SEISMOLOGY Lecture 11: Earthquake source mechanisms and radiation patterns I
Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1)...
-
date post
21-Dec-2015 -
Category
Documents
-
view
214 -
download
0
Transcript of Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1)...
![Page 1: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/1.jpg)
Lecture-14 1
Lecture #14- Earthquake Size
![Page 2: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/2.jpg)
Lecture-14 2
How is Earthquake Size Determined ?
(1) Maximum Seismic Intensity Outdated methodDoes not use seismometersMany problems
(2) MagnitudesModern methodUses seismometersFewer problems
![Page 3: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/3.jpg)
Lecture-14 3
Maximum Intensity
Maximum Intensity is used to estimate the size of historical earthquakes, but suffers from dependence on depth, population, construction practices, site effects, regional geology, etc.
Maximum Intensity = VII
![Page 4: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/4.jpg)
Lecture-14 4
1906 SF and 1811-12 New Madrid
These earthquakes were roughly the same size, but the intensity patterns in the east are broader than in the west.
![Page 5: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/5.jpg)
Lecture-14 5
Earthquake Magnitude
Earthquake magnitude scales originated because of
– the desire for an objective measure of earthquake size
– Technological advances -> seismometers
![Page 6: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/6.jpg)
Lecture-14 6
Earthquake Magnitudes
In the 1930’s, Wadati in Japan and Richter in California noticed that although the peak amplitudes on seismograms from different events differed, the peak amplitudes decreased with distance in a similar manner for different quakes.
![Page 7: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/7.jpg)
Lecture-14 7
Seismogram Peak Amplitude
The peak amplitude is the size of the largest deflection from the zero line.
![Page 8: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/8.jpg)
Lecture-14 8
Richter’s Observations
![Page 9: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/9.jpg)
Lecture-14 9
Richter’s Local Magnitude
Richter used these observations to construct the first magnitude scale, ML (Richter’s Local Magnitude for Southern California).
He based his formula for calculating the magnitude on the astronomical brightness scale - which was logarithmic.
![Page 10: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/10.jpg)
Lecture-14 10
Logarithmic Scales
In a logarithmic scale such as magnitude
– A change in one magnitude unit means a change of a factor of 10 in the amplitude of ground shaking.
• •
![Page 11: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/11.jpg)
Lecture-14 11
Richter’s Formula
He defined a reference value Ao
ML = logA - log Ao
– A is the amplitude on the seismogram, Ao is the amplitude observed for a reference event.
For Southern California ML = logA - 2.48 + 2.76 x log(distance)
– The distance is in kilometers and this formula works for southern California.
![Page 12: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/12.jpg)
Lecture-14 12
Wood-Anderson Seismometer
Richter also tied his formula to a specific seismic instrument.
![Page 13: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/13.jpg)
Lecture-14 13
Richter’s Magnitude Scale
Only valid for Southern California earthquakes
Only valid for one specific type of seismometer
Has not been used by professional seismologists in decades
Is much abused by the press today
![Page 14: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/14.jpg)
Lecture-14 14
Modern Seismic Magnitudes
Today seismologists use different seismic waves to compute magnitudes
These waves have lower frequencies than those used by Richter
These waves are generally recorded at distances of 1000s of kilometers instead of the 100s of kilometers for the Richter scale
![Page 15: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/15.jpg)
Lecture-14 15
Teleseismic MS and mb
The two most common modern magnitude scales are:
MS, Surface-wave magnitude (Rayleigh Wave)
mb, Body-wave magnitude (P-wave)
![Page 16: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/16.jpg)
Lecture-14 16
Magnitude Discrepancies Ideally, you want the same value of
magnitude for any one earthquake from each scale you develop, i.e.
MS = mb = ML
But this does not always happen: Turkey 8/17/99: MS = 7.8, mb = 6.3
Taiwan 9/20/99: MS = 7.7, mb = 6.6
![Page 17: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/17.jpg)
Lecture-14 17
Why Don’t Magnitude Scales Agree?
Simplest Answer:– Earthquakes are complicated physical
phenomena that are not well described by a single number.
– Can a thunderstorm be well described by one number ? (No. It takes wind speed, rainfall, lightning strikes, spatial area, etc.)
![Page 18: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/18.jpg)
Lecture-14 18
Why Don’t Magnitude Scales Agree?
More Complicated Answers: The distance correction for amplitudes depends on
geology.
Deep earthquakes do not generate large surface waves - MS is biased low for deep earthquakes.
Some earthquakes last longer than others, even though the peak amplitude is the same.
![Page 19: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/19.jpg)
Lecture-14 19
Why Don’t Magnitude Scales Agree?
Most complicated reason:– Magnitude scales saturate
– This means there is an upper limit to magnitude no matter how “large” the earthquake is
– For instance Ms (surface wave magnitude)
never gets above 8.2-8.3
![Page 20: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/20.jpg)
Lecture-14 20
Example: mb “Saturation”
mb seldom gives values above 6.7 - it “saturates”.
mb must be measured in the first 5 seconds - that’s the rule.
![Page 21: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/21.jpg)
Lecture-14 21
Saturation
![Page 22: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/22.jpg)
Lecture-14 22
What Causes Saturation? The rupture process.
– Small earthquakes are rupture small areas are relatively enriched in short period signals.
– Large earthquakes rupture large areas and are relatively depleted in high frequencies.
![Page 23: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/23.jpg)
Lecture-14 23
Are mb and Ms still useful?
YES!– Many (most) earthquakes are small enough that
saturation does not occur– Empirical relations between energy release and
mb and Ms exist
– The ratio of mb to Ms can indicate whether a given seismogram is from an earthquake or a nuclear explosion (verification seismology)
![Page 24: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/24.jpg)
Lecture-14 24
Is there anything better?
Yes, the seismic moment – Mo
– Invented in the 1960s to circumvent magnitude limitations
– Has physical units of energy (Nm, cal, J)– Is the product of three factors that indicate the
size of the earthquake:
Mo = (shear modulus) x (rupture area) x (slip offset)
![Page 25: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/25.jpg)
Lecture-14 25
Moment Magnitude, MW
To allow comparisons, we use moment magnitude, which is calculated from the seismic moment.
Estimating MW takes more work than the others, but it’s often the best measure of size.
MW = 2 / 3 * log(Seismic Moment) - 10.73
![Page 26: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/26.jpg)
Lecture-14 26
Magnitude Summary
Magnitude Symbol Wave Period Local (Richter) ML S or Surface Wave* 0.8 s Body-Wave mb P 1 s Surface-Wave Ms Rayleigh 20 s Moment Mw Rupture Area, Slip 100’s-1000’s
• Magnitude is a measure of ground shaking amplitude.
• More than one magnitude scales are used to study earthquakes.
• All magnitude scales have the same logarithmic form.
• Since different scales use different waves and different period vibrations, they do not always give the same value.
![Page 27: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/27.jpg)
Lecture-14 27
The Largest Earthquakes
MW is the appropriate choice for comparing the largest events, it does not saturate.
1960 Chile 9.5
1964 Alaska 9.2
1952 Kamchatka9.1
1965 Aleutians 9.0
![Page 28: Lecture-14 1 Lecture #14- Earthquake Size. Lecture-14 2 How is Earthquake Size Determined ? (1) Maximum Seismic Intensity Outdated method Does not use.](https://reader034.fdocuments.in/reader034/viewer/2022042821/56649d595503460f94a391e0/html5/thumbnails/28.jpg)
Lecture-14 28
The largest earthquake ever recorded by seismometers (the
1960 Chilean event with 9.5 Mw ) had a seismic moment equivalent to the energy of 9 trillion kilotons
of TNT!