Topic – Earthquakes and their causes.
Essential Question: What causes earthquakes and how can they be predicted?
Earthquakes
• Earthquakes are natural vibrations of the ground caused by plate ___________ in
Earth’s crust or by volcanic eruptions
• In some instances a single earthquake has
killed more than 100,000 _________ and destroyed entire cities.
• Anyone living in an area prone to earthquakes should be aware of the potential
danger and how to minimize the damage that they cause.
Stress and Strain
• Most earthquakes occur when rocks fracture, or break deep within Earth
• Fractures form when stress (forces acting on a material) ____________ the
strength of the rock
• 3 kinds of stress:
– Compression – decreases the volume of a material
– Tension – pulls a material apart
– Shear – causes a material to twist
Strain – the deformation of materials in response to stress
- Rocks fail when stress is applied too quickly or stress is too great.
Fault - the _______________ or system of fractures along which movement occurs.
• The surface along which the movement takes places is called the fault plane.
• 3 basic types of faults:
– Reverse fault – form as a result of horizontal compression
– Normal fault – caused by _____________ tension
– Strike-slip fault – caused by horizontal shear
Earthquake Waves
• The vibration of the ground during an earthquake are called seismic waves.
• Every earthquake generates 3 types of seismic waves:
– Primary waves (___________) – causes rock particles to move back and
forth as it passes
– Secondary waves (S-waves) causes rock particles to move at right angles
to the direction of the wave
– Surface waves – causes rock particles to move both up and down and side
to side.
– Surface waves travel along Earth’s surface and produce the most serious
damage.
– P-waves and S-waves (___________ waves) pass through Earth’s interior
– The first waves generated by an earthquake spread out from the point of
failure of rocks
Focus – Point where an earthquake originates, usually several km below the surface
Epicenter – Point on Earth’s surface directly above the focus is the earthquake’s.
Liquifaction – when earthquake vibrations make the loose soil behave like a liquid.
Measuring Energy Released and Locating Earthquakes
• More than one million earthquakes occur each year!
• More than ____% are not felt and cause little to no damage
• The ones that make the news are major seismic events that cause much damage.
Locating the Epicenter of an Earthquake
In order to determine the ________________ of an earthquake, the earthquake needs
to be recorded on three different seismographs that are at significantly different
locations.
The other piece of information needed is the __________ it takes for P-waves and Swaves to travel through the Earth and arrive at a seismographic station.
Triangulation - A mathematical method for locating the epicenter of an earthquake
using __________ or more data sets from seismic stations.
Seismograph - Earthquake monitoring instrument that records the ____________ waves
of the earthquake.
Earthquake Magnitude & Intensity
• The amount of energy released during an earthquake is measured by its
magnitude.
• Measured using the _____________Scale – based on the size of the largest
seismic waves made by the quake
10-Fold (logarithmic) Scale:
– meaning seismic waves of a magnitude-8 earthquake on the Richter scale
are 10 times larger than a magnitude-7 and 100 times larger than a
magnitude-6.
•
Most ____________ use the moment magnitude scale – takes into account the
size of the fault rupture, the amount of movement along the fault, and the rock’s
stiffness
– Moment magnitude values are estimated from the size of several types of seismic
waves produced by an earthquake.
• Another way to assess earthquakes is to measure the amount of damage.
• Modified ____________ Scale - measures the amount of damage done to the
structures involved and is used to determine the intensity of an earthquake.
• uses the Roman numerals I to XII to designate the degree of intensity.
• Specific effects or damage correspond to specific numerals; the higher the
numeral, the worse the damage.
Depth of Focus
• Another factor that determines a quake’s intensity is the depth of the quake’s
focus
– Can be classified as __________, intermediate, or deep
– Deep-focus = smaller vibrations at epicenter
– Shallow-focus = larger vibrations at epicenter
• A shallow-focus, magnitude-6 quake will have greater intensity than a deepfocus, magnitude-8 quake
• _____________ quakes with high intensities are almost always shallow-focus
quakes
Summary
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