Earthquakes
Earthquakes
Natural vibrations of the ground caused by movement at faults
Faults: Fracture in the Earth’s crust Faults form when rocks break
deep within the Earth’s surface due to stress
In conclusion…
Earth’s rocks deform when stress is exerted on them. If the stress exceeds a certain limit, the rocks fracture to form faults. Movement along faults causes most earthquakes.
Earthquake Waves
Seismic Waves – The vibrations of the ground during
an earthquake
3 Types of Seismic wavesPrimary waves (aka P waves)
Push/pull rocks same way waves are traveling
Travels through Earth’s interior
Secondary waves (S waves) Move perpendicular to the
direction of the waves Travel through interior of Earth
Surface waves Move in 2 directions
Like the ocean and like a snake at the same time
Travel on the surface of Earth
Some more vocabulary
Focus Point of failure of the rocks Where the earthquake startsUsually several kms below the
surface
EpicenterThe point on Earth’s surface directly
above the focus
Locating Earthquakes
There are more than 1 million earthquakes each year…
However, 90% of them are not felt by people.
The exact location of the epicenter and the time of occurrence are initially unknown.
For any distance from the epicenter, the P waves always arrive first.
The further the recording instrument is from the epicenter, the greater the difference in arrival times of the 1st P wave and the 1st S wave.
This information is used to determine the exact location of the epicenter.
Gathering data….
5 10 15 20 25
P wave
S wave
Need data from several locations: At least 3 locations
A travel-time graph is used to determine the distance from a recording station to the epicenter Based on travel time of the P and S
waves The time between when the P wave
arrives and the S wave arrives is called the S-P lag
Epicenter Distance (km)
S-P
lag (m
in)
Using the info…
Once you figure out the distance of the epicenter to the recording instrument you plot a CIRCLE on a map with the radius being the distance discovered.
Where the 3 intersect is the epicenter
Measuring Earthquakes
There are several different scales used to measure earthquakes.Measure size of seismic wavesMeasurements of the fault
Size, amount of movement, type of rock occurred in
Measure amount of damage done on surface of Earth
CD-ROM video clips
Slides on CD-ROM Types of seismic waves
369-371
Locating an epicenter 375-387
Factors Influencing Intensity
Size of seismic waves produced
Depth of the focusDistance from the epicenter
of the quake
Using Earthquakes for Info
Most of the knowledge of Earth’s interior comes from the study of seismic waves.These waves change speed and
direction when they encounter different materials.
Scientists track where they are detected and at what strength.
Understanding Stress
Fractures form when the stress exceeds the strength of the rocks
3 Kinds of Stress: a forceCompression – push motionTension – pull motionShear – twist motion
Strain – deformation in material due to stress.
Stre
ss
Strain
Elastic Deformation – not permanent
Ductile Deformation- permanent damage
Can Vary Based on Temperature!
Failure