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

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 100-300 km Deep focus earthquakes occur at depths of 300-680 km

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

Seismic data indicate Lithosphere is split into several plates Plates are 60-100 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

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

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

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

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

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

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

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