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ISNS 4359
EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 4. Faults & Seismic Waves
Earthquake (EQ) - sudden (sec.~min.) shaking of the ground due to two sides of a fault sliding past one other. EQs release
elastic energy stored in rocks and generate seismic waves. Elastic waves radiate outward from the "source" and vibrate the
ground. A fault is a large fracture in rocks, across which offset has occurred [fracture=joint=crack].
SEISMOLOGY [meaning ‘the study of earthquakes’]- initiated in 1755 by documentation of Lisbon, Portugal & 1783
Calabria, Italy Earthquakes. Poisson (1831) developed theory of elastic wave transmission in solids. Hopkins (1847)
suggested earthquakes consisted of elastic waves. John Milne (1880) began recording earthquakes and had seismograph
stations on 6 continents by 1900. Richard Oldham (~1900) distinguished P & S waves. Seismograms record ground motion
at a specific location (in 3 directions: X-Y-Z). [Greek, seismos=earthquake].
The Earth is constantly undergoing and responding to Stresspressure=force per unit area (compressional, tensional, shear).
Strain  defines the changes of size and shape (1-, 2-, 3-dimensional deformation) of a body resulting from stress.
Earth rocks behave elastically or plastically. In an elastic substance, stress is proportional to strain, materials return to
their original shape after being stressed. In a plastic (ductile) substance, under a given stress, strain is not constant
but is dependent on time; deformation is permanent (material does not return to original shape after being
stressed). If the stress is large enough (the elastic limit is reached), brittle failure (fracturing) will occur, suddenly
releasing stress and producing elastic waves, which travel through the Earth (an earthquake).
Tectonic earthquakes are initiated on faults. The EQ hypocenter or focus is the point or zone where a rupture initiates and
offset is greatest, usually below the surface. The point on Earth's surface directly above the hypocenter is the epicenter. The
amount of offset that occurs across a fault during an earthquake varies widely from less than a cm to greater than several
meters. The area of the fault that ruptures increases with earthquake magnitude, for example: M4~1.2x1.2 km; M6~10x10 km;
M8~300x20 km. Most earthquakes occur within the brittle crust and mantle at depths less than 30-50 km, with the abundance
generally decreasing with increasing depth, although events as deep as 300-700 km can occur, typically in subduction zones.
Most shallow earthquakes occur on active faults, those with recent offset in the last 10,000 years. Inactive faults do not have
observed earthquakes or evidence of slip in the last 1.6 million years. However, some faults have large earthquakes only once
every thousands of years. Reactivated faults form when movement occurs along formerly inactive faults due to a change in
stress. Deformation in the New Madrid seismic zone in the central USA is an example of fault reactivation. Rift structures
that formed about 500 million years ago have been responding to recent stresses in the mid-continent.
Orientation (attitude) of fault planes is defined by two parameters: strike & dip.
Strike - (line) intersection of a plane with the horizontal.
Dip – direction is measured perpendicular to strike: angle (0-90°) between the plane and horizontal.
Hangingwall block is above the fault, Footwall block is below the fault.
Fault Style reflects the type of forces pushing (compressional), pulling (tensional), or shearing a region.
Normal faults (hanging wall moves down) typically occur in regions undergoing stretching, and on the continents in
relatively high plateaus (Basin & Range Province, W N America) or in Rift valleys (Rio Grande, East Africa).
Reverse faults (hanging wall moves up) result from compressive forces squeezing a region, commonly in mountain
ranges. The largest earthquakes are generally low-angle reverse (shallow-dipping, thrust) faults in subduction zones.
Strike-slip faults indicate neither extension nor compression, but shear resulting from blocks sliding past each other, such
as the San Andreas system where part of coastal CA is sliding to the NW relative to the rest of North America.
Fault Slip describes the relative motion of the two fault blocks; 3 types: dip-slip (all slip is parallel to dip),
strike-slip (all slip is horizontal, parallel to stike), or oblique-slip (combination of strike-slip & dip-slip)
Stress
Tensional
Dominant Fault Style
Normal
Dominant Plate Boundary
Divergent / Rift / Spreading Center
Map View Relative Motion
 (=fault)

Compressional
Reverse
Convergent / Subduction / Collision
Shear
Strike-Slip
Conservative / Strike-Slip / Transform
 or 
Rightlateral
Leftlateral
A wave is a disturbance that transfers energy through a medium.
Seismic waves are generated by many different processes: Meteor Impacts, Earthquakes, Volcanoes, Explosions
(especially nuclear bombs), Wind (severe weather), Planes (especially supersonic), Jumping People, Vehicles.
Waves have measurable quantities: amplitude (height of peak),
period (time between peaks) = 1/frequency, frequency units: 1 Hertz= 1 wave per sec, and
wavelength (peak to peak distance measured at a given time = speed * period).
Seismic waves have many frequencies (~0.0001-20 Hertz), just as white light and sound waves have many frequencies.
Visible wavelengths: Red-0.65 microns, OYBGI Violet-0.40 microns.
Other parts of the electromagnetic spectrum: X-rays-0.3 nm, FM radio & TV- 2-12 m, AM radio-30 meters.
Audible sounds: ~0.02-15 meters (frequency~20-20,000 Hertz; velocity in air~0.34 km/sec).
Five most important types of seismic waves:
Body waves (pass through the Earth and radiate outward like a sphere)- 0.5-20 Hertz frequency
1. P-waves Compressional (primary, push-pull, longitudinal) analogous to sound waves in air.
[velocity Vp=1.5 km/sec (water), ~5-6 km/sec (crust), ~8-13 km/sec (mantle), ~17 km/sec (diamond)]
2. S-waves Transverse (secondary, shear, shake) [Vs=2-3 km/sec (crust), ~4-6 km/sec (mantle)] rotational, no volume
change. P-waves travel about twice as fast as S waves.
Surface waves (shallow waves created by body waves disturbing the surface, analogous to ripples on a pond) - lower
velocity, higher amplitude, lower frequency (0.005-0.1 Hertz), longer periods (10-200 sec) than body waves.
3. R-waves Rayleigh waves (advance in a backward-rotating elliptical motion) VR ~3.5 km/sec (crust) (freq.=0.04/sec)
4. L-waves Love waves (transverse advance in a horizontal plane) VL ~4 km/sec (crust)
5. Free oscillations motions of the Earth as a whole; ringing like a bell.
P-waves: Can travel through any material-solid, liquid or gas. Wave motion is in the direction of wave propagation
(usually the smallest amplitude waves). Velocity depends on density and elastic constants of material through which the
wave travels (rigidity or shear modulus & bulk modulus).
S-waves: Can only travel through solids, not through liquids or gases. Wave motion is at right angles to the direction of
wave propagation. An S-wave with all vertical particle motion = SV; one with all horizontal motion is SH.
S-waves can't travel through fluids because the rigidity () is zero, Vs is zero [Vs=(/)1/2].
P-waves always move faster than S-waves. Both P & S waves are transmitted through the crust & mantlesolid
Love waves produce a transverse (side to side) motion at right angles to the direction of advance; move only through
solids (not liquids or gases). Rayleigh waves advance in a backward-rotating elliptical motion; travel only along the free
surface of an elastic solid or liquid; amplitude decreases with depth below surface; move slower than Love waves.
As waves move, they can experience refraction, reflection, dispersion, diffraction, and attenuation. When a wave meets a
discontinuity surface part will be reflected and part refracted (bent). Every reflection or refraction generates additional waves,
producing an incredibly complex situation. Surface waves are dispersive - different periods travel at different velocities. The
longer the distance a wave travels, the more it is spread out (dispersing the energy). Usually, the long periods arrive first because
they are sensitive to the higher velocities deeper in Earth. “Beachballs” Moment tensor plots show fault type & stress orientation
http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/faults.html