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Slide 1
Earth Science Fall 2013
Slide 2
Earthquake- vibration of the earths crust Usually occur when
rocks under stress suddenly shift along a fault
Slide 3
Theory that rocks that are strained past a certain point will
fracture and spring back to their original shape. Energy is
released by the rocks along the fault in the form of vibrations
called seismic waves. The release of energy usually increases the
stress in other rocks along the fault causing aftershocks.
Aftershocks usually follow a major earthquake and are a series of
tremors
Slide 4
Focus- area along a fault where slippage first occurs; under
the surface of the earth Epicenter- point on the earths surface
directly above the focus When an earthquake occurs seismic waves
radiate outward in all directions from the focus
Slide 5
Earthquake depth statistics Shallow focus quakes occur within
70km of the earths surface 90% of continental earthquakes are
shallow focus Usually cause the most damage Intermediate focus
quakes occur at a depth of 70km 300 km Deep focus quakes occur in
subduction zones at a depth of between 300km 650km Usually occur
farther inland from the subduction zone than shallow and
intermediate focus earthquakes
Slide 6
Elastic Rebound Theory Elastic Rebound Theory Earthquakes 101
Earthquakes 101
Slide 7
Most earthquakes occur on or near the edges of the earths
lithospheric plates Some plate boundaries contain groups of
interconnected faults called fault zones Ex: San Andreas Fault
Slide 8
Pacific Ring of Fire Includes the west coasts of North &
South America, east coast of Asia, and the western Pacific islands
of the Philippines, Indonesia, New Guinea, and New Zealand
Subduction is occuring Mid-ocean ridges Oceanic crust is pulling
away from both sides of the ridge Eurasian-Melanesian mountain belt
Collision of plates
Slide 9
Seismograph- instrument used to detect and record seismic waves
Consists of 3 separate sensing devices: One records vertical motion
One records horizontal motion in an east-west direction One records
horizontal motion in a north-south direction
Slide 10
The major types of seismic waves: S waves P waves Surface
waves
Slide 11
P waves (primary waves) Move the fastest First to be recorded
by a seismograph Can travel through solids & liquids The more
rigid the material the faster they travel Compression waves (cause
rock particles to move together and apart along the direction of
the waves)
Slide 12
S waves (secondary waves) Second waves recorded by the
seismograph Only travel through solid material Cannot be detected
on the side of the earth opposite the earthquakes epicenter. Why?
Shear waves ( they cause rock particles to move at right angles to
the direction in which the waves are traveling)
Slide 13
Surface waves When P waves and S waves reach the earths surface
their energy is converted in surface waves. Slowest-moving waves
Last to be recorded by a seismograph Move similar to that of ocean
waves Cause the earths surface to rise & fall Particularly
destructive when traveling through loose earth
Slide 14
To find the epicenter of an earthquake scientists analyze the
difference between the arrival time P waves and S waves P waves
travel 1.7 times faster than S waves So is S waves arrive soon
after P waves the earthquake originated really close and vice versa
To determine how far an earthquake is from a given seismograph
Scientists plot the difference between the arrival times of the two
waves Need data from at least three seismograph stations to do
this
Slide 15
Magnitude- amount of ground motion Seismologists report the
magnitude of an earthquake by using a magnitude scale like the
Richter scale or the moment of magnitude scale Scientists prefer
the moment of magnitude scale because it is related more to cause
of the earthquake, where the Richter scale is more related to the
effect of the earthquake
Slide 16
Major earthquake Magnitude of 7 or above Moderate earthquake
Magnitude between 6 and 7 Minor earthquake Magnitude between 2.5
and 6 Microquake Magnitude less than 2.5 Usually not felt by
people
Slide 17
Mercalli Scale Expresses the intensity of an earthquake or the
amount of damage it causes Expressed as Roman numerals from I to
XII and a description Ex: rating of II (low intensity) Felt only by
a few persons at rest, especially on the upper floors of buildings.
Delicately suspended objects may swing. Ex: rating of XII Total
Destruction
Slide 18
A moderate earthquake that continues for a long time often
causes more damage than an earthquake of high magnitude that only
lasts for a short time.
Slide 19
Destruction of Buildings & Property Type of ground beneath
a building can affect the way a building responds to seismic waves.
Loose soil & rock is much more likely to be damaged during an
earthquake than one built on solid ground
Slide 20
Tsunamis Giant ocean wave caused by a major earthquake with an
epicenter on the ocean floor Most are caused by 2 events related to
under sea earthquakes Faulting Underwater landslides Pacific
Tsunami Warning Center (PTWC) Network of seismograph stations
around and in the Pacific Ocean that alerts scientists to the
location and magnitude of earthquakes
Slide 21
Safety Rules Before an earthquake occurs, be prepared. During
an earthquake, stay calm. After an earthquake, be cautious
Slide 22
Identified faults in populated areas Along some faults there
are zones of immobile rock called seismic gaps A seismic gap is an
area where the fault is locked and unable to move Scientists
believe that seismic gaps are likely the location of future
earthquakes
Slide 23
Sometimes there is slight tilting of the ground shortly before
an earthquake Some earthquakes are preceded by a decrease in the
speed of local P waves Can last for several days or several years
Longer the decrease lasts the stronger the earthquake will be Japan
Tsunami Japan Tsunami Earthquake & Tsunami Relationship
Earthquake & Tsunami Relationship