TYPES OF SEISMIC WAVES IN EARTHQUAKE SHAKING:

2 basic types:  Body Waves vs. Surface Waves

          both = "elastic waves" – mechanical energy transmitted through elastic materials

          analogs:  sound waves, water waves

Body waves: 

   P-Wave:  Compressional (like sound)

          --travel through solid & liquid

          --rapid velocity à Primary (first arrival)

   S-wave:  Shear wave (like rope)

          --travel through solid only

          --slower velocity à secondary arrival

Travel-time curve:  Plot distance vs. time; slope ~ 1/Velocity

Surface waves:  Only at outer (10s to 100s km) of Earth surface

          --like water waves

   Love wave: ~ S-wave in horizontal plane

          --damaging to building foundations

   Rayleigh wave:  elliptical motion (like ocean waves)

Velocity << body waves (plus longer travel path)

Observed pattern on seismogram:  P-wave, S-wave, surface waves

Wave interaction in Earth interior:

          Earth interfaces (Moho, CMB) à change in wavefield

          ~ effect of different materials on light (optics)

   Reflection: ~ light on mirror

          -- angle of incidence = angle of reflection

          --important applications:  echo sounding, oil exploration

   Refraction = bending of seismic waves @ boundary w/ different materials

          --analogy:  bending of light at acquarium

          --change of angle = f(velocities)

   Conversion = change of phase (PàS, SàP)

          --incident P-wave à reflected, refracted P, S (only at solid-solid)

   Attenuation:  absorption of wave energy by earth

          --Earth = imperfect elastic medium:  wave energy à heat

          --spherical spreading (water waves)

   Amplification:  resonance in unconsolidated materials

          --cf. Lessons from Mexico City eq, Loma Prieta eq.

Recording earthquakes

A few definitions:

•   Seismograph:  Instrument for recording tiny motions of the Earth surface produced by seismic waves

•   Seismometer:  A suspended pendulum used to sense ground motion

          Principal:  inertia

          Suspend mass from rigid frame by spring

          Must add some sort of damping

          3-components:  reconstruct ground motion in 3-D

•   Strong-motion accelerograph: low-magnification device for measuring strong ground motions associated with nearby earthquakes

          Built with withstand large forces (>1g accel!)

          Information for engineers (acceleration)

•   Seismogram:  recording from seismograph

Modern seismograph: 

     electromagnetic recording

     magnification:  thousands--> millions ==> record motions to microns

     record on paper, magnetic tape, computer

Seismograph arrays:

     clusters of individual seismographs= seismic array

     add signals==> higher resolution of small signals

          ~phased-array radar

     detection of nuclear explosions

Earth Internal Structure

Principal information from seismology

          --deepest drillhole = 13 km

          --only info on deep earth = 'remote sensing'

Slice Earth in half, then take pie slice

3 main compositional layers:  crust, mantle core (egg analogy)

Crust:  Near-surface rocks, rigid outer shell

          = sediments, granite, crystalline rocks

          --composition: Si + Al + O (+ Fe, Mg)

          --thickness

                   oceans = 8-10 km

                   continents = 30-45 km

                   mountains = 50-75 km

          --base = Moho

Mantle: change to denser crystalline rocks

          --composition:  Si + Mg + Fe + O

          --sample:  harzburgite

          --thickness = ½ of Earth à 2880 km

          --base = sudden change in physical properties:  Gutenberg Discontinuity

Core:  very high density, metallic

          --composition:  Fe + Ni

          --outer core:  molten

          --inner core:  solid

          --boundary = Lehmann Discontinuity 5140 km

1960s:  New twist:  variations in physical properties of mantle

          --important role in discovery of plate tectonics

          --rigid layer = crust & uppermost mantle (to 100 km) = Lithosphere

          --100-300 km = "weak layer" almost molten = Asthenosphere

          --below that = stronger layer:  Mesosphere