2. Engineering Geology and Seismology
Lecture#13
CE-312
Engineering Geology and Seismology
Instructor:
Dr Amjad Naseer
Department of Civil Engineering
N-W.F.P University of Engineering and Technology, Peshawar

3. Outlines of the Presentation
Earthquake Intensity and Magnitude
Different Intensity and Magnitude Scale

4. Magnitude and Intensity
How Strong Earthquake Feels to Observer
Qualitative assessment of the kinds of damage done by an earthquake
Depends on distance to earthquake & strength of earthquake
Determined from the intensity of shaking and damage from the earthquake
Magnitude
Related to Energy Release.
Quantitative measurement of the amount of energy released by an earthquake by modern seismograph.
Depends on the size of the fault that breaks
Determined from Seismic Records

5. Intensity Scale The different intensity scale are:
Rossi-Forel (RF) scale (1880s)-intesities ranges from I-IX
Modified Mercalli Intensity (MMI) Scale (1931)-intensities ranges from I-XII
Japanese Meteorological Agency (JMA)
Medvedev-Spoonheuer-Karnik (MSK) scale
Earthquake intensities are usually obtained from interviews of observers after the event.
Isoseisms
Isoseismal map
Epicentral intensity

6. Intensity How Strong Earthquake Feels to Observer Depends on:
Distance to Epicenter
Geology
Type of Building
Observer!
Varies from Place to Place

9. Earthquake Magnitude ML - Local (Richter) magnitude
MS - Surface wave magnitude
MB- Body wave magnitude
MW - Seismic Moment magnitude

10. Richter Scale Amplitude (mm) Magnitude 1 3 10 4 100 5 1000 6 10000 7
It is a measure of the amplitude of ground vibration using a seismometer.
Amplitude scale is logarithmic (10-fold increase for every whole number increase)
With the seismometer a standard distance of 100 km away from the epicenter, one gets:
Amplitude
(mm) 1 10
100
1000
10000
Magnitude 3 4 5 6 7
A magnitude of 3 is barely felt at the epicenter.

11. Richter Scale Drawbacks:
Based on Antiquated Wood-Anderson Seismographs
Measurement Past Magnitude 7.0 ineffective – Requires Estimates

12. Local Magnitude of Earthquake, ML
Richter scale measures the magnitude of an earthquake, based on seismogram independent of intensity
Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale
The equation for Richter Magnitude is:
ML = log10A(mm) + (Distance correction factor)
Here A is the amplitude, in millimeters, measured directly from the photographic paper record of the Wood-Anderson seismometer, a special type of instrument. The distance factor comes from a table given by Richter (1958).

13. Richter’s Local Magnitude, ML
Right side diagram (nomogram) demonstrates how to use Richter's original method to measure a seismogram for a magnitude estimate.
After you measure the wave amplitude you have to take its logarithm and scale it according to the distance of the seismometer from the earthquake, estimated by the S-P time difference. The S-P time, in seconds, makes t. The equation behind this nomogram, used by Richter in Southern California, is:
ML = log10A(mm) +3 log10[8 t (sec)]-2.93

14. Relationship between Richter Scale magnitude and energy released
Magnitude in Richter Scale
Energy Released in Joules
Comment
2.0
6.3 x 10 7
Smallest earthquake detectable by people.
5.0
2.0 x 10 12
Energy released by the Hiroshima atomic bomb.
6.3 x 1013 to 1.4 x 1015
About 120 shallow earthquakes of this magnitude occur each year.
6.7
7.1 x 1014
Northridge, California earthquake 1994.
7.0
2.0 x 1015
Major earthquake.
7.4
7.9 x 1015
Turkey earthquake August 17, More than 12,000 people killed.
7.6
1.6 x 1016
Deadliest earthquake this century. Tangshan, China, About 250,000 people died.
8.3
1.8 x 1017
San Francisco earthquake of 1906.
8.6
5.0 x 1017
Most powerful earthquake recorded in the last 100 years. Southern Chile Claimed 5,700 lives.

15. Richter Scale: Related to intensity
M=1 to 3: Recorded on local seismographs, but generally not felt
M= 3 to 4: Often felt, no damage
M=5: Felt widely, slight damage near epicenter
M=6: Damage to poorly constructed buildings and other structures within 10's km
M=7: "Major" earthquake, causes serious damage up to ~100 km (recent Gujarat earthquake).
M=8: "Great" earthquake, great destruction, loss of life over several 100 km
M=9: Rare great earthquake, major damage over a large region over 1000 km
Reference:

16. Below given figures describe the spatial distribution of small and large earthquakes respectively.
Distribution of earthquakes with a magnitude less than 5 on the Richter Scale.
Distribution of earthquakes with a magnitude greater than 7 on the Richter Scale.

17. These maps indicate that large earthquakes have distributions that are quite different from small events. Many large earthquakes occur some distance away from a plate boundary.
Some geologists believe that these powerful earthquakes may be occurring along ancient faults that are buried deep in the continental crust.
Recent seismic studies in the central United States have discovered one such fault located thousands of meters below the lower Mississippi Valley.
Some large earthquakes occur at particular locations along the plate boundaries. Scientists believe that these areas represent zones along adjacent plates that have greater frictional resistance and stress.

18. Surface Wave Magnitude,MS
Richter’s local magnitude does not distinguish between different types of waves.
At large distances from epicenter, ground motion is dominated by surface waves.
Gutenberg and Richter (1936) developed a magnitude scale based on the amplitude of Rayleigh waves.
Surface wave magnitude Ms = log10A log10 +2
A = Maximum ground displacement in micrometers
= Distance of seismograph from the epicenter, in degrees.
Surface wave magnitude is used for shallow earthquakes

19. Body Wave Magnitude, MB
For deep focus earthquakes, reliable measurement of amplitude of surface waves is difficult.
Amplitudes of P-waves are not strongly affected by focal depth.
Gutenberg (1945) developed a magnitude scale based on the amplitude of the first few cycles of P- waves, which is useful for measuring the size of deep earthquakes.
Body wave magnitude, MB = log10A – log10T  + 5.9
A = Amplitude of P-waves in micrometers
T = Period of P wave
= Distance of seismograph from the epicenter, in degrees.

20. Seismic - Moment Magnitude, MW
A Seismograph Measures Ground Motion at One Instant But --
A Really Great Earthquake Lasts Minutes and Releases Energy over Hundreds of Kilometers
Moment magnitude scale based on seismic moment (Kanamori, 1977) and doesn’t depend upon ground shaking levels.
It’s the only magnitude scale efficient for any size of earthquake.

21. Moment Magnitude Moment-Magnitude Scale
Seismic Moment = Strength of Rock x Fault Area x Total amount of Slip along Rupture
M0 =  A D
Moment Magnitude ,Mw = 2/3 x [log10M0(dyne-cm) –16]
Measurement Analysis requires Time

22. Seismic Energy
Both the magnitude and the seismic moment are related to the amount of energy that is radiated by an earthquake.
Dr. Gutenberg and Richter 1956), developed a relationship between magnitude and energy. Their relationship is:
Log ES = Ms Energy ES in ergs from the surface wave magnitude Ms .
ES is not the total ``intrinsic'' energy of the earthquake, transferred from sources such as gravitational energy or to sinks such as heat energy. It is only the amount radiated from the earthquake as seismic waves, which ought to be a small fraction of the total energy transferred during the earthquake process.

23. Local Magnitude - Seismic Energy correlation
Gujarat (2001)
Local Magnitude - Seismic Energy correlation
Size of an earthquake using the Richter’s Local Magnitude Scale is shown on the left hand side of the figure above. The larger the number, the bigger the earthquake. The scale on the right hand side of the figure represents the amount of high explosive required to produce the energy released by the earthquake.    

24. Frequency of earthquakes

25. Some Important Earthquakes
Lisbon, Portugal
Killed 70,000, Raised Waves in Lakes all over Europe
First Scientifically Studied Earthquake
New Madrid, Missouri
Felt over 2/3 of the U.S.
Few Casualties
Charleston, South Carolina
Felt All over East Coast, Killed Several Hundred.
First Widely-known U.S. Earthquake
San Francisco
Killed 500 (later studies, possibly 2,500)
First Revealed Importance of Faults

26. Some Important Earthquakes
Tokyo
Killed 140,000
Alaska
Killed about 200
Wrecked Anchorage.
Tsunamis on West Coast.
Tangshan, China
Hit an Urban Area of Ten Million People
Killed 650,000

27. THE TOP FIVE
Magnitude
Date
Location
9.2
1964
Prince William Sound, Alaska
9.1
1957
Andreanof Islands, Alaska
8.6
1899
Yakutat Bay, Alaska
8.3
1900
Kodiak Island, Alaska

28. STATISTICS OF SOME EARTHQUAKES
NAME OF EARTQUAKE
YEAR OF OCCURENCE
RICHTER MAGNITUDE
MMI INTENSITY
MAX acceleration
Imperial Valley, California
1940
6.7 – 7.1 X
0.33g
Kern county, California
1952
7.7 XI
0.18g
Park field, California
1966
5.5
VIII
0.5g
Konya, India
1967
6.25 – 7.5
0.63g
San fernando, California
1971
6.6
1.20g
Olympia, Washington
1949
7.1
0.31g

29. Mercalli Intensity scale Richter Scale- (logarithmic scale)
Seismogram is visual record of arrival time and magnitude of shaking associated with seismic wave. Analysis of seismogram allows measurement of size of earthquake.
Mercalli Intensity scale
Measured by the amount of damage caused in human terms- I (low) to XII (high);
Drawback: inefficient in uninhabited area
Richter Scale- (logarithmic scale)
Magnitude- based on amplitude of the waves
Related to earthquake total energy