Chapter 11: Earthquakes
I. What is an earthquake?
A. vibration of the Earth produced by the rapid release of energy from rocks
that rupture because they have been subjected to stress that exceeds their
1. energy released radiates in all directions from its source, the focus and
energy waves propagate through the earth in the form of seismic
2. sensitive instruments called seismometers record the event
B. usually caused by sudden movements on faults; don’t happen at the
surface of the earth because there’s not enough pressure. Less frequently,
due to fractures on the ground, explosions etc. Large earthquakes usually
leave some type of scar in center of earth (faults)
II. Basic Terminology
A. focus point where earthquake initiates (hypocenter). 3D location latitude,
B. epicenter point on the earth’s surface directly above the epicenter as
energy travels away from the focus, it gets larger and larger and less
intense (called wave fronts). Epicenter is not superimposed on the fault at
surface; instead, its above the focus at depth
C. fault scarp break in fault (cliff) in relief, happens when they displace and
break earth’s surface
III. How faults generate earthquakes
A. elastic rebound mechanism for earthquakes was first explained by HF
Reed. Rocks on bot sides of an existing fault are deformed by tectonic
forces; rocks bend and store elastic energy; frictional resistance holding
rocks together is overcome. Earthquake occurs when stored stress exceeds
strength of the fault
B. earthquake mechanism slippage at the weakest point (focus) occurs.
Vibrations (earthquakes) occur as the deformed rock “springs back” to its
original shape. Earthquakes most often occur along existing faults
whenever the frictional focus on the fault surfaces are overcome
IV. Foreshocks and Aftershocks
A. adjustments that follow a major earthquake often generates smaller
earthquakes called aftershocks
B. small earthquakes, called foreshocks often precede a major earthquake by
days, or in some cases, by years. Earthquakes never happen in isolation
A. seismology study of earthquake waves
B. seismographs instruments recording seismic waves. Record movement of
earth in relation to a stationary mass on a rotating drum or magnetic tape.
VI. Seismic Waves
A. Body waves travel through earth’s interior 1. P waves pushpull (compress and expand) by changing volume of
intervening material. Faster velocity than Swaves. Travel through
solid, liquid and gas. Longitudinal.
2. Swaves shear motion at right angles to their direction of travel.
Travel only through solids. Transverse (slower).
B. Surface waves cause the most damage due to undulations in ground
1. Rayleigh waves
2. Love waves
VII. Locating Earthquakes further we are from epicenter, longer lag time between
P and S waves.
•locating the source of