Where do earthquakes tend to occur?
Earthquakes can occur anywhere, but they tend to occur on and near tectonic
plate boundaries.
San Andreas Fault, California
What is a fault?
A fault is a break or crack in Earth’s crust along which rocks shift their position.
What is seismology?
Seismology: a science that deals with earthquakes and with artificially produced vibrations of Earth
• Seismograph: an instrument that records vibrations in the ground.
• Seismogram: a tracing of earthquake motion that is recorded by a seismograph.
~300 miles long!
• shaking lasted ~1 minute
• felt from southern Oregon to central Nevada, and south of Los Angeles (maybe in SD?)
1906 San Francisco Earthquake M 7.7 to M7.9?
1906 San Francisco Earthquake M 7.7 to M7.9?
Massive fires followed the earthquake and burned for several days
• Magnitude: a measure of its strength of an earthquake.
• Intensity: (in Earth Science) the amount of damage caused by an earthquake.
1906 San Francisco Earthquake M 7.7 to M7.9?
Casualties and Damage
Casualties 3000+
People left homeless 225,000
Buildings destroyed 28,000
At the plate boundary
Elastic Rebound: the sudden return of elastically deformed rock to its undeformed shape
Aftershocks…
• Aftershocks are smaller earthquakes that occur after a larger earthquake
– Happen near the mainshock– Are smaller magnitudes than the mainshock
Seismograph: The instrumentApparatus to measure and record vibrations
Seismogram: The actual wiggleRecord of an earthquake recorded by a seismograph
The difference between a “Seismograph” and a “Seismogram”
FuturePast
Seismic Waves• Body Waves: in geology, a seismic wave that
travels through the body of a medium.
• Surface Wave: in geology, a seismic wave that travels along the surface of a medium and that has a stronger effect near the surface of the medium that it has in the interior.
P-waves• primary wave• compression wave• fastest of the seismic
waves• can travel through solids,
liquids, and gases
Surface waves• 2 types: Love waves and
Rayleigh waves• Slowest seismic waves• May cause the greatest
damage in an earthquake
S-waves• secondary wave• shear wave• second-fastest seismic wave• can only travel through
solids
Seismic wavesSeismic waves
Surface Waves• Love Waves: cause rock
to move side to side and perpendicular to the direction in which the waves are traveling.
• Rayleigh Waves: cause the ground to move with an elliptical, rolling motion.
Travel time from San Diego to Los Angeles
P-waves 25 seconds 25 seconds
S-waves 41 seconds 41 seconds
Surface waves 50 seconds50 seconds
Seismic Waves and Earth’s Interior• Shadow Zone: an area on
Earth’s surface where no direct seismic waves from a particular earthquake can be detected.
• Shadow zones exist because the materials that make up Earth’s interior are not uniform in rigidity.