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EARTHQUAKES
I. What is an Earthquake?
A.
An earthquake is the trembling or shaking of the ground caused by the sudden release of energy from
the rocks in Earth’s subsurface.
B.
Large releases of energy accompany:
1.
__________________________________
2.
__________________________________
3.
__________________________________
II. Causes of Earthquakes
A.
________________________________________
1. Tectonic Forces act on a rock over a long period of time
2.
Stored energy eventually exceeds the breaking strength of the rock.
3.
Rock suddenly breaks, causing an earthquake.
B.
_______________________ are the most common source of earthquakes felt by people.
1.
Fault motions don’t necessarily break the surface.
a. Some recent California earthquakes have been caused by buried thrust faults with no surface
displacement.
b. Eastern U.S. earthquakes are not associated with surface displacment.
2.
Fault motions at Earth’s surface can cause hazards.
C.
________________________________________
1.
New theory that faults are weak and only need small stress to cause and earthquake.
2.
Conflicts with traditional theory that faults are strong and very large stress is necessary to break
rocks.
D.
1.
2.
3.
4.
Brittle behavior of rocks is confined to rocks near Earth’s surface.
There is a limit to the depth at which faults can occur.
Rocks at depth are subjected to increased temperature and pressure which causes them to deform
plastically so faulting can’t occur.
But, deep quakes occur along subducting plates where the cold sinking rock should behave in a
ductile manner due to high temperature and pressure.
a. Pressure causes minerals to transform into denser forms.
b. Lab results show bodies of the new, denser minerals along fractures.
III. Seismic Waves (Vibrations caused by energy released by an earthquake)
A.
Origin
1.
a. Also called the _________________________.
b. The point of ___________________ of an earthquake.
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Earthquakes
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c. The center of an earthquake, this is the point of initial breakage and movement along the fault.
d. Seismic waves radiate outward, three dimensionally in all directions from the focus.
e. Intensity of seismic waves is greatest near the focus and decreases with increasing distance.
2.
a. The point on _________________________________________________
b. Often the location of __________________ damage.
3.
B.
of Focus (Also called focal distance or hypocenter distance)
a. The distance between the focus and the ________________________.
b. Maximum depth is about 670 km.
c. Three categories:
i Shallow Focus:
1-70 km deep
ii Intermediate Focus:
70-350 km deep
iii Deep Focus:
350-670 km deep
Types of Seismic Waves
1.
Waves
a. Seismic waves that travel within Earth’s subsurface.
b. Two Types of Body Waves
i ___________________ (Primary Wave)
(a) ______________________ Wave
(b) Rock particles vibrate back and forth ___________________________ to the
direction of wave propagation.
(c) Travel very fast (4 to 7 km/sec (9,000 to 15, 000 mph) and is the first to reach a
recording station following an earthquake.
(d) Can travel through ____________________________of matter.
ii _______________________ (Secondary Wave)
(a) ________________________-Wave generated by shearing motion.
(b) Rock particles vibrate at right angles to the direction of wave propagation.
(c) Travels _____________________ than a P Wave (2 to 5 km/sec.)
(d) Easily travels through solid rock but will not be transmitted through
________________________________________________.
2. _
Waves
a. The _______________________________waves generated by earthquakes
b. Responsible for more _______________________________ because they:
i produce more ground movement
ii travel more slowly than body waves.
c. Two Important Types (Named after geophysicists who identified them)
i _________________Waves
(a)
(b)
(c)
(d)
ESC 101: Professor Vorwald
No ____________________displacement
Ground moves ___________________________in a horizontal plane at right angles
to the direction of wave travel
The horizontal movement is responsible for knocking buildings off their foundations
and for the destruction of highway bridge supports.
Do not travel through water.
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ii ___________________Waves
(a)
(b)
(c)
IV.
Behave like rolling ________________ waves
Ground moves in an ________________________path opposite the direction the
wave passes (unlike ocean waves where the elliptical movement is the other way)
The cause great ground movement and are slower than Love waves. As a result, they
are very destructive to buildings.
Locating and Measuring Earthquakes
A.
Instruments Used to Measure Seismic Waves
1. _____________________________
a. Measures seismic waves
b. Heavy weight suspended from a frame by springs are as a pendulum remains motionless (due
to inertia) as ground vibrates the frame.
c. The weight serves as a point of reference in determining the amount of ground motion.
d. Can be configured to measure vertical or horizontal ground motion.
2.
a. A seismometer that can record ground vibrations.
b. The record is usually a squiggly line drawn on a moving strip of paper.
c. A seismogram is the paper record of the vibrations.
B.
Determining Earthquake Location
1. A minimum of ________________separate locations is required.
2. For each location
a. Determine arrival time of the P-wave and the S-wave.
b. Determine the distance to the epicenter using a travel-time curve.
c. Around each location on a map construct a circle whose radius is equal to the distance to the
epicenter for that location.
d. The circles will intersect at a common point which is the location of the epicenter.
C.
Measuring Earthquake Magnitude (Intensity)
1. Modified
Scale
a. Measures the earthquake’s _______________ on people and buildings.
b. Expressed as Roman numerals ranging from I to XII (the higher the number, the greater the
damage)
c. Can be affected by:
i Integrity of structures
ii Type of surface on which buildings are constructed (bedrock vs unconsolidated materials)
iii Subjectivity of observers
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Earthquakes
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2.
a.
b.
c.
d.
e.
3.
Scale
Developed by Charles Richter in 1935
A numerical open-ended logarithmic scale with no lower and upper limit for earthquake
intensity.
i Higher numbers indicate stronger earthquakes
ii Difference between two consecutive numbers represents increase in vibration amplitude of
10 times which is approximately 32 times the energy released.
Defined on a particular type of seismograph (Wood-Anderson seismograph) at 100 km from
the epicenter.
Based on shallow focus earthquakes in southern California.
Maximum reported earthquake is 8.6.
Scale (Moment Magnitude)
a. Currently used to determine an earthquake’s magnitude.
b. Based on:
i length of the fault rupture
ii surface area of the rupture
iii amount of rock displacement
iv tensile strength of the rock
c. The media often still uses Richter Scale for this method
V. Earthquakes in the United States
A.
Western States
1. Location of most U.S. earthquakes
2. Alaskan earthquakes are mostly below the Aleutian Islands
B.
East of the Rocky Mountains
1. Not as common
2. Usually smaller and deeper
3. May be associated with deeply buried inactive faults of old divergent plate boundaries and failed
rifts (called aulacogens)
4. While large quakes are rare, when they occur they are very destructive and widely felt.
VI.
Effects of Earthquakes
A.
1. The trembling and shaking of Earth’s land surface
2. Cause of building failure
B.
1. Broken gas and electric lines
2. Broken water mains
C.
1. Can be triggered by ground motion.
2. __________________________
a. Water saturated soil turns from solid to “liquid” due to ground vibrations.
b. May occur several minutes after the earthquake.
c. Once solid sediment flows like water.
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Earthquakes
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D.
Permanent Displacement of the Land Surface
1. Rocks may move vertically or horizontally or even diagonally.
2. May affect very large areas.
3. ____________________: A fault trace that may appear as a low cliff or as a closed tear in the
ground.
E.
_____________________________
1. Small earthquakes following the main trembler.
2. May cause considerable damage to already weakened structures.
F.
________________________(Seismic Sea Waves)
1. Often referred to as _______________________________but have nothing to do with ocean tides.
2. The preferred term is the Japanese term tsunami which means “harbor wave.”
3. Caused by vertical displacement of the ocean floor (but can also originate from submarine
landslides or volcanic explosions)
4. With the rise or fall of the ocean floor during a quake, the water above it is either lifted or dropped
momentarily.
5. As water returns to sea level low waves are generated that spread out radially from the point of
origin.
6. Unlike wind-generated waves that have wavelengths that are shorter (400 m or about 1,500 feet, a
tsunami may have a wavelength of 160 km (approx. 100 miles).
7. The speed of a tsunami may be 725 km/hour (450 mi/hr).
8. In deep water the wave height may be only 0.6 to 2 meters but near shore it may peak to heights of
15 to 30 meters
a. Caused by bottom topography nearshore
b. Record height was 85 meters (279 feet) in the Ryukyu islands south of Japan in 1971.
9. Damage from tsunamis is confined to coastal locations.
VII.
A.
B.
World Distribution of Earthquakes
Most earthquakes are located in narrow geographic belts
Belts correlate with plate boundaries
1. Circum-Pacific Belt
a. Encircles the rim of the Pacific Ocean
b. Most important concentration of earthquakes
c. Correlates with the Pacific Ring of Fire
2. Mediterranean-Himalayan Belt
a. Runs through the Mediterranean Sea
b. Passes through the Mideast and the Himalayas, through the East Indies, meeting the circumPacific belt north of Australia
3. Mid-Ocean Ridges
4. Oceanic Trenches
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VIII.
A.
Earthquakes and Plate Boundaries
Using Seismograms for First Motion Earthquake Studies
1. Used to determine overall movement along a plate boundary.
2. Pen motion represents the reaction of the P-Wave to rocks being pulled apart or compressed.
3. From the pen drawing on a seismogram first motion as a push or pull is determined.
a. Push
i First arrival waves cause the pen drawing to be deflected _______.
ii Rocks are moved ____________the seismograph location.
iii Indicates the seismograph location experienced ________________.
b. Pull
i First arrival waves cause the pen drawing to be deflected _________.
ii Indicates rocks experienced _____________(were pulled apart).
iii Rocks are moved ___________ from the seismograph station.
4. If the orientation of the fault is known the pattern of pushes and pulls will facilitate determination
of rock movement .
5. If the orientation of the fault is not known there will be two possible solutions.
B.
Divergent Boundaries
1. Mid-Ocean Ridges:
a. Earthquakes shallow and are located on the sides of the rift valley.
b. Normal faulting parallel to the rift valley.
c. First motion studies indicate horizontal extension along the ridge causing the sea floor to split
apart and open.
2. Continental
a. Same characteristics as with mid-ocean ridges.
b. African Rift Valley is currently active.
C.
Transform Boundaries
1. Shallow earthquakes are located where plates move past each other.
2. First motion studies indicate strike-slip motion.
3. Earthquakes may be aligned in a narrow band along one faults forming a plate boundary (as with
the San Andreas fault).
4. Earthquakes may form a broader zone along a system of parallel faults.
a. Basin and Range faults in western U.S.
b. System of faults parallel and at an angle to the San Andreas forms a plate boundary at that
location.
D.
Convergent Boundaries
1. Two kinds
a. Continent-Continent
b. Ocean-Continent
2. The collision boundary is characterized by broad zones of shallow earthquakes.
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3. Subduction
a. As the plate sinks it stretches at the top of the bend.
b. Tension causes normal faults which cause the outer part of the trench to be block-faulted.
c. Below the trench
i The subducting plate is in contact with the overlying plate.
ii Underthrusting is the occurrence of shallow quakes occur due to shallow-angle thrustfaulting
d. Greater depths
i Subducting plate is not in contact with the overlying plate
ii Earthquakes are confined to a thin zone with no obvious cause
e. Earthquake distribution with depth could be causing mineral transformations that may cause or
control earthquakes.
i Sudden loss of water
ii Phase transitions of olivine to spinel, and finally perovskite (denser minerals).
f. Subduction Angle: Plotting the vertical and horizontal distribution of earthquakes allows the
subduction angle of the plate to be determined.
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Earthquakes
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