Lecture #13- Focal Mechanisms

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

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)

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

Faulting and Plate Tectonics

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?

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.

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

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

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

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.

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

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

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 ?

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

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

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)