Download Earthquakes

VII. Earthquakes
A.
B.
C.
D.
E.
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
http://news.nationalgeographic.com/news/2003
Geology in the News
California, 2003


Two die in 6.5
magnitude
Earthquake near
San Lois Obispo
California
Earthquake triggers
mudslides
San Andreas Fault
Earthquake
Bam, Iran

A Magnitude 6.5
Earthquake hits a
stone- and mudhouse 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
Volcanoes
Landslides
Tsunamis
Mudslides
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 Earthquake
Time of Earthquake
Reading a Seismogram

Ground motion vs. Time


See Fig. 8.44
Each tick mark is 1 minute
P-S Time interval indicates distance to epicenter
P-S Interval
First P wave
Arrival
First S wave
Arrival
First Surface wave
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
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



Shallow quakes at
mid ocean ridges
(<33km) and
Oceanic trenches
Deep quakes over
the subduction
zone (>70 km)
0
33
70
150
300
500
800
Depth
(km)
Earthquakes, Plate Interior

New Madrid
Fault 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”
Geologic Hazards
• Assessing Risks
• Avoiding Risks
• Preventing Damage
• Predicting Impact
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