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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.

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VII. Earthquakes


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vii earthquakes

VII. Earthquakes

Introduction

Source of seismic energy

Propagation of seismic energy

Recording earthquakes

Magnitude scales

san francisco 1906
San Francisco, 1906
  • Building design could not withstand accelerations
  • $Millions of damage
  • Thousands of people killed
geology in the news

http://news.nationalgeographic.com/news/2003

San Andreas Fault

Geology in the News
  • Two die in 6.5 magnitude Earthquake near San Lois Obispo California
  • Earthquake triggers mudslides

California, 2003

earthquake bam iran

Photos

from AP

EarthquakeBam, 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
geological hazards related to earthquakes
Geological Hazards Related to Earthquakes

Volcanoes

Landslides

Tsunamis

Mudslides

b sources of seismic energy
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
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
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 seismic energy1
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 surface waves
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)
recording earth motion
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
Measuring Velocity of Seismic waves
  • Because the P wave travels faster the the S wave
  • The S-P interval increases with distance

Time of Earthquake

Time of Earthquake

reading a seismogram

P-S Interval

Reading a Seismogram

See Fig. 8.44

  • Ground motion vs. Time
    • Each tick mark is 1 minute
    • P-S Time interval indicates distance to epicenter

First S wave

Arrival

First P wave

Arrival

First Surface wave

time distance relationships
Time-Distance Relationships

See Fig. 8.45

  • Use P-S interval to determine distance to focus

Distance from focus (Kilometers)

seismic waves and velocities
P-waves

a = (k + ¾m)r

k: Bulk modulus

m: Mod. of rigidity

r: Density

S-waves

b = m/r

Body Waves and Surface Waves

Seismic Waves and Velocities
locating the epicenter
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
Finding the Depth of Earthquakes Using 4 or more seismograph stations

Seismicity of the Pacific Rim 1975-1995

0

33

70

150

  • Shallow quakes at mid ocean ridges (<33km) and
  • Oceanic trenches
  • Deep quakes over the subduction zone (>70 km)

300

500

800

Depth

(km)

earthquakes plate interior
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
New Madrid Earthquake, 1811
  • Modified Mercalli Intensity Scale
    • Subjective observations of
      • Damage and
      • Ground motion
    • Is not a quantitative measure
earthquake intensity scales
Earthquake Intensity Scales
  • Modified Mercalli Scale
    • Subjective observations of
      • Damage and
      • Ground motion
    • Is not a quantitative measure
  • RichterScale
    • Indicates Ground Motion Amplitude
    • Logarithmic (e.g., 6 is ten times stronger than 5)
    • Does not directly indicate energy or destruction
assessing risk
Assessing Risk

“Major Quake Likely to Strike San Francisco Bay Region Between 2003 and 2032”

Geologic Hazards

  • Assessing Risks
  • Avoiding Risks
  • Preventing Damage
  • Predicting Impact
seismic risk analysis
Seismic Risk Analysis
  • Quake history (statistics)
  • Locations of active faults
  • Competency of surficial materials (soil and rock)
  • Ocean basin source Tsunamis
profiling earth s interior
Profiling Earth’s Interior

Velocities of seismic waves vs depth Fig. 19.19 & 20

imaging earth s interior
Imaging Earth’s Interior
  • P and S waves are refracted (bent) within the earth
  • S waves do not travel through fluids

Fig. 9.21