Seismology Measuring the Interior-2Presentation Transcript
Solid Earth Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Today’s class: Seismology Measuring the Interior Reading: Fowler Chapter 4
Compressional α and β are termed for the P- wave and S- wave velocities. Often, the symbols Vp and Vs are used instead of α and β . Θ is the scalar displacement potential, but U vector displacement. , are the Lamé coefficients, where is bulk modulus (incompressibility) , shear modulus (rigidity) and ρ density . Wave Equations Rotational
Seismic velocities How and depend on density ? P wave velocity a and S wave velocity b depend on physical properties of medium through which they travel: where: K = the bulk modulus, or the reciprocal of compressibility. = the shear modulus, or the second Lam é constant, and = density. = k - = 2 E 3 ( 1 + ) ( 1 – 2 )
P and S-Wave Velocities
Unlike density, seismic velocity involves the deformation of a rock as a function of time. As shown below, a cube of rock can be compressed , which changes its volume and shape or sheared , which changes its shape but not its volume.
Last Updated: January 2007
An important empirical relation exists between P wave velocity and density.
Cross-plotting velocity and density values of crustal rocks gives the Nafe-Drake curve after its discoverers.
Only a few rocks such as salt (unusually low density) and sulphide ores (unusually high densities) lie off the curve.
It is a linear relationship between density and seismic velocity
V = a ρ + b
where a and b are constants.
Elastic Constants and Velocity Grifts and King, 1981
Locating an Earthquake’s Epicentre
Travel-times for location
Measure time between P and S wave on seismogram
Use travel-time graph to get distance to epicenter
Body wave phases
P : P wave in mantle
K : P wave in OC
I : P wave in IC
S : S wave in mantle
J : S wave in IC
c : reflection off CMB
i : reflection off ICB
s,p : upgoing surface reflection
Global travel time curves
There are special graphics, called focal mechanisms or “beach balls” that we use as shorthand to describe the style of faulting.
A seismograph is an instrument which writes a permanent continuous record of earth motion.[ 1 ] Measuring the three orthogonal components of ground motion at a seismic.
Simplified motions of seismic waves Seismic waves are waves of energy that travel through Earth and along its surface. The waves are produced by earthquakes , explosions , or some other disturbance. Seismic waves are studied to locate and understand earthquakes and to determine the structure of Earth's interior.
Body and surface wave paths from an earthquake located SSE of a station. b) Seismograms from each of the three seismometers, responding to arrivals of the body (P= compressional, S= shear) and surface (L=Love, R= Rayleigh) waves
Polarity : P-waves
Initial arrival as a compression pushes the ground up; Z-component shows an upward first motion.
Initial arrival as a dilatation pulls the ground down; Z-component shows an downward first motion
Pushes up Pulls down
Initial P-wave Radiation Pattern:
Waves radiate outward in quadrants of compression and dilatation
The Z-component seismograms for the three stations highlighted in (a).
Map view of radiation pattern for right-lateral, strike-slip fault occurring along the San Andreas transform plate boundary
First-motion information for arrival at stations indicated in (a), plotted as a focal mechanism solution .
c) Auxiliary fault interpretation of the first-motion in (a), showing that the same radiation pattern could have resulted from a left-lateral fault. d) Focal mechanism solution for (c) is exactly the same as that resulting from right-lateral fault in (a).