A. Seismology

A. Deformation Processes

“compression” (leads to folding); “tension” (leads to stretching & faulting); & “shearing” (leads to transform faulting)

Folding:

as a result of compression, the original structure is bent & deformed
“anticline” - along the ridge of a fold the layers slope downward away from the axis of the ridge
“syncline” - along the trough of a fold layers slope upward away from the axis of the trough

Faulting:

occurs when rock strata are strained beyond ability to remain as one solid unit
they fracture & one side of the fault is displaced relative to the other side
at the moment the fault line shifts, there is an intense release of energy (an earthquake)

Types of faults (& fault terminology):

“reverse/thrust" (compression) fault - vertical movement along an inclined plane - here stress is due to compression

“strike-slip" (transform or lateral) fault - movement along fault line is horizontal - e.g., San Andreas Fault

B. Seismology and Earth's Interior

(1) Terms

“focus” - the subsurface area along the fault plane where the quake (seismic wave) is initiated

“foreshock” - precedes the main shock - may be related to the breaking of a few asperities prior to the main quake

(2) Basic Principles

as rock is deformed it bends, storing elastic energy
once the rock is strained beyond its breaking point it ruptures releasing the stored-up energy in the form of earthquake waves
the waves occur as the rock elastically returns to its original shape

(3) Wave Types

two main types of waves: "surface waves" - travel along the Earth's outer layer; and "body waves", which travel through the Earth's interior

two types of body waves: "primary" & "secondary" - based on their mode of travel through materials

P waves - are "push-pull" waves - they push (compress) and pull (expand) rocks in the direction the wave is traveling

solids, liquids, and gases resist a change in volume when compressed, and elastically spring-back after the force is removed; P-waves can thus move through all 3 materials

S-waves: "shake" the rock particles at right angles to the direction of wave travel

S-waves do not change the volume of the rock, but instead, change the "shape" of the rock material

Since gases and liquids (fluids) don't respond elastically to changes in shape, S-waves can't be transmitted through fluids

Surface waves: more complicated - an up-and-down motion (like ocean swells); plus a side-to-side motion similar to S-wave motion in the horizontal (very damaging to foundations)

differences in rock density and elasticity influence wave velocity. In solid rock:

P-waves travel fastest - 1.7 X S-waves; Surface waves travel at 90% the velocity of S-waves

(4) Determining Earth's Structure With Seismic Waves

the velocity of seismic waves depends on the density & elasticity of Earth's material - waves move with the greatest speed through rigid materials (e.g., faster through crystalline rocks than unconsolidated material)
in the same layer, wave speed increases w/depth, because pressure increases and squeezes the rock into a more compact material
P-waves travel through solids as well as liquids
S-waves can not travel through liquids, because unlike solids, liquids have no shear strength (they simply flow)
in all materials P-waves move faster than S-waves
when seismic waves pass from one material to another, the wave is refracted (bent); (some energy is reflected from the discontinuity)
when passing from one layer to the next waves can be: speeded-up, slowed-down, refracted, or reflected

....The Moho....

Yugoslavian seismologist, Andrija Mohorovicic in 1909 presented the first convincing evidence for layering within Earth
From seismic records, he found that the velocity of seismic waves increases abruptly below ~ 50 km depth - this boundary separates the crust from the mantle - the "Moho"

..... The P-wave Shadow Zone ....

German seismologist Beno Gutenberg noted a 35 degree zone where P-waves are absent - the "P-wave shadow zone"
the shadow zone is produced by the bending of P-waves as they enter material unlike that of the overlying mantle
since S-waves could not move through this area - the outercore - it was concluded that at least a portion of it was liquid
the greater velocities of P-waves in the inner core provided evidence that it was comprised of solid rock
in the asthenosphere (~ from100-350 km down), S waves have a lower velocity, indicating that this zone consists of hot, weak rock - a "plastic" - easily deformed

(5) Earthquake Rating Scales

“intensity scales”: classify & describe damage to terrain & structures - “Mercalli Scale” - I-XII - barely felt - to - total destruction

“magnitude scale”: “Richter Scale” - based on wave amplitude at least 100 km from epicenter - it’s open ended & logarithmic - each whole # represents a ten fold increase in measured wave amplitude

(6) Occurrence of Earthquakes