Transcript Where earthquakes?

Earthquake facts
• Earthquakes are caused by a build-up of stress
within the crust, causing rocks to fail suddenly.
•Some 80 percent of all the planet's earthquakes
occur along the Pacific rim.
• On average, a magnitude 8 (M8) quake strikes
somewhere every year.
• Most deaths during an earthquake are due to
collapsing buildings.
•Why earthquakes? Where earthquakes?
•Why earthquakes? Where earthquakes?
Distribution of earthquakes with R.S. mag > 5 from 1980-1990.
•Why earthquakes? Where earthquakes?
Honshu tsunami videos
http://www.youtube.com/watch?v=w3AdFjkl
R50&feature=related
http://www.youtube.com/watch?v=DSSssHxm4Y&feature=related
http://www.youtube.com/watch?v=s8L2kiky
Vzk
Locating EQs
Locating EQs
epicenter
M=9.0
11 March 2011
5:46:23 UTC
focus
Faults are where rocks break and move suddenly
30-40 m movement along
subduction zone
fault in Honshu
North American Plate
Pacific Plate
Tsunami (seismic sea wave) caused by seafloor displacement .
Tsunamis slow and steepen (up to 30 m) as they enter shallow water.
Tsunami wave predictions
Measurement of Earthquakes
• Magnitude
-an index of the amount of energy
released, proportional to amount of
ground motion;
-Richter Scale - a logarithmic scale:
each number represents >30X more
energy and 10X more ground motion
than preceding number; examples….
Richter magnitude depends on:
1) Separation time (distance)
2) Amplitude (ground shaking)
Measurement of Earthquakes
• Intensity
-based on damage, effects
-will vary from place to place
Destructive Effects of
Earthquakes
• Structural damage from seismic waves-
• fire
• mass wasting - landslides, liquefaction
• tsunami (seismic sea waves)
Kobe, Japan,
1995
http://environment.nationalgeographic.com/envir
onment/natural-disasters/earthquakeprofile.html?fs=video.nationalgeographic.com
(National Geographic)
http://www.youtube.com/watch?v=SgRry6GZxU
o&feature=related(hole in highway)
http://www.youtube.com/watch?v=6iNUvlU0SJY
(global EQs for 2-month period)
http://www.youtube.com/watch?v=reDKxgb_IB
Q&feature=related (7.2 quake hits Japan, 2008)
http://www.youtube.com/watch?v=7TQDQAesJv
M&feature=related
(NYC EQ)
F =100 lbs
10 lbs/ft2
Stress is force concentrated over an area
Types of Stress
• Confining
A =10 ft2
Plate Tectonics Setting
None
• Compressive
Convergent
• Extensional
Divergent
• Shear
Transform
Strain is the response of a material to stress
• Elastic
*Rubber ball
*Earthquake
• Plastic
*Modeling clay
*Folded rocks
• Brittle
*Pencil
*Faults
Type of
Strain
depends on:
•
Temperature
• Pressure
• Material
• Time
Plastic flow
Folding: plastic
response to stress
• strain energy used
up in folding
• no earthquakes
Faulting: brittle response to stress
•slow or sudden release of small or large
amounts of energy
•may cause earthquakes
Elastic Rebound – causes earthquakes
Elastic Rebound
Stick-slip Faults
• Fault zone may have segments characterized by creep
• These may altternate with segments which are
stuck (snagged)
• As elastic strain accumulates, energy is stored;
Subsequently released as large earthquake
• Creeping segments store little energy (less dangerous)
Stick-slip Faults
Seismic Waves
• body waves- move only through earth’s
interior;
-p wave: primary (compressional) wave,
fastest, first to arrive at seismic
station;
2.4 – 3.2 miles / second
~10,000 mph
compare to F-15 ~0.5 miles / second
-s wave: secondary (shear) wave
-1.8 – 2.4 miles / second
- cannot be transmitted through liquid (ex molten rock)
P WAVE (compressional wave)
S WAVE (SHEAR WAVE)
Surface Waves
• move only along the surface;
• - slowest wave type
• Rayleigh, Love waves
Surface Waves
Determining Distance to Point of
Energy Release
• Example : thunder and lightning, same source
1) light travels 186,000 miles per sec,
instantaneous over 1-25 miles
2) sound travels ~ 1,100 ft per sec
Ex. Light-sound time interval of 20 sec
indicates lightning was ~ 4 miles away
Basic principles of a seismograph…
Determining EQ Epicenter
S-P interval
Record (seismogram) of an earthquake; time separation of arrivals
based on wave type. S-P interval shown is ~ 5 minutes.
Travel-time graph, used to determine distance from seismic station
to epicenter.
The location of an epicenter on the midoceanic ridge in the South
Atlantic Ocean. S-P time intervals are used from 3 seismic stations.
MEASUREMENT OF
EARTHQUAKES
• Magnitude
-an index of the amount of energy
released, proportional to amount of
ground motion;
-Richter Scale - a logarithmic scale:
each number represents >30X more
energy and 10X more ground motion
than preceding number; examples….
Richter magnitude depends on:
1) Separation time (distance)
2) Amplitude (ground shaking)
Measurement (cont.)
• Intensity - an index of structural damage
-based on modified Mercalli Scale I-XII
(see Table );
structural damage depends on:
-magnitude and duration of EQ
-distance to epicenter
-structural design
-character of surface materials
upon which buildings rest.
USGS, 2005
San Francisco following the 1906 earthquake; the R.S. magnitude was
estimated at 7.8; There were 1,500 deaths.
Area in southern Alaska affected by “Good Friday” earthquake
of 1964; R.S. magnitude = 9.2, making it the greatest earthquake
to strike North America on record (i.e. for ~ the last 2 centuries).
Earthquake Prediction
• Basis
-historical patterns
-precursors
• Future major and great EQS in California
-probability of major quakes (Fig. 15.22)
-waiting for the “Big One” (>8 R.S.)
• Great quakes in the Midwest….
Statistical predictions for EQs of 7-8 magnitude (major EQs) based on
historical patterns and presumption of cyclical behavior involving
plate stresses, strain buildup and periodic release (fault rupture).
Low Velocity Zone/Asthenosphere
Low
Velocity
Zone
Mohorovicic Discontinuity
(“Moho)
Guttenburg Dis.
Divisions of earth’s interior; boundaries are defined by
behavior of body (i.e. p and s) waves.
Seismic Character of Earth’s
Internal Divisions
• Mohorovicic Discontinuity (crust-mantle)
-defined by abrupt acceleration of body
waves, increased rock density/change in
composition.
• Guttenburg Discontinuity (mantle-core)
defined by absorption of s-waves;
shadow zone; must be molten (iron)
Mohorovicic Discontinuity
(“Moho)
Guttenburg Dis.
Compositional divisions of interior of earth; boundaries are defined by
behavior of body (i.e. p and s) waves.
Seismic waves, like light waves, will
bend, or refract, when they
encounter different material
P-wave Shadow Zone
See animation at:
http://earthquake.usgs.gov/learn/glossary/?termID=170&alpha=S
S Wave Shadow Zone
http://dl.ccc.cccd.edu/classes/internet/geology100/IntroLecture_files/image030.jpg
END