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Lecture#13

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

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Lecture#13

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  1. 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

  2. Outlines of the Presentation • Earthquake Intensity and Magnitude • Different Intensity and Magnitude Scale

  3. 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 earthquakeby modern seismograph. Depends on the size of the fault that breaks Determined from Seismic Records Magnitude and Intensity

  4. 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

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

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

  7. Itis 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: Richter Scale Amplitude (mm) 1 10 100 1000 10000 Magnitude 3 4 5 6 7

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

  9. Local Magnitude of Earthquake, ML Magnitude • 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).

  10. 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

  11. Relationship between Richter Scale magnitude and energy released

  12. 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

  13. 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.

  14. 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.

  15. 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 + 1.66 log10 +2 • A = Maximum ground displacement in micrometers • = Distance of seismograph from the epicenter, in degrees. • Surface wave magnitude is used for shallow earthquakes

  16. 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 +0.01  + 5.9 • A = Amplitude of P-waves in micrometers • T = Period of P wave • = Distance of seismograph from the epicenter, in degrees.

  17. 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.

  18. 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

  19. Seismic Energy • Both the magnitude and the seismic moment are related to the amount of energy that is radiated by an earthquake. • Dr. GutenbergandRichter1956), developed a relationship between magnitude and energy. Their relationship is: Log ES = 11.8 + 1.5MsEnergy 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.

  20. 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.    

  21. Frequency of earthquakes

  22. Some Important Earthquakes • 1755 - Lisbon, Portugal • Killed 70,000, Raised Waves in Lakes all over Europe • First Scientifically Studied Earthquake • 1811-1812 - New Madrid, Missouri • Felt over 2/3 of the U.S. • Few Casualties • 1886 - Charleston, South Carolina • Felt All over East Coast, Killed Several Hundred. • First Widely-known U.S. Earthquake • 1906 - San Francisco • Killed 500 (later studies, possibly 2,500) • First Revealed Importance of Faults

  23. Some Important Earthquakes • 1923 - Tokyo • Killed 140,000 • 1964 - Alaska • Killed about 200 • Wrecked Anchorage. • Tsunamis on West Coast. • 1976 - Tangshan, China • Hit an Urban Area of Ten Million People • Killed 650,000

  24. THE TOP FIVE

  25. STATISTICS OF SOME EARTHQUAKES

  26. 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

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