Earth Science Fall 2013
 Earthquake-
vibration of the earth’s crust
• Usually occur when rocks under stress suddenly
shift along a fault
 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
 Focus-
area along a fault where slippage
first occurs; under the surface of the
earth
 Epicenter- point on the earth’s surface
directly above the focus
 When an earthquake occurs seismic
waves radiate outward in all directions
from the focus
 Earthquake depth statistics
• Shallow focus quakes occur within 70km of the
earth’s 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
 Elastic
Rebound Theory
 Earthquakes 101
 Most
earthquakes occur on or near the
edges of the earth’s lithospheric plates
• Some plate boundaries contain groups of
interconnected faults called fault zones
 Ex: San Andreas Fault
 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
 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
 The
major types of seismic waves:
• S waves
• P waves
• Surface waves
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)
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 earthquake’s epicenter. Why?
• Shear waves ( they cause rock particles to move
at right angles to the direction in which the
waves are traveling)
 Surface
waves
• When P waves and S waves reach the earth’s
•
•
•
•
•
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 earth’s surface to rise & fall
Particularly destructive when traveling through
loose earth
 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
 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
 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
 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
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.
 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
 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
 Safety
Rules
• Before an earthquake occurs, be prepared.
• During an earthquake, stay calm.
• After an earthquake, be cautious
 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
 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
 Earthquake & Tsunami Relationship