1. Lecture #13- Focal Mechanisms

2. Faulting We have learned that the three basic styles of faulting:
Normal
Reverse
Strike-slip
are associated with the three main types of plate boundaries:
Divergent
Convergent
Conservative

3. Faulting We also have learned that these three mechanisms:
Normal
Reverse
Strike-slip
are associated with the three types of stress regimes:
Extension (vertical stress greater than horizontal stresses)
Compression (vertical stress less than horizontal stresses)
Translation (vertical stress in between horizontal stresses)

4. Faulting
There are special graphics, called focal mechanisms or “beachballs” that we use as shorthand to describe the style of faulting.

5. Faulting and Plate Tectonics

6. Faulting
So far we have talked about the faulting of shallow earthquakes, which are well explained by plate tectonics.
What about the faulting style of deep earthquakes ?
Do similar principles hold true?

7. Faulting
We sometimes see “normal” faulting at depths of 100 km or so in subduction zones:
Earth’s surface
The slab can break under the extensional bending stresses.

8. Faulting
We sometimes see “reverse” faulting for the deepest earthquakes at about 600 km depth:
Earth’s surface
670 km Discontinuity

9. Faulting So, what are focal mechanisms (beachballs) anyway?
Graphical shorthand for a specific faulting process (strike, dip, slip)
Projections of a sphere onto a circle (the lower focal hemisphere)
Representations of the first motion of seismic waves

11. Focal Mechanism
When mapping the focal sphere to a circle (beachball) two things happen:
Lines (vectors) become points
Planes become curved lines

13. Focal Mechanisms
Beachballs always have two curved lines separating the quadrants. That means that beachballs show two planes.
But there is only one fault plane.
The other plane is called the auxiliary plane.
Seismologists cannot tell which is which from seismograms alone, so we always show both of the possible solutions.

14. Focal Mechanisms
Although seismologists cannot tell which plane ruptured from the seismograms, we can tell the type of faulting (reverse, normal, strike-slip)

15. Focal Mechanisms
It is often possible to make an educated guess as to which of the two possible planes is the actual fault plane:
Normal earthquakes – work with gravity so are often steep
Thrust events – work against gravity so are often shallow

16. A normal faulting mechanism
A normal faulting mechanism. Which plane do you think is the fault plane ?
A reverse faulting mechanism. Which plane do you think is the fault plane ?

19. Focal Mechanisms
For strike slip focal mechanisms we can often determine the fault plane by its orientation with respect to the fault.
Ridge
Plate A
Plate B
(right or left lateral ??)
Ridge

20. Summary
The focal mechanism of an earthquake is a graphical description of the type of faulting
Although the fault type can be determined from seismograms, the particular fault plane cannot be determined.
There are always 2 possibilties:
The real plane, known as the fault plane
The fake plane, known as the auxiliary plane

21. Summary
The fault plane can be distinguished from the auxiliary plane by considering:
The steepness of the fault
Reverse = shallow
Normal = steep
Nearby morphological features
The strike of the transform segment for instance
The distribution of aftershocks
Aftershocks tend to line up along the fault plane (but not always of course)