Earthquake Mechanisms Flashcards
What are earthquakes?
A sudden release of elastic energy stored in the lithosphere.
What’s the epicenter?
The point on the surface above the origin of the earthquake (hypocenter).
What are shortcomings of the Richter scale function to quantify earthquake magnitude?
The function becomes non linear at large distances and for large magnitudes.
What are the two types of seismic body waves?
P-wave (compressional)
S-wave (shear)
What are the two types of surface waves?
Rayleigh wave (circular motion) Love wave (across surface motion)
How are tectonic regimes usually defined?
By terms of their relative magnitudes of vertical stress (Sv) and two mutually perpendicular horizontal stresses (Shmax & Shmin).
What are the earthquake “beach balls” called?
Focal mechanisms - give you an idea of fault orientations and the direction seismic waves move in (on the balls, they move from white to black). Different colours indicate different depth.
What determines the amount of energy released in an earthquake?
The slip area, and the slip vector. The large the slip area, the more energy will be released.
Is it possible to determine absolute stress magnitude values at an earthquake?
No. While the relative magnitude can be determined by fault type, orientation and motion, there is no measure for the absolute magnitude.
What does this Earthquake Wave Notation mean?
Pg or P
P - Direct wave
What is the Wadati Benioff zone?
A planar zone of seismicity that corresponds with the downgoing slab at a subduction zone, and develops beneath volcanic island arcs and continental margins above subduction zones.
Why is the Wadati Benioff zone useful?
Provides 3D map of the downgoing slab
How deep can the Wadati Benioff zone be?
Up to 670km deep
What does this Earthquake Notation mean?
Pn or Sn
Head wave in the mantle (the refracted wave)
What does this Earthquake Notation mean?
PmP
PmP: Reflected wave from Moho
What does this Earthquake Notation mean?
PP, SS, PS
PP, SS, PS: surface reflections that do not pass through boundary
What does this Earthquake Notation mean?
PcP, ScP etc
Reflected from the core boundary
What does this Earthquake Notation mean?
PKP
Refracted wave through the core
What does this Earthquake Notation mean?
PKIKP
Refracted wave through the inner core
What are the maximum and minimum stress orientations of:
- reverse fault
Maximum stress (S1): Horizontal Minimum stress (S3): Vertical
What dip angle does Anderson’s Faulting Theory predict for a reverse fault?
~ 30 degrees
What dip angle does Anderson’s Faulting Theory predict for a normal fault?
60 degrees
What dip angle does Anderson’s Faulting Theory predict for a strike-slip fault?
90 degrees
What are the assumptions of Anderson’s Faulting Theory?
- Surface of earth is unconfined, and no shear stresses act on it
- Homogenous rocks
- Coloumb behaviour
What are the maximum and minimum stress orientations of:
- normal fault?
Maximum stress (S1): vertical Minimum stress (S3): horizontal
What are the maximum and minimum stress orientations of:
- strike slip fault?
Maximum (S1): Horizontal
Minimum (S3): Horizontal
What are exceptions to Anderson’s Faulting Theory?
- Thrust faults
- Low angle faults
What do nodal planes show?
Orientation of the fault (from north) - always gives two possible solutions
What do auxiliary planes show?
The dip angle of the fault
Where does the peak of the emitted energy occur?
At 45 degrees to the source (in all directions)
What is the change in composition from the upper mantle to lower mantle?
In the upper mantle, change from olivine to spinel at the 410km discontinuity, then at the 660km discontinuity with the lower mantle it changes again to perovskite and magnesio-wuestite
Where do extensional earthquakes occur at a subduction zone?
Shallow crustal zone, on the subducting plate
What is Snell’s Law of Refraction?
Sin theta 1/ sin theta 2 = v1/ v2
