Lecture-4 EQ.pdf
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Magnitude measures the amount of energy released by an earthquake at its source, using the Richter Scale. Intensity measures the strength of shaking produced by an earthquake at a certain location, using the Mercalli Intensity Scale. Magnitude is a quantitative measure while intensity is qualitative and accounts for location and building resilience. Higher magnitudes indicate a more powerful quake with exponentially greater energy release.
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Lecture-4 EQ.pdf
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- 3. 3 Measuring A Quake Magnitude – estimates the amount of energy released at the source of the earthquake. The Richter Magnitude Scale Intensity – a measure of the degree of earthquake shaking at a given location based on the amount of damage. The Mercalli Intensity Scale
- 4. 4 Magnitude Magnitude is a quantitative measure of the actual size of the earthquake. An increase in magnitude (M) by 1.0 implies 10 times higher waveform amplitude and about 31 times higher energy released. For instance, energy released in a M7.7 earthquake is about 31 times that released in a M6.7 earthquake, and is about 1000 (31×31) times that released in a M5.7 earthquake. (Did you know? The energy released by a M6.3 earthquake is equivalent to that released by the 1945 Atom Bomb dropped on Hiroshima!!) Source: http::/www.nicee.org/nicee/EQReports/Bhuj/isoseismal.html
- 5. 5 What is Richter Magnitude? Seismologists use a Magnitude scale to express the seismic energy released by each earthquake. Here are the typical effects of earthquakes in various magnitude ranges: Earthquake Severity Richter Magnitudes Earthquake Effects Less than 3.5 Generally not felt, but recorded. 3.5-5.4 Often felt, but rarely causes damage. Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions. 6.1-6.9 Can be destructive in areas up to about 100 kilometers across where people live. 7.0-7.9 Major earthquake. Can cause serious damage over larger areas. 8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across.
- 6. 6 Intensity Although each earthquake has a unique Magnitude, its effects will vary greatly according to distance, ground conditions, construction standards, and other factors. Intensity should not be confused with magnitude. It is an arbitrary ranking based on observed effects. Intensity is a qualitative measure of the actual shaking at a location during an earthquake, and is assigned as Roman Capital Numerals.
- 7. 7 Isoseismal Map of the 2001 Bhuj (India) Earthquake (MSK Intensity) Source: http::/www.nicee.org/nicee/EQReports/Bhuj/isoseismal.html
- 8. 8 I. Not felt except by a very few under especially favorable conditions. II. Felt only by a few persons at rest, especially on upper floors of buildings. III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. Earthquake intensities are rated with Roman numerals ranging from I (not felt) to XII (buildings nearly destroyed). The abbreviated description of the 12 levels of Modified Mercalli intensity is shown as follows…….. Source: Poster MMI Scale – NSET, Nepal
- 9. 9 IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. Source: Poster MMI Scale – NSET, Nepal
- 10. 10 VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Source: Poster MMI Scale – NSET, Nepal
- 11. 11 IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent. Source: Poster MMI Scale – NSET, Nepal
- 12. 12 XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air. Source: Poster MMI Scale – NSET, Nepal
- 13. 13 The Modified Mercalli and Richter Scale – side by side Text Source: Microsoft Encarta Encyclopedia
- 15. 15 Frequency of Occurrence of Earthquakes Descriptor Magnitude Average Annually Great 8 and higher 1 ¹ Major 7 - 7.9 17 ² Strong 6 - 6.9 134 ² Moderate 5 - 5.9 1319 ² Light 4 - 4.9 13,000 (estimated) Minor 3 - 3.9 130,000 (estimated) Very Minor 2 - 2.9 1,300,000 (estimated) ¹ Based on observations since 1900. ² Based on observations since 1990. The US Geological Survey (USGS) estimates that several million earthquakes occur in the world each year Source: http://neic.usgs.gov/neis/eqlists/eqstats.html
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