Transcript Earthquakes 2

EARTHQUAKES:
WHY? AND HOW?
EARTHQUAKES
sudden movement or shaking of the Earth
• Caused by plate tectonic stresses
• Located at plate boundaries
• Resulting in breakage of the Earth’s brittle crust
PLATE TECTONIC STRESSES
•
Plate boundaries and faults (= cracks where plate sections
are moving in different directions) cause friction as plates move
•
Plates in a fault zone have STICK-SLIP motion
–
Periods of no movement (stick)
and fast movement (slip)
–
Energy stored as plates stick,
–
Energy released as plates slip
EARTHQUAKE DAMAGE
• Landsides
• Building damage
• Liquefaction
LIQUEFACTION
when a solid (sand and soil) becomes saturated
with water and acts like a heavy liquid
• Results in a loss of soil strength & the ability of the soil to
support weight
EARTHQUAKE DAMAGE
Most caused by SURFACE waves (arrive last)
EARTHQUAKE INTENSITY
measures damage to man-made structures at
certain location
Modified Mercalli scale= measurement of damage to structures
• From I to XII
(Roman numerals)
• Descriptive, changes with
distance from epicenter
• Can change from location
to location
What you need:
• Your senses!
ISOSEISMIC MAPS
show the distribution of intensities
• Connects areas of with the
same Modified Mercalli
number
• Areas are colored
according to Modified
Mercalli number
Loma Prieta Earthquake
1989
EARTHQUAKE WAVES
• FOCUS = place deep within the Earth and along the fault where
rupture occurs
• EPICENTER = geographic point
on surface directly above focus
• SEISMIC WAVES produced by the release of energy
– move out in circles from the point of rupture (focus)
– 2 types: surface & body (travel inside & through earth’s layers)
• P waves: back and forth movement of rock; travel thru solid, liquid, gas
• S waves: sideways movement of rock; travel thru solids only
EARTHQUAKE WAVES
Seismographs record earthquake waves
Seismograms show:
• Amplitude of seismic waves (how much rock
moves or vibrates)
• Distance to the epicenter
• Earthquake direction
EARTHQUAKE WAVES
• 3 types of seismic waves show up on seismogram
– P waves: shake earth in same direction as wave;
travel thru solid, liquid, gas
– S waves: Shake earth sideways to wave direction;
travel thru solids only
– Surface waves: circular movement of rock;
travel on surface – cause most damage!!
EARTHQUAKE WAVES
P waves move through solids & liquids
S waves move through solids only!!!
EARTHQUAKE WAVES
Body
P waves
S waves
waves
AKA
Primary (1st to arrive)
Secondary (2nd to arrive - larger)
Longitudinal, Compression
Transverse, Shear
Moves
through
all states of matter
(solid, liquid, gas)
Can go through solids only
Movement
of rock
back and forth movement of rock
• push/pull or compression/stretch out
• Like slinky down stairs
Move sideways
•
•
Vibration is same as the direction of
travel
perpendicular to direction of wave
travel
Like snake
EARTHQUAKE WAVES
Lets test your understanding!!
Is this a P or an S wave?
S Wave
P wave!
EARTHQUAKE MAGNITUDE
measures the size of seismic waves 
the energy released by the earthquake
Richter scale=measurement of energy released
based upon wave amplitude (size of vibration)
• <2 to ~10
• Amplitude of wave goes up
by 10 (Logarithmic scale)
What you need:
• Amplitude (size of vibration = wave height)
• Time between arrival of 1st P and 1st S waves
HOW TO READ SEISMOGRAMS
P & S (body waves) move through earth & arrive first
• P & S waves used to calculate magnitude of earthquake
• Amplitude = height of wave (how much the rock moves; size of vibration)
MERCALLI VS. RICHTER
Let’s try a simulation
•
Simplified Epicenter Triangulation
•
P waves generally travel between 5.95 and 6.75 kilometers per second in the crust,
depending on compressibility, rigidity, uniformity, and density of the materials
traversed.
•
S waves tend to move at velocities between 2.9 and 4.0 km/sec in the crust.
•
Rayleigh waves travel somewhere between 2.7 and 3.7 km/sec.
•
Time and distance can be graphed for each of the wave types. By comparing the time
between pairs of wave types on the graph, a rough estimate of the distance to the
earthquake focus can be made. In each of the following sets of problems, you need
to convert the time difference given between the arrivals of each of the two wave
types into kilometers. To do this, measure the difference in time on the Y axis,
marking it on a ruler or piece of paper. Then, find the distance on the X axis where
the two relevant curves diverge by just that time difference on your marked paper or
•
In each of the following sets of problems, you need to convert the time difference
given between the arrivals of each of the two wave types into kilometers.
•
To do this, measure the difference in time on the Y axis, marking it on a ruler or piece
of paper. Then, find the distance on the X axis where the two relevant curves diverge
by just that time difference on your marked paper or ruler. Give your answer rounded
to the nearest 250 km.
Simplified Graph of P, S, and R Waves through Time
Calculate together
• Notebooks…… see handout