GEOL 1001 : A EXAM 3 Lecture Materials
3/22/12 Chapter 11
- An earthquake is the vibration of Earth, produced by the rapid release of energy.
o Energy released radiates in all directions from its source, the focus.
o Energy is in the form of waves.
o Sensitive instruments around the world record the event.
- Earthquakes and faults
o Movements that produce earthquakes are usually associated with faults.
- Elastic Rebound
o Rocks on both sides of an existing fault are deformed by tectonic forces.
o Rocks bend and store elastic energy.
o Frictional resistance holding the rocks together is overcome.
▪ Earthquake mechanism
• Slippage at the weakest point (the focus) occurs.
• Vibrations (earthquakes) occur as the deformed rock “springs back” to its
original shape (elastic rebound).
▪ Earthquakes most often occur along existing faults whenever the frictional forces
on the fault surfaces are overcome.
- Foreshocks and aftershocks
o Adjustments that follow a major earthquake often generate smaller earthquakes called
aftershocks.
o Small earthquakes, called foreshocks, often precede a major earthquake by days or, in
some cases, by as much as several years.
o San Andreas is the most studied fault system in the world
o Displacement occurs along discrete segments 100 to 200 km long.
▪ Some portions exhibit slow, gradual displacement known as fault creep.
▪ Other segments regularly slip, producing small earthquakes
o Displacements along the San Andreas Fault
▪ Still other segment store elastic energy for hundreds of years before rupturing in
▪ great earthquakes
• Process as stick-slip motion
• Great earthquakes should occur about every 50 to 200 years along these
sections.
- Seismology
o Seismographs are instruments that record seismic waves.
▪ Records the movement of Earth in relation to a stationary mass on a rotating
drum or magnetic tape.
o Seismographs
▪ More than one type of seismograph is needed to record both vertical and
horizontal ground motion
▪ Records obtained are called seismograms
o Types of seismic waves
▪ Surface waves
• Travel along outer part of the Earth
• Complex motion
• Cause greatest destruction
• Exhibit greatest amplitude and slowest velocity
• Waves have the greatest periods (time interval between crest)
• Often referred to as long waves, or L waves
▪ Body Waves
• Travel through Earth’s interior
• Two types based on mode of travel
• Primary (P) waves
• Secondary (S) waves
o Primary (P) waves
▪ Push-pull (compress and expand) motion changing the
volume of the intervening material
▪ Travel through solids, liquids, and gases
▪ P waves travel faster than (S) waves.
o Secondary (S) waves
▪ “Shake” motion at right angles to their direction of travel
▪ Travel only through solids
▪ Slower velocity than P waves
▪ Slighter greater amplitude than P waves
3/27/12
Continuing chapter 11-
- The three different types of seismic waves move at different speeds
o Travel-time curves
▪ The S-P time interval depends on the distance from the epicenter
o Locating the epicenter of an earthquake
▪ A circle with a radius is equal to the distance to the epicenter is drawn around
each station
▪ At least 3 stations are needed to find the epicenter
▪ The point where all three circles intersect is the earthquake epicenter
o Measuring the size of Earthquakes
▪ Two measurements that describe the size of an earthquake are:
• 1. Intensity- a measure of the degree fo earthquake shaking at a given
locale based on the amount of damage
• 2. Magnitude estimates the amount of energy released at the source of
that energy
o Magnitude scales
▪ Richter scale
• Based on the amplitude of the largest seismic wave recorded
• Accounts for the decrease in wave amplitude with increased distance
o Earthquakes depths
▪ Earthquakes originate at depths ranging from 5 to nearly 700 kilometers
▪ Earthquake foci are arbitrarily classified as:
• Shallow –surface to 70 km
• Intermediate- 70-300 km
• Deep- over 300 km
▪ Definite patterns exist
• Shallow- focus earthquakes occur along the oceanic ridge system
• Almost all deep-focus earthquakes occur in the circum-pacific belt,
particularly in regions situated landward of deep-ocean trenches
▪ Amount of structural damage attributable to earthquake vibrations depends on:
• Intensity and duration of vibrations
• Nature of the material upon which the structure rest
• Design of the structure
▪ Tsunamis, or seismic sea waves
• Result from vertical displacement along a fault located on the ocean floor
or a large undersea landslide triggered by and earthquake
▪ Can earthquakes be predicted?
• Short-range predictions
• Currently , no reliable method exist for making short-range
3/29/12 Chapter 12
- About the Earth’s interior
o The center of the Earth is about 6400km below us… the deepest well is 10km
o Earth’s interior is explored by using information from seismic waves and their passage
through the body of the Earth
o Heat inside the Earth drives the core’s geodynamo and the mantle’s convection
- Most of our knowledge of Earth’s interior comes from the study of earthquake waves
o Travel times of P (compressional) and S (shear) waves through the Earth vary depending
on the properties of the materials.
o Variations in the travel times correspond to changes in the materials encountered
- The Nature of seismic waves
o Velocity depends on the density and elasticity of the material
o Within a given layer, the speed increases with depth due to pressure forming a more
compact elastic material.
o Compressional waves (P waves) are able to propagate through liquids as well as solids.
o Shear waves (S waves) cannot travel through liquids.
o In all materials, P waves travel faster than do S waves
o When seismic waves pass from one material to another, the path of the wave is refracted
(bent).
- Layers are deined by physical properties
