Transcript Earthquakes PPT - Engineering the Future Workshop

Earthquakes
Chapter 8
Earthquakes
▪ Seismology – study of earthquakes
▪ Seismologist – the scientists that study earthquakes
▪ Caused by a sudden movement between two tectonic
plates
Earthquakes
▪ Happen near the edges of tectonic plates
▪ Because tectonic plates move different ways and at
different speeds, they cause faults
▪ Earthquakes occur along faults because of the sliding
What causes earthquakes?
▪ Stress builds up as plates push, pull, or slip past each
other and causes rock to deform
▪ Deformation – change in the shape of a rock due to stress
▪ The rock in plates moving past each other deforms in
one of two ways
▪ Plastic deformation – rock stretches like a rubber band;
forces push opposite directions but rock doesn’t break
▪ Elastic deformation – rock stretches, but only so far before it
breaks. Energy is released and rock returns to its normal
shape.
▪ Elastic rebound – sudden return of elastically deformed rock to
its normal shape
Faults at Plate Boundaries
Plate Boundary
Transform
Convergent
Divergent
Major Fault Type
Strike-Slip
Reverse
Normal
Faults on Plate Boundaries
▪ Earthquake zones – places where a large number of faults are located
▪ San Andreas Fault Line!
▪ Sometimes earthquakes happen at faults in the middle of tectonic plates
Divergent – blocks of crust
pulled away; normal faults
Convergent – blocks of crust
pushed together; reverse
faults
Transform – blocks of crust
slide horizontally; strike-slip
faults
How Earthquake Waves Travel
▪ Seismic Waves – waves of energy that travel through the Earth after
an earthquake
▪ Body waves – seismic waves that travel through Earth’s interior
▪ Surface waves – travel along Earth’s surface
How Earthquake Waves Travel
▪ P waves – pressure waves, primary waves; travel through solids,
liquids, gases
▪ Travel FASTER than S waves, always the first to be detected
▪ S waves – shear waves, secondary waves; cannot travel through
liquids
▪ Travel SLOWER than P waves
▪ Move side to side, shear/stretch rock sideways
How Earthquake Waves Travel
▪ Surface Waves
▪ Travel along Earth’s surface, mostly in the upper few kilometers
▪ Two types of waves:
▪ Up and down and around
▪ Back and forth
▪ More slowly and cause more destruction
Section 8.1 Review!
▪ Page 229
▪ 1-11
Section 8.2
Earthquake Measurement
Locating Earthquakes
▪ Seismographs – instruments that record seismic waves
▪ Seismogram – tracing of earthquake motion by a seismograph
Locating Earthquakes
▪ After an earthquake, different seismograms from around the world
compare the arrival times of P and S waves to locate where the
earthquake happened
▪ Epicenter – point on Earth’s surface above an earthquake’s starting
point
▪ Focus – point inside the Earth where an
earthquake starts
Locating Earthquakes
▪ S-P Time Method
▪ Using different seismograms on the same earthquake, scientists
can determine where an earthquake originated
▪ Measure the seconds between the first P wave and the first S
wave
▪ The seconds are converted to a distance, which
shows how far the epicenter was from the
seismograph
Earthquake Strength
▪ Magnitude – measure of the strength of an earthquake
▪ Richter scale – created by Charles Richter in 1930s
▪ Measures the ground motion from an earthquake and adjusts for
distance to find its strength
▪ For each unit, ground motion is 10x larger
Earthquake Strength
Magnitude (Richter Scale)
Effects
1.0
Detected only by seismographs
2.0
Hanging object swing
3.0
Feels like a truck passing by
4.0
Breaks windows, small things fall
5.0
Furniture moves
6.0
Damage to structures
7.0
Cracks in earth, underground pipes cracked, buildings
displaced from foundations
8.0
Bridges destroyed, buildings toppled over
>9.0
Complete damage, waves visible moving through earth
Earthquake Strength
▪ Intensity – a measure of how much an earthquake is felt by
people and the amount of damage caused by the
earthquake
▪ Mercalli Intensity Scale – uses Roman Numerals I through
XII to indicate the intensity of an earthquake
▪ XII – total destruction of an area
▪ One earthquake would have different intensities in different areas
Section 8.3
Earthquakes and society
Earthquake Hazard
▪ Earthquake hazard – how likely an area is to have damaging
earthquakes in the future
▪ Determined by past and present seismic activity
Earthquake Forecasting
▪ Strength and Frequency
▪ The strength of earthquakes is related to how often they occur!
Worldwide Earthquake Frequency
Magnitude of Earthquake
Average Number per Year
8.0 or higher
1
7.0-7.9
18
6.0-6.9
120
5.0-5.9
800
4.0-4.9
~6,200
3.0-3.9
~49,000
2.0-2.9
~365,000
Earthquake Forecasting
▪ Gap hypothesis – sections of active faults that have had few
earthquakes in the past are likely to have strong earthquakes in the
future
▪ Seismic gap – areas along a fault where few earthquakes have
occurred
Earthquakes and Buildings
▪ Retrofitting – making older structures more earthquake
resistant
Retrofitting
▪ Mass damper – weight placed in the room of a building.
Motion sensors detect building movement in an
earthquake and send information to a computer. The
computer moves the weight to counteract the building’s
movement.
Retrofitting
▪ Cross braces – steel beams placed between floors of a
building to counteract pressure that pushes and pulls at the
side of a building
Retrofitting
▪ Active tendon system – work like the mass damper in the
roof; a computer moves a large weight in the bottom of a
building to counteract movement
Retrofitting
▪ Flexible pipes – prevent waterlines and gas lines from
breaking; pipes can twist and bend without breaking
Retrofitting
▪ Base isolators – shock absorbers made of layers of rubber
and steel
Get Prepared!
▪ Make an action plan for an earthquake.
▪ What would you do beforehand to prepare for an earthquake?
▪ What would you do during an earthquake to protect yourself?
▪ What would you do after an earthquake to recover?