Earthquakes A Violent Interlude

1
EarthquakesA Violent Interlude
2
Definition of earthquake -vibration of the
Earth's by shock waves generated by energy
released from rocks rupturing under stress -most
(not all) earthquakes and ruptures occur along
faults -most faults and stress occurs along
active plate tectonic boundaries
3
Fault Types
Dip-Slip normal
Strike-Slip right-lateral
4
Elastic rebound theory Crustal blocks move
through time -locked segment along the
fault no slip Rocks are strained
deformation of shape -the strain can be
released catastrophically Energy released
causes earthquake Analogy breaking rubber band
5
(No Transcript)
6

• Other causes of earthquakes
• Landslides, rockslides, or slumping of rocks.
• Movement of magma, gases, or rocks associated
  with volcanism

7
Morphology of earthquakes and related terms
Focus or Hypocenter -exact location of the
earthquake disturbance-with few exceptions is
located below the surface of the Earth
Epicenter -the position on the surface of the
Earth directly above the focus Slip the
distance of displacement along the fault
8
(No Transcript)
9
(No Transcript)
10
Foreshock(s) -Earthquake(s)
generated at or very near the focus of the main
earthquake prior to the main shock -smaller
magnitude than the main shock and precede main
quake by a short or substantial time interval
11
Main shock -is the main earthquake disturbance
generated at the focus
Aftershock(s) -Earthquake(s) generated at or
very near the focus of the main earthquake
following the main shock -smaller in magnitude
than the main shock. Follow the main earthquake
by a short or substantial time interval
12
Seismic waves
-waves which travel within
the Earth -P (primary) wave-moves in a straight
line path with alternating compression and
expansion -S (secondary) wave-moves in a
sinusoidal motion along its path of movement
13
Principles of P and S Waves
14
P-Wave Propagation
P waves Compressional waves similar to sound
waves Series of contractions and relaxations
Fastest, 5 km/sec (depends on rock type)
Travel through solid, liquid and gas
15
S-Wave Propagation
S waves Shear waves motion is right angles to
direction of wave Half speed of P waves
Travel only through solid
16
P-wave
S-wave
17
Surface Waves -move along the Earths surface
radiating out from epicenter
two types Rayleigh Love
18
(No Transcript)
19
(No Transcript)
20
Seismographs -seismograph is the machine that
detects seismic waves. -seismogram is the
printed record.
21
(No Transcript)
22
Seismograms from multiple Seismographs around
San Francisco Bay Area
23
Locating a focus/epicenter 1. Need a seismogram
from at least 3 different seismic stations. 2.
Examine the seismograms and determine the elapsed
time between the arrival of the first P-wave and
the first S-wave.
24
3. Use a travel-time graph -knowing the S-
P-wave travel-time, you can determine the
distance to the focus/epicenter from the seismic
station.
25
4. On a map, draw a circle around the seismic
station. 5. Repeat for two other seismic
stations. (Triangulation.)
26

• Measuring Earthquakes
• Several different scales exist to measure
  earthquakes.
• Modified Mercalli Scale
• Measures earthquake intensity.
• -describes damage to structures.-ranges from I
  (felt by only a few) to XII (total destruction).
• Intensity depends on
• -Distance to epicenter
• -Building materials and design
• -Type of ground material - soil, rock, etc.

27
Modified Mercalli Intensity Scale I. Not felt
except by a very few under especially favorable
conditions. II. Felt only by a few persons at
rest, especially on upper floors of buildings.
III. Felt quite noticeably by persons indoors,
especially on upper floors of buildings. IV.
Felt indoors by many, outdoors by few during the
day. At night, some awakened. Dishes, windows,
doors disturbed walls make cracking sound. V.
Felt by nearly everyone many awakened. Some
dishes, windows broken. Unstable objects
overturned. VI. Felt by all, many frightened.
Some heavy furniture moved a few instances of
fallen plaster. Damage slight. VII. Damage
negligible in buildings of good design and
construction slight to moderate in well-built
ordinary structures considerable damage in
poorly built or badly designed structures. VIII.
Damage slight in specially designed structures
considerable damage in ordinary substantial
buildings with partial collapse. Damage great in
poorly built structures. Fall of chimneys,
factory stacks, columns, monuments, walls. Heavy
furniture overturned. IX. Damage considerable in
specially designed structures well-designed
frame structures thrown out of plumb. Damage
great in substantial buildings, with partial
collapse. Buildings shifted off foundations. X.
Some well-built wooden structures destroyed most
masonry and frame structures destroyed with
foundations. Rails bent. XI. Few, if any
(masonry) structures remain standing. Bridges
destroyed. Rails bent greatly. XII. Damage
total. Lines of sight and level are distorted.
Objects thrown into the air.
28
Richter Scale -Measures earthquake
magnitude.-determined by measuring the amplitude
of the largest waves on the seismogram.
-logarithmic scale. -each number on the
Richter Scale is ten times greater in wave
amplitude. -each number on the Richter Scale
involves an energy release about 32 times as
great.
29
(No Transcript)
30

• Moment magnitude scale
• -measures the amount of energy released
• Designed to differentiate large earthquakes
• May be used to calculate energy of old events by
  slip along fault
• Moment µ A D
• µ shear modulus A LW areaD average
  displacement during rupture

31
Moment Magnitude for Indonesian Quake 2004
32
Locations of Earthquakes Most occur along
tectonic plate boundaries -around the Pacific
Ocean (circum-Pacific belt is major site of large
earthquakes) -Mediterranean Sea area through
Iran and on to the Himalayas -Asia (Indonesia,
Himalayan region) -Mid-ocean ridges Some occur
far from plate edges -intraplate earthquakes
33
(No Transcript)
34
(No Transcript)