1. Earthquakes A manifestation of rock deformation
Occur when one mass of rock slips past another mass of rock on a discrete surface separating the two rock masses
Discrete surface = geological fault
Slip is catastrophic

2. Stick-slip mechanism
One of several mechanisms by which slip occurs on faults
Prior to slip event, rock distorts elastically
In elastic deformation, bonds bend, stretch, and otherwise distort but do not break
Elastic deformation is not permanent - it is recoverable
Elastic deformation stores energy
Stored energy is released during catastrophic slip event

3. Catastrophic slip occurs at hypocenter
Hypocenter = focus
Stored elastic energy released, creating wave-like distortions that emanate from focus as seismic waves
Elastic distortions consist of:
Compression/dilatation of bonds in rocks
Shearing of bonds in rocks
Rotary motion of particles in rock, etc.
Components disperse with distance from focus
Distinguish body wave & surface wave components

4. Body wave component I Longitudinal or compressional component
Travels through rock as regions of compressed and dilated rock
Atoms/molecules move back & forth along lines parallel to direction in which wave travels
Travels through solids and liquids
Travels at higher speeds, & so arrives at distant recording station sooner
Called P wave component = primary wave

5. Body wave component II Transverse or shear component
Travels through rock as regions of sheared rock
Atoms/molecules move back & forth along lines perpendicular to direction in which wave travels
Travels through solids but not through liquids
Travels at lower speeds, & so arrives at distant recording station later
Called S wave component = secondary wave

6. Like all waves, seismic waves reflect and refract when they travel from a medium where they have one speed to a medium where they have a different speed

7. Through reflection & refraction of seismic waves, we ‘see’
Earth is composed of layers with different characteristics & compositions
Crust - regions of low seismic velocity; 5-7 km thick in oceans & km thick in continents
Moho - discontinuity at base of crust; discovered early in the 20th century
Mantle - region of high seismic velocity below crust; dense substrate on which crust floats

8. Taking larger view, we ‘see’ that Earth is composed of concentric shells of distinctive seismic character

9. Concentric shell in earth, I
Lithosphere
Outermost shell
Consists of crust & cool, relatively rigid mantle immediately below it
Characterized by high seismic velocity & low attenuation of seismic waves
Is usually km thick
Is <5 km thick under MOR axial valley
Is >300 km thick in some continental areas

10. Concentric shell in earth II
Asthenosphere
Warm, relatively ductile layer within mantle
Has relatively low seismic velocity and high attenuation of seismic waves
Usually about 250 km thick; it occurs between 60 & 350 km below surface
Absent beneath some continental regions
Ductility may result from partial melting; compare geothermal gradient against solidus temperature

11. Concentric shell in earth III
Mesosphere
Lower portion of the mantle
Has relatively high seismic velocities & low attenuation of seismic waves; velocity increases with depth
Rock is warm, but high ambient pressure makes melting unlikely & increases strength of rock
We know little of the details of the composition or behavior of the mesosphere

12. Concentric shell in deep in earth
Outer core
P wave velocities drop dramatically; leads to P wave shadow zone
S waves do not penetrate outer core; leads to S wave shadow zone
Infer that outer core is liquid, probably composed of mixture of iron & nickel

13. Center of the earth Inner core
P wave velocities increase significantly
S waves (generated when P waves traveling through outer core intersect inner core boundary) travel through inner core
Infer that outer core is solid mixture of iron & nickel

14. Have two systems for characterizing Earth’s interior
Distinguish crust, mantle, & core on the basis of chemical composition of rock
Outer layers more silica-rich
Inner layers enriched in iron & magnesium
Distinguish lithosphere, asthenosphere, mesosphere, outer and inner core on basis of material behavior
Have variations in strength, ductility, seismic wave speed, etc.

15. Earthquake locations - where do earthquakes occur?
Time intervals between the arrivals of different components of a seismic disturbance gives the distance from the earthquake focus to a seismic recording station
Measure the time (in seconds) between the arrival of P & S components (S-P interval)
S-P intervals from three or more recording stations fix the location of earthquake focus (hypocenter) or epicenter

16. Compilations of earthquake locations indicate
Earthquakes can occur anywhere on earth
Most earthquakes occur in discrete zones
In mid-ocean ridges, mainly beneath axial valleys
Along oceanic fracture zones, usually between offset segments of axial valleys
In Benioff-Wadati zones near deep-ocean trenches
Beneath active, modern mountain ranges

17. Compilations of earthquake locations indicate
Earthquakes usually within 100 km of surface
Most of these shallow focus earthquakes occur at depths <20 km
Some earthquakes occur at depths >100 km
Intermediate focus earthquakes occur at depths of km
Deep focus earthquakes occur at depths of km