Body Waves: S wave
Surface Waves
• Rayleigh waves
• Love waves
>
• Free body oscillations
Free Body Oscillations
From
IRIS
Torroidal Modes
Spherical Modes
Foreshocks, Aftershocks
h
USGS
USGS
Bolivar Peninsula, Texas
Bolivar Peninsula, Texas
Earthquake Magnitude
Richter Scale, ML
Each increment in log
scale implies 10-fold
change in linear scale
ML is logarithmic in
amplitude of shaking
• i.e., if A = 2 cm for ML = 5,
what is A for ML = 6?
ML is logarithmic in
amplitude of shaking
• i.e., if A = 2 cm for ML = 5,
what is A for ML = 6?
A = 20 cm
ML is logarithmic in
amplitude of shaking
• i.e., if A = 2 cm for ML = 5,
what is A for ML = 6?
A = 20 cm
• i.e., if A = 10 μm for ML = 3.7,
what is A for ML = 1.7?
ML is logarithmic in
amplitude of shaking
• i.e., if A = 2 cm for ML = 5,
what is A for ML = 6?
A = 20 cm
• i.e., if A = 10 μm for ML = 3.7,
what is A for ML = 1.7?
A = 0.1 μm
ML is logarithmic to base 32 for
energy of earthquake
• i.e., how much more energy does ML = 5
earthquake have than ML = 4?
M=5
E
M=4
ML is logarithmic to base 32 for
energy of earthquake
• i.e., how much more energy does ML = 5
earthquake have than ML = 4?
32 × more energy!
M=5
E
M=4
ML is logarithmic to base 32 for
energy of earthquake
• i.e., how much more energy does ML = 5
earthquake have than ML = 4?
32 × more energy!
• i.e., how much more energy does ML = 6.5
earthquake have than ML = 4.5?
ML is logarithmic to base 32 for
energy of earthquake
• i.e., how much more energy does ML = 5
earthquake have than ML = 4?
32 × more energy!
• i.e., how much more energy does ML = 6.5
earthquake have than ML = 4.5?
32 × 32 ~ 1,000 × more energy!
– Rule: energy increases by order of 1,000 for
every increment of 2 in Magnitude
Frequency Spectrum of Waves
Analogy: Music
Analogy: Music
• Earthquake waves = mixture of S, P,
surface waves of various frequencies
• Music = mixture of P waves of various
frequencies
– Bass = low frequency
– Midrange = medium frequency
– Treble = high frequency
• Most important point: big earthquakes
make different music than small
earthquakes
Bass
Midrange
Treble
Problem with measuring large earthquakes
Richter scale tuned to
these earthquakes
Small earthquake
Big earthquake
Louder Longer Lower frequency
Saturation
Moment
Magnitude
Scale, MW
Energy ~
Rupture Area ×
Slip Distance ×
Rock Strength
Gutenberg-Richter
• Plots LOG(frequency) or LOG(RI) vs. magnitude
RI (years)
Magnitude M
Gutenberg-Richter
RI (years)
Magnitude M
Magnitude vs.
Intensity
Earthquake Intensity
I Not felt at all
II
III
IV Hanging objects swing
V
VI Felt by all
VII Difficult to stand
VIII
IX
X Rails bent
XI Bridges destroyed
XII Total destruction
I 0.1% G
II Barely felt 0.2% G
III 0.3% G
IV 0.7% G
V 1.5% G
VI 3% G
VII 7% G
VIII 15% G
IX 32% G
X 70% G
XI 100% G
XII Objects fly >124% G
1811 New
Madrid
1906 San (M=7.5)
Francisco
(M = 8.3)
Epicenter
S waves
Close
L waves R waves
Far
Auroville Earth Institute
Amplification
Sedimentary
basin forming
Amplification from wave slowing
Energy depends on:
Speed
Amplitude
Amplification from focusing
Amplification from reflection
Example:
1989 Loma Prieta earthquake, California
Fort Mason
Marina built on solid
district bedrock
built on
fill
Amplification damage, Marina District, San Francisco
Liquefaction
Soil Liquefaction web site,
University of Washington
Total Stress Effective Water Pressure
(load) Stress (grain-
to-grain
contacts)
Soil Liquefaction web site,
University of Washington
Liquefaction Simulation
(Soil Liquefaction web site, University of Washington)
P
Total Stress Effective Water Pressure
(load) Stress (grain-
to-grain
contacts)