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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 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 Catastrophic slip occurs at hypocenter • Hypocenter = focus • Stored elastic energy released, creating wavelike 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 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 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 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 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 & 35-70 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 Taking larger view, we ‘see’ that Earth is composed of concentric shells of distinctive seismic character 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 60-100 km thick –Is <5 km thick under MOR axial valley –Is >300 km thick in some continental areas 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 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 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 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 Have two systems for characterizing Earth’s interior • Distinguish crust, • Distinguish lithosphere, mantle, & core on the asthenosphere, basis of chemical mesosphere, outer and composition of rock inner core on basis of material behavior • Outer layers more silica-rich • Have variations in strength, ductility, • Inner layers enriched seismic wave speed, in iron & magnesium etc. 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 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