REVIEW OUTLINE: SEISMIC
WAVES & EQ MONITORING
Classification of Seismic Waves
- Body waves
- P (Primary, compressional) wave
- Most rapid velocity
- Travels through solid & liquid
- S (Secondary, shear) wave
- Slower velocity, always arrives after P-wave
- Only passes through solid
- Surface waves
- Only travel in outer (10s to 100s km) of Earth surface
- Rayleigh waves (vertical motion, like water wave)
- Love waves (side-to-side motion)
- damaging to building foundations
- Velocity << body waves (plus longer travel path)
- Observed pattern on seismogram: P-wave, S-wave,
surface waves
Travel-time curve: Plot distance vs.
time; slope ~ 1/Velocity
Wave interaction in Earth interior:
- Earth interfaces (Moho, CMB) produce change in wavefield
- 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 (P to S, S to P)
- incident P-wave produces reflected, refracted P, S (only
at solid-solid)
- Attenuation: absorption of wave energy by earth
- Earth = imperfect elastic medium: wave energy absorbed
(turns to heat)
- Amplification: resonance in unconsolidated materials
- cf. Lessons from Mexico City eq, Loma Prieta eq.
Recording earthquakes
- Seismograph: Instrument for recording tiny motions
of the Earth surface produced by seismic waves
- Seismoscope: Primitive seismograph designed to record
ground motions, but with no record of time history of movement
- Seismometer: A suspended pendulum used to sense
ground motion
- Principal: inertia
- Suspend mass from rigid frame by spring
- 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