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 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

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, 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.

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