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VII. Earthquakes. Introduction Source of seismic energy Propagation of seismic energy Recording earthquakes Magnitude scales. San Francisco, 1906. Building design could not withstand accelerations $Millions of damage Thousands of people killed. - PowerPoint PPT Presentation
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VII. Earthquakes
A. Introduction
B. Source of seismic energy
C. Propagation of seismic energy
D. Recording earthquakes
E. Magnitude scales
Building design could not withstand accelerations
$Millions of damage
Thousands of people killed
San Francisco, 1906
Geology in the News
Two die in 6.5 magnitude Earthquake near San Lois Obispo California
Earthquake triggers mudslides
http://news.nationalgeographic.com/news/2003
California, 2003
San Andreas Fault
Earthquake Bam, Iran
A Magnitude 6.5 Earthquake hits a stone- and mud-house city of 100,000 in Iran December 26, 2003 30,000 Dead 30,000 Refugees
US sends aid and releases sanctions
Relations improved
Photos from AP
Geological Hazards Related to Earthquakes
Landslides
Volcanoes
Mudslides Tsunamis
B: Sources of Seismic Energy
Elastic Rebound Buildup of elastic energy
during elastic strain Sudden release due to
slippage along a fault or brittle rupture
Anatomy of an Earthquake
Fig. 8.35
Focus: Source of energy Epicenter: Location directly above focus at the surface
(ground motion is greatest) Fault Trace: Shows intersection of fault and the surface
of the land Fault Scarp: Indicates vertical motion of fault
Propagation of Seismic Energy
Body Waves travel through the earths interior (crust, mantle, core) P wave: Compression
and expansion of rock S Wave: Shearing motion
of particles
Fig. 8.42
Propagation of a P-Wave
Propagation of Seismic Energy
Body Waves travel through the earth’s interior (crust, mantle, core) P wave: Compression
and expansion of rock S Wave: Shearing
motion of particles Surface Waves
Propagation of an S-Wave
Propagation of Surface Waves
Surface Waves travel along the earth’s surface Love Wave: Lateral
movement of the surface Rayleigh Wave: Rolling
movement of the surface (similar to an ocean wave)
Propagation of a Surface Wave
Propagation of a Surface Wave
Recording Earth Motion Seismograph:
An instrument that measures the horizontal or vertical motion of Earth’s surface
Seismograms: The plot of the motion
Measuring Velocity of Seismic waves
Because the P wave travels faster the the S wave
The S-P interval increases with distance
Time of EarthquakeTime of Earthquake
Reading a Seismogram Ground motion vs. Time
Each tick mark is 1 minute P-S Time interval indicates distance to epicenter
First P waveArrival
First S waveArrival First Surface wave
P-S Interval
See Fig. 8.44
Time-Distance Relationships
Use P-S interval to determine distance to focus
See Fig. 8.45
Distance from focus (Kilometers)
Seismic Waves and Velocities P-waves
= ( + ¾): Bulk modulus
: Mod. of rigidity : Density
S-waves
= /
Body Waves and Surface Waves
Locating the Epicenter
Triangulation using 3 seismograph stations
Depth can be determined with four or more stations
Finding the Depth of Earthquakes
Using 4 or more seismograph stations
Seismicity of the Pacific Rim 1975-1995
Depth(km)
03370
300
150
500
800
Shallow quakes at mid ocean ridges (<33km) and
Oceanic trenches Deep quakes over
the subduction zone (>70 km)
Earthquakes, Plate Interior
New MadridFault Zone Faults activated by
crustal warping Bowling Green
Fault Largest Earthquake
on the N. American Continent
New Madrid Earthquake, 1811
Modified Mercalli Intensity Scale Subjective
observations of Damage and Ground motion
Is not a quantitative measure
Earthquake Intensity Scales
Modified Mercalli Scale Subjective observations of
Damage and Ground motion
Is not a quantitative measure Richter Scale
Indicates Ground Motion Amplitude Logarithmic (e.g., 6 is ten times stronger than 5) Does not directly indicate energy or destruction
Earthquakes around the World
Assessing Risk“Major Quake Likely to
Strike San Francisco Bay Region Between 2003 and 2032”
• Assessing Risks• Avoiding Risks• Preventing Damage• Predicting Impact
Geologic Hazards
Seismic Risk Analysis
Quake history (statistics) Locations of active faults Competency of surficial
materials (soil and rock) Ocean basin source
Tsunamis
Solomon Islands Earth Quake and Tsunami 4-1-07
Profiling Earth’s Interior
Velocities of seismic waves vs depth Fig. 19.19 & 20
Imaging Earth’s Interior
P and S waves are refracted (bent) within the earth
S waves do not travel through fluids
Fig. 9.21
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