05-07-2013, 12:02 PM
Earthquakes
Earthquakes.ppt (Size: 3.1 MB / Downloads: 150)
Earthquakes
Shaking of earth due to movement of rocks along a fault.
Rocks under stress accumulate strain energy over time.
When stress exceeds strength of rocks, rock breaks.
Strain energy is released as seismic waves. The longer that energy is stored up and is maintained without release, the more likely that a strong earthquake will occur.
Types of seismic waves
1. Body waves -- travel through interior
2. Surface waves -- travel on surface of earth
Specific Body Waves
Primary or "P" Waves: Primary waves Highest velocity
Causes compression and expansion in direction of wave travel.
Secondary or "S" Waves: Secondary or shear waves
Slower than P waves but faster than surface waves.
Causes shearing of rock perpendicular to direction of wave propagation
Cannot travel through liquids
Surface Waves or "Love" (“L”) Waves
Cause vertical & horizontal shaking
Travel exclusively along surface of earth
Determining the location of an earthquake
First, distance to earthquake is determined.
1. Seismographs record seismic waves
2. From seismograph record called the seismogram, measure time delay
between P & S wave arrival
3. Use travel time curve to determine distance to earthquake as function
of P-S time delay
Now we know distance waves traveled, but we don't know the direction from
which they came.
We must repeat the activity for each of at least three (3) stations to
triangulate a point (epicenter of quake).
Plot a circle around seismograph location; radius of circle is the distance to the
quake.
Quake occurred somewhere along that circle.
Do the same thing for at least 3 seismograph stations; circles intersect at epicenter. Thus, point is triangulated and epicenter is located.