1. 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

2. 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

3. All earthquakes occur in lithosphere
Intermediate & deep focus earthquakes occur where slabs of lithosphere extend to depth
Recognize lithosphere by its relatively high seismic velocity & the relatively low attenuation of seismic waves

4. Seismic data indicate Lithosphere is split into several plates
Plates are km thick but may be 100’s or 1000’s of km across
Plates are composed of oceanic crust, continental crust, or combinations of oceanic & continental crust
To fit on curved earth’s surface, lithospheric plates have 3D shape that resembles a dinner plate
Locate plate boundaries by occurrence of earthquakes
Other geological phenomena, such as volcanic activity and mountain building, occur along plate boundaries

5. Earthquake first-motion studies
Seismogram may indicate that a rock mass initially moved toward or away from a recording station
If first-motion is toward recording station, leading edge of P wave is a region of compressed rock
If first-motion is away from recording station, leading edge of P wave is a region of dilated rock
Every fault slip event generates a systematic pattern of compressions and dilations

6. Each pattern of compressions and dilations is consistent with fault slip on one of two mutually perpendicular faults

7. First motion studies indicate
Plates move away from each other at MOR ridge crest/axial valleys
Creates new lithosphere
Plates move past each other in oceanic FZs & across continental transcurrent faults
Lithosphere neither created nor destroyed
Plates move toward each other at DOTs and in active mountain belts
Lithosphere consumed by subduction

8. Create new lithosphere at MOR
Plate divergence causes mantle upwelling, which promotes mantle melting by reducing pressure
Melt migrates toward surface, & solidifies to form oceanic crust
Generates layered oceanic crust, with residual mantle at base, gabbroic layer = intrusive magma chambers, sheeted dike complex = feeders of extrusive material, & basaltic layer = extrusive carapace
Oceanic crust cooled by infiltrated sea water
Divergent plate boundaries are constructive plate boundaries

9. Subduction
Plate convergence causes slab of oceanic lithosphere to subduct, i.e. bend & descend into the underlying asthenosphere & eventually the mesosphere beneath another plate
Overriding lithosphere may be oceanic or continental
Subducted oceanic lithosphere is heated & subjected to great pressure, i.e. metamorphosed

10. Subduction II
Metamorphism of subducted slab causes a dramatic increase in its density, enhancing its tendency to sink
Metamorphism releases water from subducted slab; water helps both the slab & the overlying asthenosphere to melt, causing volcanism seen on landward side of DOT
Bending, shortening, shearing, & compression in subducted slab causes seismicity unlike that seen elsewhere in earth - intermediate & deep focus earthquakes are not like shallow earthquakes

11. Plate tectonics theory
Geometrically precise description of relative motion of lithospheric plates
Data from Very Long Baseline Array, satellite telemetry, & GPS surveys confirm earth movements consistent with the predictions of plate tectonic theory
Provides rational explanation for the volcanism, plutonism, metamorphism, deformation, & seismicity observed on earth
Processes concentrated at plate boundaries
Successfully predicts the relative likelihood of seismic activity, volcanism
Understand the formation of mountain belts through Wilson cycle; now understand the geology of continents
Plate movement is a process by which heat escapes from earth's interior