip Direction = East +

33. Strike & Dip: The Stereonet Way
Dip Direction = East +

34. Strike & Dip: The Stereonet Way
Dip Direction = East +

35. Strike & Dip: The Stereonet Way
Dip Direction = SE +

36. Strike & Dip: The Stereonet Way
Dip Direction = SW +

37. Strike & Dip: The Stereonet Way
Dip Direction = NE +

38. Beach Balls For Standard Fault Types
For faults with pure dip-slip or pure strike-slip motion the focal mechanisms are relatively straightforward

39. Focal Mechanisms For Oblique Slip
Focal mechanisms can also determine the direction of slip
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 = ? = ?

40. 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

41. Calculating Focal Mechanisms

42. 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?