earthquakes. earthquake: the shaking or vibrating of earth caused by a release of energy
TRANSCRIPT
EARTHQUAKES
earthquake: the shaking or vibrating of Earth caused by a
release of energy
elastic rebound: as two plates move past each other they get stuck and then slip; the plates suddenly move, release energy and then snap back into place
epicenter: point on the earth’s surface directly above the focus
focus: point at which rupture occurs along the fault
If you hear about an earthquake on the news,
you will always be told the location of the epicenter. If
you are told the depth of the earthquake, that is the
focus.
fault: a break in the
earth’s surface where
movement has
occurred
TYPES OF FAULTS
normal fault: blocks of rock move apart- found at divergent boundaries
reverse fault: blocks move toward one another
- found at convergent boundaries- also called thrust fault
strike slip: rocks move horizontally past one another
- found at transform boundaries
seismology: the study of earthquakes
seismograph: device that measures earthquakes
seismogram: earthquake graph made by a seismograph
Seismic Waves
As energy is released from an earthquake, it is distributed in waves
Body Waves: travel through the interior of the Earth
Surface Waves: travel along Earth’s surface
Types of Body Waves
P-wave (primary wave):- longitudinal wave- can travel through any material- 1st to arrive (6-7 km/s)- shortest period, smallest amplitude
S-wave (secondary wave)
- transverse wave- cannot travel through liquids (outer core)- 2nd to arrive (3.5 km/s)
S waves cannot travel through the Earth. P waves refract as they pass through the
liquid part of the core.
Surface Waves
- most damaging to structures- largest amplitude; slowest- last to arrive (2.5 km/s)
Types of Surface Waves
R- WavesRayleigh WavesMake the ground ripple up and
down
L-WavesLove WavesMake the ground ripple side to side
Determining the Location of the
Epicenter
epicenter
Shallow earthquakes tend to occur along transform
boundaries.
Deep earthquakes tend to occur along subduction zones.
* the deeper an earthquake, the less damage it causes at
the surface
How do we measure
earthquake magnitude?
Richter Scale
- measures amplitude of largest seismic wave 100 km from the epicenter
- wave height is ten times greater for each step that increases
The energy released is 33 times greater for each step that is increased
How many magnitude 6 earthquakes would you need to
release the same amount of energy in one magnitude 8
earthquake?
Richter Scale values
> 8 = total destruction (about 1/year)7-7.9 = bad (18/year)6-6.9 = moderate-serious (120/year)5-5.9 = felt by everyone (minor damage) (800/year)4-4.9 = not felt by everyone (6,000/year)< 3.9 very small (about 1 million per year)
Moment Magnitude Scale
-measures energy released-used by scientists- factors in amount of movement and rigidity of rocks
Comparison:
1964 Alaskan EQRichter- 8.4, Moment – 9.2
1906 San FranciscoRichter – 8.3, Moment – 7.9
Mercalli Intensity Scale
-measures damage done by the earthquake- scale of I – XII
I – felt by few people
V – felt by everyone (dishes broken)
VIII – slight damage to structure
X – wooden structures destroyed
XII – total destruction; see waves at surface
Earthquakes caused by HumansHoover Dam –
600 tremors over the 1st 10 years; some as high as magnitude 5
Rocky Mountains – In the 1960’s the US Army drilled a well
and injected liquid waste into the mountains
This lubricated the fault and earthquakes began
When the Army stopped injecting the waste, the earthquakes stopped too
Earthquake Prediction
long term years, not days or hours
short term Good Luck!
In order to study earthquake predictions, you need to predict an
earthquake.
(in order to set up equipment, etc.)
Recurrence Interval
- how often an earthquake of a specific magnitude occurs
Seismic Gap Method
-looking at gaps along faults where there is no
seismic activity
Earthquake Damage
1. Ground Displacementexamples: 1906 San Fran – 7 m of displacement 1964 Alaska – 12 m of displacement
(vertical)1989 Loma Prieta 2.2 m1994 Northridge 1 foot
Alaska, 1964
2. Landslides
1970 Peru – killed 70,000 people
3. Liquefaction
- sediments act as fluids
- Kobe, Japan
4. Tsunami- seismic sea waveex. Sumatra 2004
5.Fireex. Tokyo, San
Francisco
1906 – San Francisco
6. Valley Fever
- disease-sewage lines break- dust associated with fires causes bronchial problems
Most Devasting EQs
1500’s China 850,000 dead1923 Tokyo 143,000 dead1976 China 250,000 dead1985 Mexico 10,000 dead1995 Kobe, Japan 5,000 dead
$100 billion in damages
Northridge, CA 1994
Northridge 1994
Northridge 1994
Alaska, 1964
San Francisco Bay Bridge, 1989
Kobe Japan, 1995