Transcript No Slide Title

Rupture, Seismic Waves, and Shaking
Earthquake
Origins and
Seismic Waves
– Focus point where earthquake rupture occurs
• Shallow focus -70 km or less (80% or more)
• Deep focus -down to 700 km; often along subduction zones
– Epicenter point projected onto
the surface from the center of the Earth through the focus
Types of
Seismic Waves
• Body waves -travel within the
Earth
– Primary or P-waves
• Compression
• Faster of the two types (5.5-1.5
km/sec)
• Passes through solids, liquids, or
gases (sound at 15 Hz)
– Secondary or S-waves
• Shear waves
• Travels only in solids
• Velocity ~60% of P-wave velocity
Shaking Amplification
Shaking is affected by:
Seismic wave
frequency
Earthquake magnitude
Directivity – direction
of fault rupture
Rock material through
which seismic wave
propagates
Comparing Earthquakes
• Earthquake Magnitude (1935)
– Richter Magnitude (MW)
• Measures of energy released (30 X increase/ magnitude)
• Maximum amplitude 100 km from the epicenter
• Logarithmic scale; a 10-fold increase per cm of amplitude
• mb = log10(A/T) + Q(D,h)
• MS = log10 (A/T) + 1.66 log10 (D) + 3.30
•
–
»
»
Seismograph
S-and P-waves
Magnitude (wave amplitude)
Distance (P-S)
Charles Richter
Richter-TNT Seismic Magnitudes
•
•
•
•
•
•
•
1.0
2.0
4.0
4.5
6.0
6.5
7.0
30 pounds
1 ton
1,000 tons
5,100 tons
1 million tons
5 million tons
32 million tons
•
•
•
•
•
•
7.5
8.0
8.5
9.0
10.0
12.0
160 million tons
1 billion tons
5 billion tons
32 billion tons
1 trillion tons
160 trillion tons
Large Blast at a Construction Site
Large Quarry or Mine Blast
Small Nuclear Weapon
Average Tornado (total energy)
Double Spring Flat, NV Quake, 1994
Northridge, CA Quake, 1994
Hyogo-Ken Nanbu, Japan Quake, 1995;
Largest Thermonuclear Weapon
Landers, CA Quake, 1992
San Francisco, CA Quake, 1906
Anchorage, AK Quake, 1964
Chilean Quake, 1960
San-Andreas type fault circling Earth
Fault Earth in half through center, OR
Earth's daily receipt of solar energy
Calculating Earthquake Magnitude
• Moment method
– Essentially the same as Richter at higher
magnitudes
– May be applicable over a wider range of ground
motions than Richter
Mo = µ A D (seismic moment)
µ = shear modulus = 32 GPa in crust, 75
GPa in mantle
A = LW = area
D = average displacement during rupture
MW = 2/3 log10(M0) - 10.7
Determining Earthquake Location
Pakistan Event (2005)
Determining Earthquake Location P-wave Travel Map From Pakistan
Determining Earthquake Location
Comparing Earthquakes
• Earthquake Intensity
– Modified Mercalli Scale
– Qualitative way of comparing earthquakes
• Based on perception by people
• Damage to buildings
• Varies with distance from the epicenter (unlike
Richter or Moment scales)
– Radio wave model
Comparison of Earthquake Scales
Richter
Magnitude
Modified
Mercalli
2
3
4
5
I - II
III
IV - V
VI - VII
6
VII - VIII
7
8+
IX - X
X - XII
Maximum Intensity (at epicenter)
Usually detected only by instruments
Felt indoors
Felt by most people; slight damage
Felt by all; many frightened and run
outdoors; damage minor to moderate
Everybody runs outdoors; damage
moderate to major
Major damage
Total and major damage
After Charles F. Richter, 1958, Elementary Seismology.
Ground Acceleration
Rate of change in
horizontal and vertical
velocity
Measure with respect to
gravity (1 g = 9.8 m/s2)
Building resistance varies
with construction
(adobe vs.concrete)
Earthquake Risk and Prediction
• Regional Changes in Land Elevation
– 1964 Alaskan Quake
– 1992 Mendocino, CA
• Long and short term prediction
• Estimation of Seismic Risk (maps)
• Conditional Probabilities of Future
Earthquakes
– Probability maps
• The Loma Prieta quake as
probable before 1989
• Failed at San Bernardino in
1992
Earthquake
Hazard Map
Florida’s Earthquake History
• Basically aseismic (i.e., little activity) since Paleozoic
• Only about 6 "real" earthquakes (until 2006!)
–
–
–
–
–
–
1879-location unknown; felt in Fla. and Ga.
1893-Jacksonville
1933-Palatka
1945-offshore Miami
1973-Merritt Island
1975-Daytona Beach (most recent)
• 1998 Panhandle (Jay fault; epicenter near
Brewton,Alabama; MR = 4.5)
2006 Gulf of Mexico
Event
More information available at:
http://www.clas.ufl.edu/users/rrusso/florida_eq.html